HK1148102B

HK1148102B - A method and an apparatus for driving electro-optic displays

Info

Publication number: HK1148102B
Application number: HK11102159.0A
Authority: HK
Inventors: R. Amundson Karl; W. Zehner Robert; A. Sjodin Theodore
Original assignee: E Ink Corporation
Priority date: 2004-08-13
Filing date: 2011-03-03
Publication date: 2012-12-21

Description

Method and apparatus for driving electro-optic display

The application is a divisional application of the same-name Chinese patent application No.2005800274744 filed on 8, 15 days of 2007 by the company Yingke.

Technical Field

The present invention relates to a method for driving an electro-optic display, in particular a bi-stable electro-optic display, and to a device (controller) for use in such a method. More particularly, the present invention relates to driving methods for enabling more accurate control of the gray state of pixels of electro-optic displays. The invention also relates to a driving method for enabling such a display to be driven in a manner that allows compensation for "dwell times" during which the pixels remain in a particular optical state until the transition, while still allowing the driving strategy for driving the display to be dc balanced. The invention is particularly, but not exclusively, intended for use with particle-based electrophoretic displays in which one or more types of charged particles are suspended in a liquid and moved in the liquid under the influence of an electric field to alter the display of the display.

Background

This application and international application publication No. wo 2005/054933; WO 2005/006290; WO 2004/090857; WO 03/107315; and WO 03/044765, to which the reader is referred for background information on the prior art of driving electro-optic displays. The following description will assume familiarity with these applications, which for convenience may be referred to hereinafter collectively as "method" (method of driving an electro-optic display) applications.

An electro-optic display using the method of the invention will typically comprise an electro-optic material which is a solid in the sense that the electro-optic material has a solid outer surface, although the material may, and typically does, have an interior space filled with a liquid or gas. Such displays using solid electro-optic materials may be referred to hereinafter for convenience as "solid electro-optic displays".

The term "electro-optic" as used herein for materials or displays is used in its conventional sense in the imaging art to refer to a material having first and second display states differing in at least one optical characteristic, the material changing from its first display state to its second display state by application of an electric field to the material. Although this optical property is typically a color perceptible to the human eye, it may also be other optical properties, such as light transmission, reflectance, brightness, or, in the case of machine-readable displays, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.

The term "grey state" as used herein in the imaging art is used in its conventional sense to refer to a state intermediate the two extreme (extreme) optical states of the pixel and does not necessarily encompass the black-to-white transition between the two extreme states. For example, several of the patents and published applications mentioned below describe electrophoretic displays in which the extreme states are white and deep blue, so that the intermediate "grey state" is effectively pale blue. In fact, as already mentioned, the transition between the two extreme states may not be a color transformation at all. The term "grey scale" is used herein to denote a possible optical state of a pixel, including two extreme optical states.

The terms "bistable" and "bistability" as used herein are used in their conventional meaning in the art to refer to such displays: comprising display elements having first and second display states which differ in at least one optical characteristic such that after driving of any given element to assume its first or second display state is completed with an addressing pulse of finite duration, that state will last at least several times, for example at least four times, the shortest duration of the addressing pulse required to change the state of the display element after the addressing pulse has terminated. It is described in published U.S. patent application No. 2002/0180687 that some particle-based, greyscale electrophoretic displays are stable not only in their extreme black and white states, but also in their intermediate greyscale states, which is equally suitable for some other types of electro-optic displays. This type of display is more suitably referred to as "multi-stable" rather than bi-stable, although the term "bi-stable" may be used herein to cover both bi-stable and multi-stable displays for convenience.

The term "impulse" as used herein in its conventional sense refers to the integral of voltage with respect to time. However, some bistable electro-optic media act as charge converters, and for such media an alternative definition of impulse, i.e. the integral of the current over time (which is equal to the total charge applied), can be used. A proper definition of the impact should be used depending on whether the medium is acting as a voltage-time impact converter or a charge impact converter.

More discussion below will focus on methods for driving one or more pixels of an electro-optic display to transition from an initial gray level to a final gray level (which may or may not be different from the initial gray level). The term "waveform" will be used to denote the entire voltage versus time curve used to affect the transition from a particular initial gray level to a particular final gray level. Typically, such a waveform will include a plurality of waveform elements, as illustrated below; where the elements are substantially rectangular (i.e., a given element comprises a constant voltage applied for a period of time), the elements may be referred to as "voltage pulses" or "drive pulses". The term "drive scheme" denotes a set of waveforms sufficient to cause all possible transitions between grey levels for a particular display.

Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member (rotating bichromal member) type, such as described in U.S. patent nos. 5,808,783; 5,777,782, respectively; 5,760,761, respectively; 6,054,071, respectively; 6,055,091; 6,097,531, respectively; 6,128,124, respectively; 6,137,467, respectively; and 6,147,791 (although this type of display is often referred to as a "rotating bichromal ball" display, the term "rotating bichromal ball" is more accurate because the rotating element is not spherical in some of the above patents). Such displays use a large number of small bodies (usually spherical or cylindrical) having two or more sections (sections) with different optical properties and an internal dipole. The bodies are suspended in liquid-filled vacuoles in a matrix, which vacuoles are filled with liquid so that the bodies are free to rotate. The appearance of such a display is altered by applying an electric field thereto, thereby rotating the bodies to various positions and altering the portion of the bodies seen through the viewing surface. This type of electro-optic medium is generally bistable.

Another type of electro-optic display uses an electrochromic medium, for example in the form of a nanochromic film (nanochromic film) comprising an electrode at least a portion of which is formed of a semiconducting metal oxide and a plurality of reversibly color-changeable dye molecules attached to the electrode; see, e.g., O' Regan, B., et al, Nature1991, 353, 737; and Information Display, 18(3), 24 (3 months 2002) of Wood, d. See also Bach, u., et al, adv.mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. patent No.6,301,038, international application publication No. wo 01/27690, and U.S. patent application No. 2003/0214695. This type of media is also generally bistable.

Another type of electro-optic display that has been studied earnestly and developed over the years is a particle-based electrophoretic display in which a plurality of charged particles move through a liquid under the influence of an electric field. Electrophoretic displays may have attributes of superior brightness and contrast, wide viewing angles, bi-stable states, and low power consumption compared to liquid crystal displays. However, problems with long-term image quality of these displays have prevented their widespread use. For example, particles forming electrophoretic displays are prone to deposition, resulting in insufficient lifetime of these displays.

As noted above, electrophoretic media require the presence of a fluid. In the recent prior art, this fluid is a liquid, but the electrophoretic medium can be generated with a gaseous fluid; see, for example, Kitamura, T.et al, "electric tuner movement for electronic Paper-like display", IDW Japan, 2001, Paper HCS1-I, and Yamaguchi, Y.et al, "tuner display using insulating specific microphone three display", IDW Japan, 2001, Paper AMD 4-4. See also european patent application 1,429,178; 1,462,847, respectively; 1,482,354, respectively; and 1,484,625; and international application WO 2004/090626; WO 2004/079442; WO 2004/077140; WO 2004/059379; WO 2004/055586; WO 2004/008239; WO 2004/006006; WO 2004/001498; WO 03/091799; and WO 03/088495. Such gas-based electrophoretic media appear to be as susceptible to the same types of problems resulting from particle deposition when the media is used in an orientation that allows such deposition, for example in indications where the media is deposited in a vertical plane, as liquid-based electrophoretic media. In fact, particle deposition appears to be more problematic in gas-based electrophoretic media than in liquid-based electrophoretic media, because the gaseous fluid is less viscous than the liquid, allowing the electrophoretic particles to deposit more quickly.

A number of patents and applications describing encapsulated electrophoretic media under or assigned to the names of the institute of technology and technology (MIT) and E Ink companies have been published in the near future. Such an encapsulation medium comprises a plurality of small capsules (capsules), each of which itself comprises an internal phase containing electrophoretically mobile particles suspended in a fluid, with capsule walls surrounding the internal phase. Typically, the capsule itself is held in a polymeric binder to form a binding layer (coherent layer) between the two electrodes. Encapsulation media of this type have been described, for example, in U.S. Pat. nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185, respectively; 6,118,426, respectively; 6,120,588; 6,120,839, respectively; 6,124,851, respectively; 6,130,773, respectively; 6,130,774, respectively; 6,172,798; 6,177,921, respectively; 6,232,950, respectively; 6,249,271, respectively; 6,252,564, respectively; 6,262,706, respectively; 6,262,833; 6,300,932, respectively; 6,312,304, respectively; 6,312,971, respectively; 6,323,989, respectively; 6,327,072, respectively; 6,376,828, respectively; 6,377,387, respectively; 6,392,785, respectively; 6,392,786, respectively; 6,413,790, respectively; 6,422,687, respectively; 6,445,374, respectively; 6,445,489, respectively; 6,459,418, respectively; 6,473,072, respectively; 6,480,182, respectively; 6,498,114, respectively; 6,504,524; 6,506,438, respectively; 6,512,354, respectively; 6,515,649, respectively; 6,518,949, respectively; 6,521,489, respectively; 6,531,997, respectively; 6,535,197, respectively; 6,538,801, respectively; 6,545,291, respectively; 6,580,545, respectively; 6,639,578, respectively; 6,652,075, respectively; 6,657,772, respectively; 6,664,944, respectively; 6,680,725, respectively; 6,683,333, respectively; 6,704,133, respectively; 6,710,540, respectively; 6,721,083, respectively; 6,724,519, respectively; 6,727,881, respectively; 6,738,050, respectively; 6,750,473, respectively; 6,753,999, respectively; 6,816,147, respectively; 6,819,471, respectively; 6,822,782; 6,825,068, respectively; 6,825,829, respectively; 6,825,970, respectively; 6,831,769, respectively; 6,839,158, respectively; 6,842,167, respectively; 6,842,279, respectively; 6,842,657, respectively; 6,864,875, respectively; 6,865,010, respectively; 6,866,760, respectively; 6,870,661, respectively; 6,900,851, respectively; and 6,922,276; and U.S. patent application publication No. 2002/0060321; 2002/0063661, respectively; 2002/0090980, respectively; 2002/0113770, respectively; 2002/0130832, respectively; 2002/0180687, respectively; 2003/0011560, respectively; 2003/0020844, respectively; 2003/0025855, respectively; 2003/0102858, respectively; 2003/0132908, respectively; 2003/0137521, respectively; 2003/0214695, respectively; 2003/0222315, respectively; 2004/0012839, respectively; 2004/0014265, respectively; 2004/0027327, respectively; 2004/0075634, respectively; 2004/0094422, respectively; 2004/0105036, respectively; 2004/0112750, respectively; 2004/0119681, respectively; 2004/0136048, respectively; 2004/0155857, respectively; 2004/0180476, respectively; 2004/0190114, respectively; 2004/0196215, respectively; 2004/0226820, respectively; 2004/0239614, respectively; 2004/0252360, respectively; 2004/0257635, respectively; 2004/0263947, respectively; 2005/0000813, respectively; 2005/0001812, respectively; 2005/0007336, respectively; 2005/0007653, respectively; 2005/0012980, respectively; 2005/0017944, respectively; 2005/0018273, respectively; 2005/0024353, respectively; 2005/0035941, respectively; 2005/0041004, respectively; 2005/0062714, respectively; 2005/0067656, respectively; 2005/0078099, respectively; 2005/0105159, respectively; 2005/0122284, respectively; 2005/0122306, respectively; 2005/0122563, respectively; 2005/0122564, respectively; 2005/0122565, respectively; 2005/0151709, respectively; and 2005/0152022; and international application publication No. WO 99/67678; WO 00/05704; WO 00/38000; WO 00/36560; WO 00/67110; WO 00/67327; WO 01/07961; and in WO 03/107,315.

A number of the above-mentioned patents and applications recognize that the walls surrounding discrete capsules (microcapsules) in an encapsulated electrophoretic medium can be replaced with a continuous phase (phase), thereby producing a so-called "dispersed polymer-dispersed" electrophoretic display in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and the discrete droplets of the electrophoretic fluid within such a dispersed polymer display can be considered as capsules or microcapsules even without the combination of a discrete capsule membrane and each individual droplet; see, for example, 2002/0131147, previously mentioned. Thus, for the purposes of this application, such dispersed polymer electrophoretic media are considered to be a sub-class of encapsulated electrophoretic media.

Encapsulated electrophoretic displays generally do not suffer from the clumping and settling failure modes of conventional electrophoretic devices and provide further advantages such as the ability to print or coat the display on a variety of flexible and rigid substrates. Use of the word "printing" is intended to include all forms of printing and coating including, but not limited to, pre-metered coating such as slot die coating, slot or extrusion coating, slide or step coating, curtain coating, roll coating such as knifeover roll coating, forward or reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, knife coating, screen printing processes, electrostatic printing processes, thermal printing processes, ink jet printing processes, and other similar techniques) whereby the resulting display may be flexible. Furthermore, since the display medium can be printed (using various methods), the display itself can be inexpensively fabricated.

A related type of electrophoretic display is the so-called "microcell electrophoretic display". In microcell electrophoretic displays, the charged particles and fluid are not encapsulated but are held within a plurality of cavities formed within a carrier medium, typically a polymer film. See, for example, international application publication No. wo 02/01281, and U.S. patent application publication No.2002/0075556, both assigned to Sipix Imaging, inc.

Other types of electro-optic media may also be used in the displays of the present invention.

Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media, the particles substantially block the transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays may be made to operate in a so-called "shutter mode" in which one display state is substantially opaque and one is light transmissive. See, e.g., the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552; 6,144,361, respectively; 6,271,823, respectively; 6,225,971, respectively; and 6,184,856. A dielectrophoretic display, which is similar to an electrophoretic display but relies on a change in electric field strength, can operate in a similar mode; see U.S. patent No.4,418,346.

The bistable or multistable nature of particle-based electrophoretic displays is in sharp contrast to the analogous nature of other electro-optic displays (such displays will be referred to hereinafter for convenience as "impact-driven displays") and the nature of conventional liquid crystal ("LC") displays. Twisted nematic liquid crystal behavior is not bistable or multistable but acts as a voltage transformer, applying a set electric field to a pixel of such a display, producing a specified grey level at the pixel, regardless of the grey level originally present at the pixel. In addition, liquid crystal displays are driven in only one direction (from non-transmissive or "dark" to transmissive or "bright"), and the reverse transition from the lighter state to the darker state is achieved by reducing or eliminating the electric field. Finally, the grey levels of the pixels of a liquid crystal display are not sensitive to the polarity of the electric field, but only to its amplitude, whereas in practice commercial liquid crystal displays often reverse the polarity of the driving electric field at frequent intervals for technical reasons. In contrast, bistable electro-optic displays, to a first approximation, act as impact transducers, such that the final state of a pixel depends not only on the applied electric field and the time for which the field is applied, but also on the state of the pixel prior to application of the electric field.

The first ideal method for addressing such impact driven electro-optic displays would be the so-called "general gray scale image flow", in which the controller schedules each write operation of the image so that each pixel transitions directly from its initial gray level to its final gray level. However, it is inevitable that there will be some error in writing the image on the impact driven display. Some of the errors faced in practice include:

(a) prior state dependencies; for at least some electro-optic media, the required impact of transitioning a pixel to a new optical state depends not only on the current and the desired optical state, but also on the previous optical state of the pixel.

(b) A dwell time correlation; for at least some electro-optic media, the required impact of transitioning a pixel to a new optical state depends on the time it takes for the pixel to be in its various optical states. The precise nature of this dependence is less well understood, but in general the more shocks required, the longer the pixel is in its current optical state.

(c) A temperature dependence; the required impact of switching the pixel to a new optical state depends strongly on the temperature.

(d) A humidity dependence; the required impact to transition the pixel to a new optical state depends, for at least some types of electro-optic media, on the ambient humidity.

(e) Mechanical uniformity; the required impact to transition the pixel to a new optical state may be affected by mechanical changes in the display, for example, changes in the thickness of the electro-optic medium or associated lamination adhesive. Other types of mechanical non-uniformity may result from unavoidable differences between media of different manufacturing batches of media, manufacturing tolerances, and material differences.

(f) A voltage error; the actual impact applied to the pixel is inevitably slightly different from the theoretically applied impact because of the small error that inevitably exists in the voltage delivered by the driver.

The general grayscale image stream suffers from the "accumulation of errors" phenomenon. For example, imagine that the temperature dependence produces 0.2L in the positive direction on each transition^*(wherein L^*With the general CIE definition:

L^*＝116(R/R₀)^1/3-16，

wherein R is the reflectance, R₀Is the base reflectance value) error. After fifty transitions, the error will accumulate to 10L^*. It may be more realistic to assume that the average error over each transition, expressed as the difference between the theoretical and actual reflectivity of the display, is 0.2L^*. After 100 consecutive transitions, these pixels will display an average of 2L deviations from their desired state^*(ii) a Such deviations are apparent to the average viewer on certain types of images.

This error accumulation phenomenon involves not only errors due to temperature but also all types of errors listed above. As described in the above-mentioned U.S. patent publication 2003/0137521, compensation for such errors is possible, but only to a limited degree of accuracy. For example, temperature errors may be compensated for by using a temperature sensor and a look-up table, but the temperature sensor has limited resolution and may read a temperature that is slightly different from the temperature of the electro-optic medium. Similarly, prior state dependencies can be compensated for by storing prior states and using a multi-dimensional transition matrix, but the controller memory limits the number of states that can be recorded and the size of the transition matrix that can be stored, thereby limiting the accuracy of this type of compensation.

Thus, the universal grayscale image stream requires very precise control of the applied shock to give good results, and it has been found by experience that it is not feasible in commercial displays in terms of the current state of electro-optic display technology.

Almost all electro-optic media have a built-in restart (error-limiting) mechanism, i.e. their extreme (usually black and white) optical states, which act as "optical guides". After a given pulse has been applied to a pixel of an electro-optic display, that pixel is unlikely to become whiter (or blacker). For example, in an encapsulated electrophoretic display, after a given pulse has been applied, all electrophoretic particles interact or interact with the capsule walls and cannot move further, thereby creating limited optical states or optical guides. Because of the electrophoretic particle size and charge distribution present in such media, some particles strike the rail before others, creating a "soft rail" phenomenon whereby the required pulse accuracy is reduced when the final optical state of the transition is close to the extreme black and white states, and the required optical accuracy is greatly increased when the end of the transition is close to the middle of the optical range of the pixel.

Various types of drive schemes for electro-optic displays using optical guides are known. For example, the previously described 2003/0137521 fig. 9 and 10 and the related description at paragraphs [0177] to [0180] describe a "slide show" drive scheme in which the entire display is driven to at least one optical rail before any new images are written. Clearly, a purely generic grayscale image stream driving scheme cannot rely on the use of optical guides to prevent errors in grayscale, because in such a driving scheme any given pixel may experience an infinite number of changes in grayscale without ever touching any of the optical guides.

Before proceeding further, it is desirable to define the slide show drive scheme more accurately. The basic slide show drive scheme is to effect a transition from an initial optical state (grey level) to a final (wanted) optical state (grey level) by transitioning to a limited number of intermediate states, where the minimum number of intermediate states is 1. Preferably, the intermediate state is at or near an extreme state of the electro-optic medium used. The transitions will vary from pixel to pixel in the display because they depend on the initial and final optical states. The waveform for a particular transition of a given pixel of the display can be expressed as:

R₂→goali→goal₂→…→goal_n→R₁(scheme 1)

Wherein in an initial state R₂And a final state R_iWith at least one intermediate or target state in between. The target state is typically a function of the initial state and the final state. The currently preferred number of intermediate states is 2, but more or fewer intermediate states may be used. Each individual transition within the overall transition is effected using waveform elements (typically voltage pulses) sufficient to drive the pixel from one state to the next in the sequence. For example, in the above-mentioned symbolically represented waveform, from R₂The transition to target i is typically achieved with waveform elements or voltage pulses. This waveform element may be a single voltage (i.e. a single voltage pulse) for a limited time, or may comprise different voltages in order to achieve the precise goali state. This waveform element is followed by a second waveform elementElement to achieve from goali to goal₂Is performed. If only two target states are used, the second waveform element is followed by a third waveform element, which drives the pixel from coarse₂State to final optical state R₁. The target state may be related to R₂And R₁Independently, or may depend on one or both.

Disclosure of Invention

The present invention seeks to provide an improved slide show drive scheme for electro-optic displays which enables improved control of grey levels. The invention has particular, but not exclusive, application to pulse width modulation drive schemes in which the voltage applied to any given pixel of the display at any given moment may be only-V, 0, or + V, where V is any voltage. More particularly, the present invention relates to two different types of improvements in slide show drive schemes, namely (a) inserting certain correction elements into the base waveform for such drive schemes; and (b) setting the drive scheme such that at least some of the grey levels are further closer to the optical guide from the desired grey level.

In another aspect, the invention relates to dwell time compensation for a drive scheme for an electro-optic display. As discussed in the "method" application, it has been found that, at least in the case of many particle-based electro-optic displays, the impulse necessary to change a given pixel by an equal change in gray scale (as judged by the human eye or by standard optical instrumentation) is not necessarily constant, nor is they necessarily interchangeable. For example, consider a display in which each pixel can display a gray level of 0 (white), 1, 2, or 3 (black), advantageously spaced apart. (the spacing between gray levels may be linear in percent reflectivity, as measured by the eye or instrument, but other spacings may be used-for example, the spacing may be linear in L, or alternatively a specific gamma value may be provided; a gamma value of 2.2 is often used for monitors, and a similar gamma value may be used when an electro-optic display is used as a replacement for a monitor.) it has been found that the pulses necessary to change a pixel from 0 to 1 (hereinafter referred to as a "0-1 transition" for convenience) are often different than those required for a 1-2 or 2-3 transition. Also, the pulses required for a 1-0 transition are not necessarily the same as the inversion of the pulses required for a 0-1 transition. In addition, some system representations exhibit a "memory" effect, such that the pulses required for, say, a 0-1 transition vary slightly depending on whether a particular pixel undergoes a 0-0-1, 1-0-1 or 3-0-1 transition. (where the symbol "x-y-z" represents a sequence of chronologically addressed optical states, where x, y, z are all optical states 0,1, 2 or 3.) although these problems can be reduced or overcome by driving all pixels of the display to one of the extreme states for a substantial period of time before driving the desired pixel to the other state, the resulting solid color "flicker" is generally unacceptable; for example, a reader of an electronic book may want the text of the book to scroll down, and if the display needs to blink solid black or white at frequent intervals, the reader may be confused or lose his position. Furthermore, such flickering of the display increases its power consumption and may reduce the lifetime of the display. Finally, it has been found that the impact required for a particular transition is, at least in some cases, affected by the temperature and the total running time of the display, and that these factors need to be compensated for in order to ensure accurate greyscale reproduction.

As briefly mentioned above, it has been found that in bistable electro-optic displays the impulse necessary for a given transition varies with the dwell time of the pixel in its optical state, a phenomenon hereinafter referred to as "dwell time dependence" or "DTD", although the term "dwell time sensitivity" has been used in some prior art documents, at least in some cases. It may therefore be desirable or even in some cases in practice necessary to vary the impact for a given transition as a function of the dwell time of the pixel in its initial optical state.

The phenomenon of residence time dependency will be explained in more detail below with reference to fig. 1 of the accompanying drawings, fig. 1 showing the dependency for the representation as R₃→R₂→R₁The reflectance of the pixel is a function of time, where (according to the nomenclature used above) each R is_kThe term represents a gray level in a gray level sequence, with the R with the larger subscript preceding the R with the smaller subscript. R₃And R₂And R₂And R₁The transitions between are also indicated. DTD is caused by the presence of the optical state R₂Last optical state R caused by a change in the time taken (called dwell time)₁A change in (c). DTD can be compensated by selecting different waveforms for different dwell times or for different ranges of dwell times in the preceding optical state. This method of compensation is referred to as "dwell time compensation," or "DTC," or simply "time compensation.

However, such DTCs may conflict with other desirable characteristics of the drive scheme. In particular, for the reasons discussed in detail in the "method" application, it is highly desirable for many electro-optic displays to ensure that the drive scheme used is Direct Current (DC) balanced, in the sense that the applied impulse (i.e. the integral of the applied voltage with respect to time) is zero for any random series of transitions that begin and end at the same optical state. This ensures that the net impulse experienced by any pixel of the display (also referred to as a "DC imbalance") is limited by a known value, regardless of the exact series of transitions that pixel experiences. For example, a 15V, 300 millisecond pulse is used to drive a pixel from a white state to a black state. After this transition the pixel experiences a DC imbalance pulse of 4.5V seconds. If a-15V, 300 ms pulse is used to drive the pixel from black to white, then the pixel is DC balanced for the entire run from white to black and then back to white. The DC balance should be maintained for all possible strokes from one original optical state to a series of optical states that are the same as or different from the original optical state and then return to the original optical state.

The drive scheme may be residence time compensated by adding or removing voltage signatures from the base drive scheme. For example, one can start with a drive scheme for a two optical state (black and white) display that includes the following four waveforms:

TABLE 1

Transformation of	Wave form
		Black to black	0V, 420 ms
Black to white	15V, 400ms, then 0V, 20ms
		White to black	+15V, 400 msec, then 0V, 20 msec
White to white	0V, 420 ms

The drive scheme is DC balanced in that any series of transitions that return the pixel to its initial optical state is DC balanced, i.e. the net area under the voltage profile (profile) is zero for the entire series of transitions.

Optical errors may arise from the DTD of the display. For example, a pixel may start in a white state, be driven to a black state, reside for a period of time, and then be driven back to the white state. The final white state reflectivity is a function of the time spent in the black state.

It is desirable to have a very small DTD. If this is not possible for a particular electro-optic display, it is desirable, in accordance with one aspect of the invention, to compensate for the DTD by selecting different waveforms for different ranges of dwell times in the previous optical state. For example, it can be seen that in the example just given the final white state is brighter after a short dwell of the previously black state than after a long dwell of the previously black state. One dwell time compensation scheme would be to modify the duration of the pulse that causes the pixel to go from black to white to cancel out this DTD of the last optical state. For example, when the dwell time of the previous state is short, the pulse length in the black to white transition may be shortened and the sustain pulse is longer than the long dwell time in the previous black state. This tends to generate darker white states for shorter prior state dwell times, which counteracts the effect of DTD. For example, a black-to-white waveform that varies with dwell time in the black state may be selected according to table 2 below.

TABLE 2

Dwell time	Wave form
		0 to 0.3 second	15V, 280 ms, 0V, 140 ms
0.3 second to 1 second	15V, 340 ms, 0V, 80ms
		1 second to 3 seconds	15V, 380 milliseconds,0V, 40ms
3 seconds or longer	15V, 400ms, 0V, 20ms

A problem with this approach to DTC of the drive scheme is that the drive scheme as a whole is no longer DC balanced. Since the impact on the black-to-white transition is a function of the time spent in the black state, and similarly, the impact on the white-to-black transition may be a function of the dwell time in the white state, the net pulse in the black-to-white-to-black sequence is typically not DC balanced. For example, assume that the sequence is implemented by: the voltage pulse with an impulse of-15V for 280 ms, i.e. -4.2V sec, transits from black to white after a short dwell time in the black state, and immediately after a long dwell time in the white state, transits from white to black with an impulse of 15V for 400ms, i.e. 6V sec. The net impact in this sequence (black white black cycle) is-4.2V sec +6V sec-1.8V sec. Repeating this cycle causes an increase in DC imbalance, which can be detrimental to the performance of the display.

Accordingly, this aspect of the invention provides a method of dwell time compensation for a DC balanced waveform or drive scheme that maintains DC balance of the waveform or drive scheme.

Another aspect of the invention relates to methods and apparatus for driving electro-optic displays that allow for rapid response to user input. The aforementioned "method" application describes several methods and controllers for driving electro-optic displays. Most of these methods use a memory with two image buffers, a first image buffer storing the first or initial image (presented on the display at the start of the transition or rewriting of the display), and a second image buffer storing the final image, which is intended to be placed on the display after the rewriting. The controller compares the initial image and the final image and, if they are different, applies drive voltages to the various pixels of the display which cause the pixels to undergo a change in optical state so that at the end of the overwrite (alternatively referred to as update) the final image is formed on the display.

However, in most of the above-mentioned methods and controllers, the update operation is "primitive (atomic)", in the sense that once the update is started, the memory cannot accept any new image data until after the update is completed. This causes some difficulties when the display is intended for applications that accept user input (e.g. via a keyboard or similar data entry device) because the controller does not respond to user input when an update is being implemented. For electrophoretic media, where the transition between two extreme optical states may take hundreds of milliseconds, the period of unresponsive time ranges from about 800 to about 1800 milliseconds, the majority of this period being attributable to the required update period of the electro-optic material. Although the duration of the unresponsive period may be reduced by removing some of the performance artifacts (artifacts) that increase the update time and by increasing the response speed of the electro-optic material. It is not possible for such a technique alone to reduce the unresponsive period below about 500 milliseconds. This is still longer than desired for interactive applications, such as the example of an electronic dictionary where a user desires to respond quickly to the user's input. Therefore, there is a need for an image update method and controller with reduced periods of non-response.

This aspect of the invention uses the principle of asynchronous image update (see Zhou et al, "drying an Active Matrix electrophoretic display," Proceedings of the SID 2004) to substantially reduce the duration of the period of unresponsiveness. The method described in this paper uses a structure already developed for grayscale image displays to reduce the unresponsive period by up to 65% compared to prior art methods and controllers, with only a modest increase in the complexity and memory requirements of the controller.

Finally, the invention relates to a method and apparatus for driving an electro-optic display in which data defining the drive scheme is compressed in a specified manner. The "methods" application described above describes methods and apparatus for driving electro-optic displays in which data defining the drive scheme (or schemes) used is stored in one or more look-up tables ("LUTs"). Such a LUT must of course contain data defining each waveform of the or each drive scheme, and a single waveform will typically require multiple bytes. As described in the "method" application, the LUT may have to take into account more than two optical states, along with adjustments for factors such as temperature, humidity, run time of the media, etc. Therefore, the capacity of the memory necessary to hold the waveform information may be considerable. It is desirable to reduce the capacity of the memory allocated to the waveform information to reduce the cost of the display controller. A simple compression scheme that can be accommodated in a display controller or host is indeed helpful in reducing the cost of the display controller. The present invention relates to a simple compression scheme which appears to be particularly beneficial for electro-optic displays.

Accordingly, in one aspect, the present invention provides a method for driving an electro-optic display having at least one pixel capable of achieving at least three different gray levels, said gray levels comprising two extreme optical states. The method comprises applying to said pixel a basic waveform comprising at least one reset pulse sufficient to drive said pixel to or near an extreme optical state, followed by at least one set pulse sufficient to drive said pixel to a grey level different from said extreme optical state. However, the basic waveform is modified by at least one of:

(a) inserting at least one balanced pulse pair into the base waveform;

(b) removing at least one balanced pulse pair from the base waveform; and

(c) at least one period of zero voltage is inserted into the basic waveform, where a "balanced pulse pair" represents a sequence of two pulses of opposite polarity such that the total impulse of the balanced pulse pair is substantially zero.

For convenience, this method of the present invention may be referred to hereinafter as the "balanced pulse pair slide show" or "BPPSS" method of the present invention. When the method includes modifying the base waveform by inserting or removing at least one balanced pulse pair ("BPP"), the two pulses of the balanced pulse pair may each be of constant voltage but opposite polarity and equal length. When the modification of the base waveform comprises removing at least one BPP, the period in the base waveform occupied by the or each removed BPP may be replaced by a period of zero voltage; alternatively, other elements of the basic waveform may be shifted in time to occupy the period previously occupied by the or each removed BPP, and the period of zero voltage may be inserted at a different point in time to that occupied by the or each removed BPP.

In a preferred form of the BPPSS method of the invention, the basic waveform comprises a first reset pulse sufficient to drive the pixel to or near one of its extreme optical states, a second reset pulse sufficient to drive the pixel to or near its other extreme optical state, and at least one set pulse in succession.

The BPPSS method may be implemented with a drive circuit capable of voltage modulation, pulse width modulation, or both. However, it has been found that a drive scheme for three levels is particularly useful in which a voltage of 0, + V or-V is applied to the pixel at any point in time, where V is a predetermined drive voltage.

For reasons explained in detail below, in the BPPSS method, it is desirable to define the total number of modifications to the basic waveform (i.e., the total number of balanced pulse pairs inserted or removed and periods of zero voltage inserted). Typically, the total number of such corrections will not exceed 6, desirably not 4 and preferably not 2.

As discussed in the aforementioned "methods" application, and as discussed below, it is desirable that the BPPSS method of the present invention be DC balanced, and as far as possible, that each individual waveform of the drive scheme used be DC balanced.

The BPPSS methods of the present invention may be used with any of the types of electro-optic displays discussed above. Thus, for example, the display may include a rotating bichromal or electrochromic medium. Alternatively, the display may comprise an electrophoretic electro-optic medium comprising a plurality of electrically charged particles in a fluid and capable of moving through the fluid on application of an electric field to the fluid. In this type of display, the fluid may be gaseous or liquid. The charged particles and the fluid may be confined within a number of capsules or microcells.

The present invention extends to a display controller, application specific integrated circuit, or software code for implementing the BPPSS method of the present invention.

In another aspect, the invention provides a method of driving an electro-optic display having a plurality of pixels, each pixel being capable of achieving at least four different grey levels comprising two extreme optical states, said method comprising applying to each pixel a waveform comprising a reset pulse sufficient to drive the pixel to or near one of its extreme optical states, followed by a set pulse sufficient to drive the pixel to a final grey level different from said extreme optical state, wherein the reset pulse is selected such that the image on the display immediately prior to said set pulse is substantially an inverted monochromatic projection of the final image following said set pulse.

For convenience, this method of the present invention may be referred to hereinafter as the "inverted monochromatic projection" or "IMP" method of the present invention. As explained in more detail below, a monochromatic projection of a gray scale image is a projection in which all pixels in the gray scale image that are in one extreme optical state or in a gray state closer to that extreme optical state than a predetermined threshold (e.g., white and light gray pixels) are changed to that extreme optical state (e.g., white) or to a state close thereto, while pixels in the opposite extreme optical state or in a state closer to that opposite extreme optical state than the threshold (e.g., black and dark gray) are changed to the opposite extreme optical state (e.g., black) or to a state close thereto. The inverted monochromatic projection is the inverse of the monochromatic projection.

In a preferred form of the IMP method of the invention, a waveform is applied to each pixel, the waveform comprising a first reset pulse sufficient to drive each pixel to or near one of its extreme optical states, a second reset pulse sufficient to drive each pixel to or near its other extreme optical state, and a set pulse, and the first reset pulse is selected such that the image on the display immediately preceding the second reset pulse is substantially a monochrome projection of the final image following the set pulse.

In the IMP method, the waveform may be modified by:

(a) inserting at least one balanced pulse pair into the waveform;

(b) removing at least one balanced pulse pair from the waveform; and

(c) inserting at least one period of zero voltage into the waveform,

wherein the "balanced pulse pair" is as defined above. In such a modified waveform, the two pulses of the balanced pulse pair may each be of constant voltage but opposite polarity and equal length. When the modification of the base waveform comprises removing at least one BPP, the period in the base waveform occupied by the or each removed BPP may be replaced by a period of zero voltage; alternatively, other elements of the basic waveform may be shifted in time to occupy the period previously occupied by the or each removed BPP, and the period of zero voltage may be inserted at a different point in time to that occupied by the or each removed BPP.

As with the BPPSS method, the IMP method of the present invention may be implemented with a driver circuit capable of voltage modulation, pulse width modulation, or both. The IMP method has been found to be particularly useful, however, for a three-level drive scheme in which a voltage of 0, + V or-V is applied to a pixel at any point in time, where V is a predetermined drive voltage. Also, as with the BPPSS method, the IMP method of the present invention may be used with any of the types of electro-optic displays discussed above. Thus, for example, the display may contain a rotating bichromal or electrochromic medium. Alternatively, the display may comprise an electrophoretic electro-optic medium comprising a plurality of electrically charged particles in a fluid and capable of moving through the fluid on application of an electric field to the fluid. In this type of display, the fluid may be gaseous or liquid. The charged particles and the fluid may be confined within a number of capsules or microcells.

The invention extends to a display controller, application specific integrated circuit or software code for implementing the IMP method of the invention.

In another aspect, the invention provides a method for driving an electro-optic display having at least one pixel, each pixel being capable of achieving at least two different gray levels, wherein at least two different waveforms are used for the same transition between particular gray levels, depending on the duration of the dwell time of the pixel in the state in which said transition begins, the two waveforms differing from each other in at least one of the following ways:

(a) inserting at least one balanced pulse pair;

(b) removing at least one balanced pulse pair; and

(c) at least one period of zero voltage is inserted,

wherein the "balanced pulse pair" is as defined above.

For convenience, this method of the present invention may be referred to hereinafter as the "dwell time compensated balanced pulse pair" or "DTCBPP" method of the present invention. In this approach, the entire drive scheme is highly desirable to be DC balanced, and preferably, all waveforms are themselves DC balanced. When the method includes modifying the basic waveform by inserting or removing at least one BPP, the two pulses of the balanced pulse pair may each be constant in voltage but of opposite polarity and equal length. When the modification of the base waveform comprises removing at least one BPP, the period in the base waveform occupied by the or each removed BPP may be replaced by a period of zero voltage; alternatively, other elements of the basic waveform may be shifted in time to occupy the period previously occupied by the or each removed BPP, and the period of zero voltage may be inserted at a different point in time to that occupied by the or each removed BPP.

As with the BPPSS and IMP methods, the DTCBPP method of the present invention can be implemented with a driver circuit capable of voltage modulation, pulse width modulation, or both. However, the DTCBPP method has been found to be particularly useful for a three-level drive scheme in which a voltage of 0, + V or-V is applied to a pixel at any point in time, where V is a predetermined drive voltage. For reasons explained in detail below, in the DTCBPP method, it is desirable to define the total number of modifications to the base waveform (i.e., the total number of balanced pulse pairs inserted or removed and periods of zero voltage inserted). Generally, the total number of such corrections will not exceed 6, desirably will not exceed 4, and preferably will not exceed 2.

Also, as with the BPPSS and IMP methods, the DTCBPP method can be used for any of the types of electro-optic displays discussed above. Thus, for example, the display may include a rotating bichromal or electrochromic medium. Alternatively, the display may comprise an electrophoretic electro-optic medium comprising a plurality of electrically charged particles in a fluid and capable of moving through the fluid on application of an electric field to the fluid. In this type of display, the fluid may be gaseous or liquid. The charged particles and the fluid may be confined within a number of capsules or microcells.

The invention extends to a display controller, application specific integrated circuit or software code for implementing the DTCBPP method of the invention.

In another aspect, the invention provides two related methods for reducing the period of non-response while an electro-optic display is being updated. A first of these methods is used to drive an electro-optic display having a plurality of pixels, each pixel capable of achieving at least two different gray levels, said method comprising:

(a) providing a final data buffer, the final data buffer being arranged to receive data defining a desired final state for each pixel of the display;

(b) providing an initial data buffer, the initial data buffer being used to store data defining an initial state of each pixel of the display;

(c) providing a target data buffer, the target data buffer being used to store data defining a target state for each pixel of the display;

(d) determining when the data in the initial and final data buffers differs, and when such a difference is found, updating the value in the target data buffer by: (i) setting the target data buffer to the value when the initial and final data buffers contain the same value for the particular pixel; (ii) setting the target data buffer to the value of the initial data buffer minus an increment when the initial data buffer contains a greater value for the particular pixel than the final data buffer; and (iii) setting the target data buffer to increase the value of the initial data buffer by the increment when the initial data buffer contains a smaller value for the particular pixel than the final data buffer;

(e) updating the image on the display with the data in the initial data buffer and the data in the target data buffer as the initial and final states, respectively, for each pixel;

(f) after step (e), copying the data from the target data buffer into the initial data buffer; and

(g) repeating steps (d) through (f) until the initial and final data buffers contain the same data.

The second of these two methods is for driving an electro-optic display having a plurality of pixels, each pixel capable of achieving at least three different gray levels, said method comprising:

(d) providing a polarity bit array, the polarity bit array being used to store a polarity bit for each pixel of the display;

(e) determining when the data in the initial and final data buffers differs, and upon finding such a difference, updating the polarity bit array and the value in the target data buffer by: (i) setting a value for a polarity bit of a pixel to transition to an opposite extreme optical state when the values in the initial and final data buffers are different for the particular pixel and the values in the initial data buffer represent extreme optical states of the pixel; (ii) setting the target data buffer to be the value of the initial data buffer plus or minus an increment according to the correlation value in the polarity bit array when the values for the specific pixels are different in the initial and final data buffers;

(f) updating the image on the display with the data in the initial data buffer and the data in the target data buffer as the initial and final states, respectively, for each pixel;

(g) after step (f), copying the data from the target data buffer into the initial data buffer; and

(h) repeating steps (e) through (g) until the initial and final data buffers contain the same data.

For convenience, these two related methods of the present invention may be referred to hereinafter as the "target buffer" or "TB" methods of the present invention. When it is desired to distinguish between the two methods, the former may be referred to as a "non-polar target buffer" or "NPTB" method, while the latter may be referred to as a "polar target buffer" or "PTB" method. The invention extends to a display controller, application specific integrated circuit or software code for implementing the TB method of the invention.

Finally, the present invention provides a method for reducing the amount of data that needs to be stored to drive an electro-optic display. Accordingly, the present invention provides a method for driving an electro-optic display having a plurality of pixels, each pixel capable of achieving at least two different gray levels, said method comprising:

storing a basic waveform defining a sequence of voltages to be applied by the pixel during a particular transition between two gray levels;

storing a multiplication factor (multiplication factor); and

said specific transition is effected by applying said sequence of voltages to said pixel over a plurality of periods depending on said multiplication factor.

For convenience, this method may be referred to hereinafter as the "waveform compression" or "WC" method of the present invention.

Drawings

As already mentioned above, fig. 1 of the accompanying drawings shows the reflectivity of a pixel of an electro-optical display as a function of time and illustrates the phenomenon of dwell time dependence.

Fig. 2A and 2B illustrate waveforms for two different transitions in a prior art triple-reset pulsed slide show drive scheme of the type described in the aforementioned "method" application.

Fig. 2C and 2D illustrate the reflectance over time for two pixels of an electro-optic display to which the waveforms of fig. 2A and 2B, respectively, have been applied.

Fig. 3A and 3B illustrate waveforms for two different transitions in a prior art two-reset pulse slide show drive scheme of the type described in the aforementioned "method" application.

Fig. 4A, 4B and 4C illustrate balanced pulse pairs that the BPPSS method according to the present invention can be used to modify prior art slideshow waveforms (such as those shown in fig. 2A, 2B, 3A and 3B).

Fig. 5A illustrates waveforms for a prior art two-reset pulse slide show drive scheme.

FIGS. 5B-5D illustrate BPPSS waveforms of the present invention generated by modifying the waveforms of FIG. 5A.

Fig. 6A illustrates the same prior art basic waveforms as fig. 5A.

FIGS. 6B-6D illustrate BPPSS waveforms of the present invention generated by removing balanced pulse pairs from the basic waveform of FIG. 6A.

FIG. 7A illustrates a BPPSS waveform of the present invention generated by inserting balanced pulse pairs between two basic waveform elements of a basic waveform.

Fig. 7B illustrates the BPPSS waveform of the present invention generated by inserting the same balanced pulse pair as in fig. 7A within a single basic waveform element of the same basic waveform as in fig. 7A.

Fig. 8A illustrates the same prior art basic waveforms as fig. 5A and 6A.

FIGS. 8B-8D illustrate BPPSS waveforms of the present invention generated by inserting periods of zero voltage at different locations in the basic waveform of FIG. 8A.

Fig. 9A and 9B illustrate prior art base waveforms that may be modified to generate the BPPSS waveforms of the present invention.

FIG. 9C illustrates a BPPSS waveform of the present invention generated by inserting two balanced pulse pairs in the basic waveform of FIG. 9B.

FIG. 9D illustrates the BPPSS waveform of the present invention generated by inserting balanced pulse pairs and periods of zero voltage in the basic waveform of FIG. 9B.

FIGS. 10A-10C and 11A-11C illustrate additional BPPSS waveforms of the present invention generated by modifying the basic waveforms of FIGS. 9A and 9B.

FIG. 12 is a symbolic representation of the inverted monochromatic projection method of the present invention.

Fig. 13 shows the way in which the grey levels of a grey scale image are mapped to a monochrome projection of the image, which can be achieved with the preferred inverse monochrome projection method of the present invention.

Figures 14 and 15 show selected waveforms used during the first inverse monochromatic projection method of the present invention.

FIG. 16 is a symbolic representation of another inverted monochromatic projection method of the present invention similar to FIG. 12.

Figure 17 illustrates modifying one of the IMP waveforms shown in figure 14 by inserting balanced pulse pairs in the waveform.

Figure 18 illustrates the modification of one of the IMP waveforms shown in figure 14 by removing balanced pulse pairs from the waveform.

Figure 19 illustrates an additional modification to one of the IMP waveforms shown in figure 17 by changing the insertion position of the balanced pulse pair.

Figure 20 illustrates an additional modification to one of the IMP waveforms shown in figure 18 by changing the position of the despibed balanced pulse pairs.

Fig. 21 illustrates in a highly schematic manner the waveforms of a further IMP drive scheme of the present invention.

Fig. 22 is a graph representing gray levels generated by the driving scheme shown in fig. 21.

Fig. 23 illustrates a modified form of the IMP drive scheme shown in fig. 21 in the same manner as fig. 21.

Fig. 24 is a graph showing gray levels generated by the modified drive scheme shown in fig. 23.

Fig. 25A-25E illustrate a set of dwell compensation waveforms used in the first dwell compensation balanced pulse pair driving scheme of the present invention.

Fig. 26A-26C illustrate a set of dwell compensation waveforms used in the second dwell compensation balanced pulse pair driving scheme of the present invention.

Detailed Description

From the foregoing summary it will be apparent that the invention provides several different methods of driving electro-optic displays, particularly bistable electro-optic displays, and apparatus and software code adapted to implement such methods. The various methods of the invention are described separately below primarily, but it will be appreciated that a single electro-optic display or component thereof may employ more than one aspect of the invention. For example, a single electro-optic display may use the BPPSS, IMP and DTCBPP aspects of the invention. It should also be noted that the preferred form of balancing pulse pairs is common to all aspects of the present invention that use such pulse pairs, which are the preferred definition of the size of such pulse pairs, and the method for adjusting the length of the waveform to accommodate the insertion or removal of such pulse pairs and/or periods of zero voltage. Finally, it should be noted that the desired DC balanced drive scheme, as well as the DC balanced waveforms, are also generic to all aspects of the invention, as discussed in the above-referenced "methods" application and below.

Part A: balanced pulse pair slide display method and device

As already mentioned, the BPPSS method of the present invention is a method for driving an electro-optical display having at least one pixel capable of achieving at least three different gray levels, said gray levels comprising two extreme optical states. The method comprises applying to said pixel a basic waveform comprising at least one reset pulse sufficient to drive said pixel to or near an extreme optical state, followed by at least one set pulse sufficient to drive said pixel to a grey level different from said extreme optical state. The basic waveform is modified by at least one of:

(a) inserting at least one balanced pulse pair into the base waveform;

(b) removing at least one balanced pulse pair from the base waveform; and

(c) at least one period of zero voltage is inserted into the basic waveform.

Also, as already mentioned, the term "balanced pulse pair" denotes a sequence of two pulses of opposite polarity, such that the total impact of the balanced pulse pair is substantially zero. In a preferred form of the BPPSS method, the two pulses of the balanced pulse pair are each constant voltage, opposite in polarity, and equal in length. The term "basic waveform element" or "BWE" may be used later to refer to a reset or set pulse of the basic waveform. The insertion of balanced pulse pairs and/or zero voltage periods (hereinafter may be referred to as "gaps") may be performed within a single basic waveform element or between two consecutive waveform elements. All of these modifications have the property of not affecting the net impulse of the waveform; the net impulse is the integral of the waveform voltage curve over the duration of the waveform. Balanced pulse pairs and zero voltage pauses of course have zero net impact. Although typically the pulses of the BPP will be inserted adjacent to each other, this is not essential and the two pulses may be inserted at separate locations.

Wherein the correction of the basic waveform of the BPPSS method according to the present invention includes removing at least one BPP, a period previously occupied by the or each removed BPP may become a period of zero voltage. Alternatively, the period may be "closed" by moving some or all of the following waveform elements to an earlier time, but in this case it is usually necessary to insert a period of zero voltage at some later stage in the waveform, typically at the end of the waveform, to ensure that the total length of the waveform is maintained, since it is usually necessary to ensure that all pixels of the display are driven with waveforms of equal length. Of course, alternatively, the period may be "closed" by moving some or all of the earlier waveform elements to a later time, with a period of zero voltage inserted at an earlier stage of the waveform (typically at the beginning of the waveform).

As already indicated, the BPPSS waveforms of the present invention are a modification of the basic slide show waveforms described in the aforementioned "method" application. As discussed above, the slideshow waveform includes one or more reset pulses that move the pixels to or at least near one of the extreme optical states (optical rail); if the waveform comprises two or more reset pulses, each reset pulse after the first will cause the pixel to move to the opposite extreme optical state, traversing substantially its entire optical extent. (for example, if the display uses an electro-optic medium with a reflectivity in the range of, say, 4% to 40%, each reset pulse after the first may cause the pixel to traverse 8% to 35% reflectivity.) if more than one reset pulse is used, successive reset pulses must of course be of alternating polarity.

The slide show waveform also contains a set pulse that drives the pixel from the extreme optical state it had been in by the last reset pulse to the desired final grey level of the pixel. It is noted that the duration of the set pulse may be zero when the desired final grey level is one of the extreme optical states and the last reset pulse brings the pixel in the desired extreme optical state. Similarly, the duration of the first reset pulse may be zero if the pixel's sleep state is in the extreme optical state prior to application of the slide show waveform.

Referring to the drawings, preferred BPPSS waveforms of the present invention will now be described, by way of illustration only.

Fig. 2A and 2B illustrate waveforms for two different transitions in a prior art (basic) slide display drive scheme of the type described in the aforementioned "method" application. The slide display driving scheme uses three reset pulses for each transition. Fig. 2C and 2D show the corresponding change in optical state (reflectance) versus time for pixels to which the waveforms of fig. 2A and 2B are applied, respectively. According to the convention used in the aforementioned "method" application, fig. 2C and 2D are drawn such that the bottom horizontal line represents the extreme optical state of black, the top horizontal line represents the extreme optical state of white, and the intervening levels represent the grey state. The start and end of the reset and set pulses of the waveform are indicated in fig. 2A and 2B by vertical dashed lines, and the different BWEs (i.e., reset and set pulses) are shown as pulses of generally or less equal length, although generally BWEs may have more arbitrary lengths, and if a series of pulses of equal length is included, typically more than ten such pulses are used for a maximum length BWE.

The basic waveform shown in fig. 2A and 2C, generally designated 100, effects a white-to-white transition (i.e., a "transition" where the initial and final states of the pixel are both extreme optical states of white). Waveform 100 includes a first negative (i.e., progressively black) reset pulse 102 that drives the pixel to its black-end optical state, a second positive (progressively white) reset pulse 104 that drives the pixel to its white-end optical state, a third negative (progressively black) reset pulse 106 that drives the pixel to its black-end optical state, and a set pulse 108 that drives the pixel to its white-end optical state. The four pulses 102, 104, 106, and 108 each have a maximum duration of ten units. (to avoid consecutively mentioning "units of duration", which may be referred to hereinafter as "time units" or "TUs")

Fig. 2B and 2D illustrate waveforms (generally designated 150) for making a transition from dark gray to light gray using the same three-reset pulse driving scheme as in fig. 2A and 2C. Waveform 150 includes a first reset pulse 152 that is negative and gradually black as the first reset pulse 102 of waveform 100. However, the transition using waveform 150 starts from a dark gray level, so the duration of the first reset pulse 152 (shown as four TUs) is shorter than the reset pulse 102, since a shorter first reset pulse is required to cause the pixel to change to its black extreme optical state at the end of the first reset pulse. For the remaining 6 TUs of the first reset pulse 152, zero voltage is applied to the pixel. (FIGS. 2B and 2D illustrate a first reset pulse 152 having a negative voltage of four TUs at the end of the relevant period, but this is arbitrary and periods of negative and zero voltages can be set as desired.)

The second and third reset pulses 104 and 106 of waveform 150 are the same as the corresponding pulses of waveform 100. The set pulse 158 of waveform 150, like the set pulse of waveform 100, is positive and gradually white. However, because the transition using waveform 150 ends with a light gray level, the duration of set pulse 158 (shown as 7 TUs) is shorter than the duration of set pulse 108 because a shorter set pulse is required to bring the pixel to its final light gray level. For the set pulse 158 for the remaining three TUs, zero voltage is applied to the pixel. (Again, the distribution of positive and zero voltage periods within the set pulse 158 is arbitrary and can be set as desired.)

As will be apparent from the foregoing, in the prior art slide show drive schemes shown in fig. 2A-2D, the duration of the first reset pulse and the duration of the set pulse will vary depending on the initial and final states of the pixel, respectively, and in some cases one or both of these pulses may be of zero duration. For example, in the drive schemes of fig. 2A-2D, the black-to-black transition may have a first reset pulse of zero duration (because the pixel is already in the black extreme optical state reached at the end of the first reset pulses 102 and 152), a set pulse of zero duration (because the pixel is already in the desired extreme black optical state at the end of the third reset pulse 106).

In general, it is desirable to keep the total duration of the waveform as short as possible so that the display can be rewritten quickly; for obvious reasons, users prefer displays that quickly display new images. Since each reset pulse occupies a substantial period of time, it is desirable to reduce the number of reset pulses to a minimum consistent with acceptable gray scale performance of the display, and one or two reset pulse slide show drive schemes are generally preferred. Fig. 3A and 3B illustrate waveforms for two different transitions in a prior art two-reset pulse slide show drive scheme of the type described in the aforementioned "method" application.

Fig. 3A illustrates a single reset pulse waveform (generally designated 200) from white to light gray, which includes a reset pulse 202 and a set pulse 208 (identical to pulse 158 in fig. 2B), where the reset pulse 202 drives the pixel from an initial white state to black and the set pulse 208 drives the pixel from black to light gray. Although waveform 200 uses only a single reset pulse, it will be appreciated that in practice it is part of a two reset pulse slide show drive scheme, the first reset pulse having a zero duration, as represented by the period of zero voltage on the left hand side of figure 3A.

Fig. 3B illustrates a two-black to light gray reset pulse waveform (generally designated 250) comprising a first reset pulse 252, a second reset pulse 254, and a set pulse 208, wherein the first reset pulse 252 drives the pixel from its initial black state to white, the second reset pulse 254 drives the pixel from white to black, and the set pulse 208, which is the same as the reset pulse in fig. 3A, drives the pixel from black to light gray.

As already mentioned, the BPPSS waveform of the present invention is derived from a basic slide show waveform (such as the waveforms shown in fig. 2A, 2B, 3A and 3B) by inserting at least one balanced pulse pair into the basic waveform, removing at least one balanced pulse pair from the basic waveform, or inserting at least one period of zero voltage into the basic waveform. In the case of BPP removal, the resulting gap may be "closed" or made to a zero voltage period. Combinations of these modifications may also be used.

FIGS. 4A-4C illustrate preferred balanced pulse pairs for use in the BPPSS waveforms of the present invention. The BPP shown in fig. 4A, generally designated 300, comprises a negative pulse 302 of constant voltage followed immediately by a positive pulse 304 of the same duration and voltage as pulse 302 but of opposite polarity. It is evident that BPP 300 imposes a zero net shock on the voxel. The BPP shown in fig. 4B, generally designated 310, is identical to the BPP 300 except that the order of the pulses is reversed. The BPP shown in fig. 4C, generally designated 320, results from the BPP 310 introducing a period 322 of zero voltage between the positive and negative pulses 304 and 302.

Fig. 5A-5D illustrate the modification of the basic two-reset pulse slide show waveform by BPP according to the present invention. Fig. 5A illustrates a basic waveform (generally designated 400) for a white to light gray transition. Waveform 400 is substantially similar to waveform 250 illustrated in fig. 3B, except that the order of the two reset pulses is reversed. Thus, the waveform 400 includes a 16-TU negative, gradually black first reset pulse 402 (which drives the pixel from its original white state to its black-extreme optical state), a 16-TU positive, gradually white second reset pulse 404 (which drives the pixel from its black-extreme optical state to its white-extreme optical state), and a 3-TU negative, gradually black set pulse 408 (which drives the pixel from its white-extreme optical state to the desired final light gray state).

Fig. 5B illustrates the BPPSS waveform of the present invention (generally designated 420) generated by inserting the BPP of fig. 4B between the second reset pulse 404 and the set pulse 408 of the waveform 400 of fig. 5A. As can be seen from fig. 5B, the effect of this insertion is that the positive pulse 304 of BPP lengthens the second reset pulse 404 to 17TU, while the negative pulse 302 of BPP lengthens the set pulse 404 to 4 TU.

FIG. 5C illustrates the BPPSS waveform of the present invention (generally designated 440) generated by inserting the BPP of FIG. 4C after the set pulse 408 of waveform 400 of FIG. 5A.

FIG. 5D illustrates the BPPSS waveform of the present invention (generally designated 460) generated from the further modified waveform 420 shown in FIG. 5B. The waveform 460 has a second BPP 304 ', 302' inserted between the first and second reset pulses 402 and 404 of the waveform 420; this second BPP is similar to the BPPs 304, 302 except that the duration of the two pulses is doubled.

As already noted and as illustrated in fig. 5D, the BPPSS waveform of the present invention may include multiple BPPs, removals, aborts, and combinations thereof (hereinafter collectively referred to as "additional waveform elements" or "AWEs"). However, it is generally preferred to use a minimum number of AWEs consistent with the desired accuracy of controlling the final gray level produced by the waveform. Both BPP and pauses lengthen the waveform, which in combination with several such BPPs and/or pauses may require an undesirably long period of time required to rewrite the display. For example, although the waveform 460 of FIG. 5D uses only one short 3-TU set pulse 408, the waveform 460 occupies the entire period of time for updating the display (the period between the vertical dashed lines in FIG. 5D), and introducing any additional BPP or pause would require that the period be extended. It is therefore desirable that the entire length of the modified waveform of the invention does not exceed the total length of the corresponding basic waveform (in which the duration of the set pulse is sufficient to drive the pixel from one extreme optical state to the other extreme optical state). In many cases (depending of course on the exact optical medium used in the display and other characteristics of the drive electronics), it has been found that good control of the grey scale can be achieved with a waveform containing no more than two AWEs; in other cases, no more than 4, or less often no more than 6 AWEs may be required, but it is generally not desirable to increase any further AWEs.

Fig. 6A-6D illustrate the modification of the basic two reset pulse waveforms by removing BPP in accordance with the present invention. For comparison, fig. 6A illustrates the same waveform 400 as fig. 5A. Note that waveform 400 is seen to terminate 7 TUs after the end of the set pulse 408, because fig. 6A assumes that, as in fig. 2A, 2B, 3A and 3B, an applied voltage of 10TU is required to fully drive the pixel between its extreme optical states, so that in another waveform of the same drive scheme, it would be necessary to lengthen the set pulse 408 to a maximum of 10 TU. Fig. 6B illustrates a modified BPPSS waveform of the invention (generally designated 520) generated by removing the BPP from the waveform 400, wherein the BPP comprises the last two TUs of the first reset pulse 402 and the first two TUs of the second reset pulse 404, resulting in a modified 14-TU first reset pulse 402 and a modified 14-TU second reset pulse 404, separated by a 4-TU pause 522 during which pause 522 a zero voltage is applied to the pixel.

FIG. 6C illustrates the BPPSS waveform of the present invention (generally designated 540) generated from the alternatively modified waveform 400 of FIG. 6A. Waveform 540 is generated by removing one BPP from waveform 400, which includes the last TU of the second reset pulse 404 and the first TU of the set pulse 408, and "closing" the period originally occupied by the removed BPP by shifting the first and second reset pulses back in time by 2 TU. Thus, waveform 540 includes a 2TU pause 544, a first reset pulse 402 for 16-TU, a second reset pulse 404 "for 15-TU, and a set pulse 408' for 2-TU; note that the set pulse 408' terminates at exactly the same time as the set pulse 408 of the basic waveform 400 at 7TU before the end of the waveform.

FIG. 6D illustrates the BPPSS waveform of the present invention (generally designated 560) generated from the further modified waveform 400 of FIG. 6A. Waveform 560 is generated by removing a BPP from waveform 400 that includes the last two TUs of the first reset pulse 402 and the first two TUs of the second reset pulse 404, and "closing" the period originally occupied by the removed BPP by moving the second reset pulse and the set pulse forward in time by 4 TUs. Thus, waveform 540 includes a first reset pulse 402 'of 14-TU (identical to the first reset pulse in FIG. 5B), a second reset pulse 404' of 14-TU (identical to the second reset pulse in FIG. 5B except for timing), and a set pulse 408 of 3-TU; note that the final period 562 of zero voltage after the set pulse 408 is extended from 7TU to 11TU because of the shift of the second reset pulse 404' and the set pulse 408.

The preferred BPPSS waveform modification discussed thus far involves the insertion or removal of BPPs between successive basic waveform elements or at the end of the basic waveform. However, the BPPSS aspect of the present invention is not limited to such modifications, but extends to modifications that insert BPPs within a single BWE, as will now be illustrated with reference to fig. 7A and 7B. Fig. 7A illustrates a BPPSS waveform 620 of the invention generated by modifying the base waveform 400 (fig. 5A or 6A) by inserting BPPs 302 ', 304' between the first reset pulse 402 and the second reset pulse 404, the BPPs 302 ', 304' being similar to the BPP shown in fig. 5D except for the opposite positive and negative pulse order. Fig. 7B shows another BPPSS waveform 640 of the invention generated by modifying the base waveform 400 by inserting BPPs 302 ', 304', but in waveform 640 the BPPs 302 ', 304' are inserted at the midpoint of the second reset pulse 404, splitting the pulse into two separate portions 404A and 404B. Thus, the waveform 640 includes a succession of 16-TU first reset pulses 402 (identical to the first reset pulses of the waveform 400), 8-TU pulses 404A (i.e., a first portion of the second reset pulse), BPP 302 ', 304', 8-TU pulses 404B (i.e., a second portion of the second reset pulse), and 3-TU reset pulses 408 (identical to the reset pulses of the waveform 400).

As already mentioned, the BPPSS aspect of the present invention includes not only the insertion or removal of BPPs from the basic waveform but also the insertion of pauses (zero voltage periods) in the basic waveform, and such insertion pauses will now be explained with reference to fig. 8A-8D. For comparison, fig. 8A illustrates the same basic waveform 400 as fig. 5A and 6A. Fig. 8B illustrates a modified BPPSS waveform of the invention (generally designated 720) generated by introducing a 2-TU pause 722 between the second reset pulse 404 and the set pulse 408 of the basic waveform 400. It should be noted that the insertion pause 722 necessarily reduces the zero voltage period following the set pulse 408 from 7TU to 5 TU. Fig. 8C illustrates another BPPSS waveform of the invention, generally designated 740, which is generally similar to waveform 720 except that a pause of 2-TU is inserted after the first 12TU of the second reset pulse 404, splitting the second reset pulse into a first portion 404C and a second portion 404D. Thus, the waveform 740 includes a succession of 16-TU first reset pulses 402 (identical to the first reset pulses of the waveform 400), 12-TU pulses 404C (i.e., a first portion of the second reset pulse), 2-TU pauses 722', 4-TU pulses 404D (i.e., a second portion of the second reset pulse), and 3-TU reset pulses 408 (identical to the reset pulses of the waveform 400).

Fig. 8D illustrates the BPPSS waveform of the present invention (generally designated 760) generated by inserting a 2-TU pause in the base waveform 400. However, in waveform 760, a pause 722 "is inserted before the first reset pulse 402. Thus, waveform 760 comprises pause 722 ", first reset pulse 402, second reset pulse 404, set pulse 408 in succession, the last three elements all being identical to the corresponding elements of basic waveform 400.

As has been shown, the BPPSS waveforms provided by the present invention are useful for improving the gray scale performance of electro-optic displays, particularly bistable electro-optic displays. The BPPSS waveforms of the present invention can achieve such improved gray scale performance while still maintaining the long-term DC balance of the display. (for reasons discussed in detail in the aforementioned "method" application, it is important that the driving scheme used to drive at least some electro-optic displays be DC balanced, in the sense that the integral of the applied voltage with respect to time for a given pixel is bounded, regardless of the sequence of optical states through which that pixel is driven). It has been found that in accordance with the BPPSS aspect of the invention, the final gray level of a pixel can be adjusted by inserting or removing BPPs and/or inserting pauses. It has also been found that the final gray level of a pixel is affected by where the BPP is inserted or removed and/or the insertion is suspended. While the final gray level can typically be well controlled by inserting BPPs between adjacent BWEs, BPPs can also be inserted within a single BWE, as illustrated in fig. 7B, to change the "tunability" of the final gray level; for example, if the addition of BPP between two reset pulses does not provide sufficiently fine adjustability of the final gray level, moving the BPP to a point in the middle of BWE may adjust the final gray level more finely.

For example, the waveform 420 of fig. 5B typically produces a gray level that is slightly deeper than the gray level produced by the corresponding basic waveform 400 of fig. 5A, because the pulse 304 in the BPPs 304, 302 has little or no effect on the gray level of the pixel, because the gray level is already in the white end optical state at the end of the second reset pulse 404, and the pulse 302 effectively lengthens the set pulse 408 to bring the final gray level further from the white end optical state (i.e., slightly darker in color). In contrast, the waveform 540 shown in FIG. 6C generally produces a gray level that is slightly shallower than the gray level produced by the corresponding base waveform 400 of FIG. 6A. Since fig. 5A, 6A and 6C are based on the assumption that: the pixel can be moved between its extreme optical states by applying a voltage 10TU as shown (as mentioned above), the 16-TU second reset pulse 404 of the basic waveform 400 effecting a substantial "overdrive" of the pixel into the white-end optical rail (white-end optical state), i.e. the second reset pulse 404 lasts for a substantial period of time after the pixel has reached its extreme white optical state. Thus, the second reset pulse 4041TU of 16-TU is shortened to produce the 15-TU second reset pulse 404 "of waveform 540, which will have little or no effect on the gray scale level at the end of the second reset pulse 404". In contrast, shortening the 3-TU set pulse 4081TU of waveform 400 to produce the 2-TU set pulse 408' of waveform 540 will significantly reduce the extent to which the white end optical state present at the end of the second reset pulse 404 "is driven to black, so that the final gray level at the end of waveform 540 will be significantly darker than the end of the basic waveform 400.

As already indicated, it was also found that pauses (periods of zero voltage) can be used to adjust the final grey level. For example, adding a pause between the last reset pulse and the set pulse affects the final gray level. Moving the pause to an earlier point of the last reset pulse also all causes a slight change in the final grey level. Thus, the pause position can be used to adjust the final gray level produced by the BPPSS waveform. In general, a pause may be added at any point in the waveform. In addition, it is advantageously possible to move all BWEs of the waveform forward or backward in time within the update time interval allocated for full overwriting of the display, thereby moving the relative time positioning of the various transitions that occur within the entire transition from the initial state to the final state. This time shifting is advantageous for several reasons, such as reducing unwanted transient behavior of the display during the transition, or producing a more satisfactory final image, for example by reducing the variation between pixels intended to be at the same grey level.

Another preferred BPPSS waveform and drive scheme of the present invention will now be described with reference to FIGS. 9A-9D, 10A-10C and 11A-11C. Fig. 9A and 9B illustrate two basic waveforms of a prior art two-reset pulse slide show driving scheme in which each of the first and second reset pulses and the set pulse may occupy a maximum of 12 TU. Fig. 9A illustrates a waveform 800 for implementing a white-to-black transition, which includes a first reset pulse 802 that is 12-TU black shaded, a second reset pulse 804 that is 12-TU white shaded, and a set pulse 808 that is 12-TU black shaded. As discussed above with reference to fig. 2A and 2B, the length of the first reset pulse and set pulse needs to be adjusted if the initial state and final state of the pixel are intermediate gray levels between the black and white end optical states of the pixel, fig. 9B shows a basic waveform 810 comprising a first reset pulse 812 for 7-TU, a second reset pulse 804 for 12-TU (identical to the corresponding pulses of waveform 800), and a set pulse 818 for 6-TU. To "fill" the waveform 810 to the same total length 36-TU as the waveform 800, a zero voltage period 822 of 5-TU precedes the first reset pulse 812 and a zero voltage period 824 of 6-TU follows the set pulse 824.

FIG. 9C shows the BPPSS waveform of the present invention (generally designated 840) generated by a modification of the waveform 810 shown in FIG. 9B. Specifically, waveform 840 is obtained by inserting a first BPP containing a positive pulse 842 and a similar negative pulse 844 of 1-TU immediately before the first reset pulse 812 and a similar second BPP846, 848 immediately after the set pulse 818 at waveform 810. Pulses 812, 804, and 818 are unchanged, but to accommodate BPP while maintaining the overall length of waveform 840, the initial period 822 'of zero voltage is reduced to 3TU and the final period 824' of zero voltage is reduced to 4 TU.

The use of two BPPs in the manner illustrated in fig. 9C may, in at least some cases, make gray level control more accurate than that achievable with a single BPP. It has been found that the BPPs (e.g., BPPs 846, 848 in waveform 840) placed after the set pulse can cause the final gray levels to vary significantly, and if the driver used only allows relatively coarse adjustment of the duration of each half of the BPP (if, for example, the duration can only be adjusted by 1TU in fig. 9C), the difference between the gray levels that can be obtained by varying the minimum increment of the duration of each half of the BPP can be unacceptably large. BPPs inserted at earlier points in the waveform (e.g., BPPs 842, 844 in waveform 840) have much less effect on the final gray level than BPPs inserted after the set pulse, thus allowing the final gray level to be changed more finely. Thus, by controlling the duration of the BPPs 846, 848 to effect a coarse adjustment to the final gray level and by controlling the duration of the BPPs 842, 844 to effect a fine adjustment to that gray level, the waveform 840 allows the final gray level to be adjusted over a substantial range.

FIG. 9D illustrates the BPPSS waveform of the present invention (generally designated 860) resulting from optional modifications to the waveform 810. Like waveform 840, waveform 860 includes BPPs 846, 848 following the set pulse 818. However, the waveform 860 does not include the second BPP located earlier in the waveform, but rather includes a pause 850 of 4-TU between the second reset pulse 804 and the set pulse 818. The effect of the pauses tends to be smaller than BPPs of the same length at the same points in the waveform, and the pauses 850 function in a similar manner to the BPPs 842, 844 of the waveform, with changes in the length of the pauses 850 being used to make fine adjustments to the final gray level. Note that the final period 824' of zero voltage in waveform 860 has the same 4-TU length as in waveform 840, while the duration of the initial period 822 "of zero voltage is reduced to 1TU to accommodate the pause 850 of the 4-TU while still maintaining the total length 36-TU of the waveform.

FIGS. 10A-10C show additional BPPSS waveforms of the present invention generated by various modifications to the waveform 810 of FIG. 9B. The waveform of fig. 10A, generally designated 920, is formed by adding BPPs 846 ', 848' after the set pulse 818 of the waveform 810 (fig. 9B), each pulse 846 ', 848' of the BPPs having a length of 2 TU. The final period 824 "of zero voltage is reduced to 2TU to accommodate a BPP of 4-TU length.

As discussed above with reference to fig. 9C, changing the length of the BPP after the set pulse may not provide a sufficiently fine adjustment to the final gray level, and fig. 10B illustrates a waveform (generally designated 940) generated by further modifying waveform 920 to overcome this fine adjustment problem. The waveform 940 incorporates second BPPs 842 ', 844' between the second reset pulse 804 and the set pulse 818. Changing the length of the BPPs 842 ', 844' has less effect on the final gray level than changing the length of the BPPs 846 ', 848' accordingly, so that the BPPs 842 ', 844' can be used for fine adjustment of the final gray level.

Although changing the length of the BPPs 842 ', 844' has less of an impact than changing the length of the BPPs 846 ', 848' accordingly, it may have a greater impact than changing the length of BPPs inserted even earlier in the waveform (e.g., BPPs 842, 844 in fig. 9C). If the BPPs 842 ', 844' in the waveform 940 do not provide sufficiently fine adjustment of the final gray level, a second BPP may be inserted earlier in the waveform; generally, the earlier the BPP is inserted into the waveform, the smaller the variation in the final gray level produced by a given variation in the BPP length. For example, fig. 10C illustrates a BPPSS waveform (generally designated 960) of the present invention that is similar to waveform 940 except that the BPPs 842 ', 844' are replaced by BPPs 962, 964 disposed between the first reset pulse 812 and the second reset pulse 804. (BPPs 962, 964 are of opposite polarity to BPPs 842 ', 844', in the sense that negative pulses 962 precede positive pulses 964; BPPs of either polarity may be used anywhere within the waveform, although the polarity of the BPPs will of course change their effect on the final gray level.)

Finally, FIGS. 11A-11C illustrate the modification of the basic waveform by introducing BPP therein and aborting. Fig. 11A illustrates a waveform (generally designated 1020) generated by modifying the base waveform 810 by inserting BPPs 842 ', 844 ' between the second reset pulse 804 and the set pulse 818, with a corresponding reduction in the length of the final period 824 ' of zero voltage to 4 TU. For the reasons discussed above, the variation in the lengths of the BPPs 842 ', 844' may not provide sufficiently fine adjustment of the final gray level, and fig. 11B shows a BPPSS waveform (generally designated 1040) generated by further modifying the waveform 1020, specifically by introducing a 2-TU pause 1042 within the second reset pulse, thereby dividing the pulse into the first portion 804A and the second portion 804B. To accommodate the abort 1042, the length of the initial period 822' of zero voltage is reduced to 3 TU; the final period 824' of zero voltage remains at 5 TU.

Pauses 1042 are used to fine tune the final gray level. Such fine adjustment may be implemented by varying the duration of the pause 1042 and/or its position within the second reset pulses 804A, 804B; as with BPP, the effect of the pause on the final gray level varies not only with its length in the waveform but also with its position in the waveform. Of course the BPPSS aspect of the invention is not limited to the use of a single abort; for example, the pause 1042 may be replaced by two separate pauses of 1TU duration, such that the second reset pulse is split into three parts instead of two.

As already mentioned, when the waveform does not occupy the full period provided for updating the display (as in the example of waveform 810 of fig. 9B, which occupies only 25TU, whereas updating the display requires at least a period of 36TU to accommodate the longer waveform 800 of the same drive scheme), it is advantageous to move the entire waveform within the update period, for example to reduce transient visual effects during the update. Fig. 11C illustrates a waveform (generally designated 1060) generated by moving the entire waveform 10402TU of fig. 11B forward in time (actually inserting a 2-TU gap just after the set pulse 818, as indicated in fig. 11C), thereby reducing the initial period 822 "of zero voltage to only 1TU, and increasing the length of the final period 824A of zero voltage to 6 TU.

And part B: reverse monochromatic projection method and device

As already mentioned, a second aspect of the invention provides a method for driving an electro-optical display having a plurality of pixels each capable of achieving at least four different grey levels comprising two extreme optical states. The method comprises applying to each pixel a waveform comprising a reset pulse sufficient to drive the pixel to or near one of its extreme optical states, followed by a set pulse sufficient to drive the pixel to a final grey level different from the extreme optical state. The reset pulse is selected such that the image on the display just before the set pulse is substantially an inverted monochromatic projection of the final image after the set pulse. This process is referred to herein as the "inverted monochromatic projection" or "IMP" method.

Using the "target state" nomenclature in scheme 1 above, the IMP method can be defined as a monochromatic projection where the final target state is almost the inverse of the desired final state (Ri) of the display. In a preferred form of the IMP method, the target state (in the nomenclature of scheme 1, gold) precedes the final target state_n-i) Almost the desired final state (R) of the display₁) Monochromatic projection of (2). Such a preferred IMP procedure may be symbolized as scheme 2 shown in fig. 12, where R_i，mRepresents R₁The upper line indicates the image inversion.

A monochromatic projection of optical states is a state in which all possible grey levels in the image are mapped to one of the two extreme optical states of each pixel or (for reasons explained below) close to one of the extreme optical states. For this purpose, the gray levels may be represented as 1, 2,3,.. N, where N is the number of gray levels, the gray level with the smallest reflectivity (typically black) is represented as 1, the gray level with the next smallest reflectivity is represented as 2, and so on until the gray level with the largest reflectivity (typically white) is represented as N. A monochromatic projection of gray levels is a projection whereby gray levels equal to or less than a threshold are mapped to a state at or near gray level 1 and gray levels greater than the threshold are mapped to a state at or near gray level N. The threshold is most desirably N/2, but in practice may effectively be set anywhere in the range from 1 to the middle half of N, i.e. the threshold is at least N/4 and at most 3N/4.

An example of a monochromatic projection is shown in fig. 13. In this example, the gray scale image (symbolically illustrated on the left hand side of fig. 13) contains 8 gray scale levels, denoted 1 to 8. In a monochromatic projection, symbolically displayed on the right-hand side of the figure, grey levels 1 to 3 are mapped to grey level 1, as indicated by the connecting lines, while grey levels 4 to 8 are mapped to grey level 8. The inverted monochromatic projection is of course generated simply by inverting the two states used in the monochromatic projection.

The IMP method mentioned above produces "substantially" inverted monochromatic projections, and such projections involve optical states close to one of the extreme optical states, as will be explained. In principle, monochromatic projection as well as inverse monochromatic projection require projection onto one of the extreme optical states. In practice, however, the drive schemes and waveforms used to drive electro-optic displays are defined in terms of the voltage pulses or other waveform elements applied to individual pixels of the display, rather than in terms of the exact optical states resulting from the application of the defined voltage pulses or other waveform elements (although the two are closely related). As discussed in detail in the above-referenced "method" application, the response of at least some bistable electro-optic media to a given waveform or waveform element depends not only on the initial optical state of the pixel and the exact waveform or waveform element, but also factors such as some previous optical state of the pixel and how long the pixel remains in the same optical state before the waveform or waveform element is applied (the previously-referenced dwell time correlation problem). Because the slide show waveform generally does not take into account all such relevant factors, the actual optical states achieved by the various pixels in a monochromatic projection or an inverted monochromatic projection may differ slightly from the extreme optical states theoretically achieved in such a projection.

This deviation of the actual optical state of the pixel from the extreme optical state can be illustrated with reference to fig. 14-15, which shows the waveforms for certain selected transitions in the two-reset impulse slide show IMP method of the present invention using four gray scale electro-optic media, driving the media from black (gray scale 1) to white (gray scale 4) with a +15V200 millisecond impulse, and driving the media from white to black with a-15V 200 millisecond impulse. A first waveform, shown generally at 1420, for a black (grey level 1) to white (grey level 4) transition, comprises a first reset pulse 1422 (driving the pixel from black to white), a second reset pulse 1424 (driving the pixel from white to black), and a set pulse 1426 (driving the pixel from black to white). Fig. 14 also shows a waveform 1440 for a gray level 2 (dark gray) to gray level 4 (white) transition; this waveform 1440 has a first reset pulse 1428 that is only 140 milliseconds in length, rather than 200 milliseconds as in the case of reset pulse 1422 of waveform 1420. The second reset 1424 and set 1426 pulses of waveform 1440 are the same as those of waveform 1420. Finally, fig. 14 also shows waveform 1460 for a gray level 4 (white) to gray level 4 transition; in this case, the first reset pulse is of zero duration (i.e., only 200 milliseconds of zero voltage period at the beginning of the waveform), but the second reset 1424 and set 1426 pulses of waveform 1460 are the same as those of waveform 1420.

Fig. 15 shows further waveforms for the same drive scheme as in fig. 14. The first waveform shown in fig. 15, generally designated 1480, is for a gray level 1 (black) to gray level 1 transition and is essentially the inverse of waveform 1460 shown in fig. 14. Waveform 1480 has a first reset pulse of zero duration (i.e., simply 200 milliseconds of zero voltage period at the beginning of the waveform), a second reset pulse 1482 (which drives the pixel from black to white), and a set pulse 1484 (which drives the pixel from white to black). Fig. 15 also shows a waveform 1500 for a gray level 1 (black) to gray level 3 (light gray) transition. The waveform 1500 has a first reset pulse 1422 which is the same as the first reset pulse of waveform 1420 shown in fig. 14 and drives the pixel from black to white. The waveform 1500 also has a second reset pulse 1502 (which drives the pixel from white to black) and a set pulse 1504 (which drives the pixel from black to gray level 3 (light gray)) for 130 milliseconds. Finally, for completeness, fig. 15 repeats the black-to-white (gray level 1 to gray level 4) waveform of fig. 14.

It will be seen from fig. 14 and 15 that the illustrated drive scheme is an IMP drive scheme, where the upper-dashed R is just before the set pulse as in various waveforms_i，mThe image on the display just before the set pulse is shown as an inverted monochrome projection of the final image after the set pulse; more specifically, in all transitions ending with gray level 3 or 4, the pixel is black immediately before the set pulse, while for all transitions ending with gray level 1 or 2, the pixel is white immediately before the set pulse. In addition, according to another preferred form of the IMP method, e.g. by R in the various waveforms just before the second reset pulse_1，mThe image on the display just before the second reset pulse is shown as a monochrome projection of the final image after the set pulse; more specifically, in all transitions ending in gray level 3 or 4, the pixel is white immediately before the second reset pulse, while for all transitions ending in gray level 1 or 2, the pixel is black immediately before the second reset pulse.

However, as can be gathered from fig. 14 and 15, the reflectivity of a given grey level achieved at each point in the various waveforms is not necessarily exactly the same, although it is assumed that the difference between pixels of the same grey level is small relative to the full dynamic range of the display (the difference between the reflectivities of the two extreme optical states). For example, just prior to the second reset pulse, the pixels subjected to waveforms 1420 and 1460 in fig. 14 should both be at gray level (white). However, the pixels subjected to waveform 1420 will have just completed the black-to-white transition at this point, while the pixels subjected to waveform 1460 may have been in the white state for some time and (as discussed in some of the aforementioned "methods" applications) the optical state of the bistable electro-optic medium tends to "drift" (i.e., change gradually over time) without them being driven. Thus, the actual white state of a pixel subject to waveform 1460 can differ slightly from the white state of the pixel subject to waveform 1420 that was just rewritten. Modifications to the IMP drive scheme, such as those discussed below, may modify the reflectivity achieved at various target states and other points in the waveform, such that the reflectivity of various target and other states may deviate significantly from the reflectivity at target states achieved without such modifications.

Although the IMP drive schemes illustrated in fig. 14 and 15 use only two reset pulses and thus only two target states, the IMP aspects of the present invention are of course not limited to a particular number of reset pulses and target states; for example, fig. 16 symbolically illustrates, in the same manner as fig. 12, an IMP drive scheme that includes intermediate black (B) and white (W) states prior to monochrome projection and reversing the monochrome projection target state.

It should be noted that not all pixels of the display necessarily reach a given target state (e.g., an inverted monochrome projection target state) at the same point in time during the rewriting of the display from the initial image to the desired final image. The points in time in the transition where the target state arrives are the initial and the desired final grey level R, respectively₂And R₁As a function of (c). Ideally (and as illustrated generally herein) for R₂And R₁Is matched to the overall display being driven through various target states, and these target states are reached by all pixels simultaneously. However, it is often desirable to relatively time the various waveform shifts of the drive scheme. The temporal shifting of the waveform may be done for aesthetic reasons, e.g. to improve the appearance of the transition or the appearance of the resulting image. Also, the correction as discussed below may move the relative temporal position of the target state, thus for R₂And R₁Reach the target state at different times during the transition.

Alternative definitions of IMP drive schemes may be given without explicit mention of inverted monochromatic projections. The IMP drive scheme is a drive scheme in which the various grey levels of the display can be divided by thresholds such that one extreme optical state and at least one non-extreme optical state are on each side of the threshold, the set pulses of the slide display drive scheme being defined such that each set pulse affects a transition across the threshold. As this definition sets forth, in an IMP drive scheme, the final set pulse of each waveform drives the pixel from an extreme optical state that is farther away from the desired final gray level to the desired final gray level, where "farther" is used to mean "on the opposite side of the threshold" rather than simply counting the difference in the number of gray levels between the desired final gray level and the two extreme optical states.

It has been found that the IMP drive scheme allows for precise control of the final grey level and provides a wide temperature performance range. It is believed (although the invention is in no way limited by this belief) that associated with these advantages are a relatively long set pulse for driving from the "farther" extreme optical state to the final grey level, and a relatively constant power consumption (power drain) generated on the drive electronics during display update.

The basic IMP drive scheme described above can be effectively modified in several different ways to make small adjustments to the final grey level achieved, thereby changing the appearance of the display during the transition and achieving the desired image quality.

The first type of modification of the IMP drive scheme is to insert or remove balanced pulse pairs, and/or insert periods of zero voltage into the waveform, in a similar manner to that implemented in the BPPSS drive scheme (as discussed in section a above). The balanced pulse pairs used may, for example, have any of the forms shown in fig. 4A-4C. Modifying the basic IMP waveform to insert or remove BPP or insert a zero voltage period (pause) may be implemented in any of the ways previously described. The BPP may be inserted between two consecutive basic waveform elements or within a single basic waveform element. In many cases this has the effect of increasing the pulse length to or away from a particular target state. The removed BPP may be replaced by a zero voltage period, or other basic waveform elements may be moved in time to "close" the period previously occupied by the removed BPP, and the zero voltage period may be inserted at other points in the waveform. As in the BPPSS driving scheme, the final gray level reached is not only sensitive to the presence of BPPs and pauses in the waveform but also to their positions in the waveform, the general rule being that the earlier a BPP is inserted or removed or a pause is inserted in the waveform, the less the effect on the change in the final gray level.

It is important to realize that such waveform modification affects not only the reflectivity of the final optical state (i.e., the final gray level) but also the reflectivity of the intermediate target states. Although the target state of the basic IMP waveform is typically close to one of the extreme optical states (optical rails) and, by definition, to the optical rail for the last target state, or the last two target states in the preferred form of the IMP drive scheme, the modifications described above may change the reflectivity of the target state away from the optical rail. It is the change in the degree of optical rail drive that gives a small adjustment to the final optical state (grey level).

It has been found desirable to keep the impulse of each voltage pulse containing BPP relatively small. The amplitude of the BPP may be defined by a parameter d, the absolute value of which describes the length of each of the two voltage pulses of the BPP, and the sign of which represents the sign of the second of the two pulses. For example, the BPPs shown in FIGS. 4A and 4B are assigned d values of +1 and-1, respectively (whereas the BPP in FIG. 4C is assigned a d value of-1 in the uniform scheme, which inserts a gap correction between the two pulses). In a preferred embodiment of the IMP drive scheme all BPPs used have a value of d which is smaller in magnitude than PL, preferably smaller than PL/2, where PL (BPP measured in the same unit) is defined as the length of the voltage pulse required to drive the pixel from one extreme optical state to the other extreme optical state, or the average value of the voltage pulse, wherein the lengths for the two direction transitions are not the same in terms of the drive voltage characteristics of the drive scheme. In the example just given, d is expressed in units of display scan frames, and the BPP of fig. 4A and 4B has a plurality of voltage pulses each having a length of one scan frame. In this case, PL will also be defined in terms of scan frames. Of course all quantities may alternatively be expressed in units of time, such as seconds or milliseconds.

Fig. 17 illustrates three waveforms generated by modifying the IMP waveform 1440 shown in fig. 14 by inserting BPP. The first waveform shown in fig. 17, generally designated 1700, is identical to waveform 1440 except that a BPP 1702 is inserted at the end of the waveform, where the BPP 1702 includes a-15V 10 millisecond pulse followed by a +15V 10 millisecond pulse. The second waveform shown in fig. 17 (generally designated 1720) is inserted into the same BPP 1722 as BPP 1702, but between the second reset pulse and the set pulse of the waveform; to accommodate the BPP 1722, the two reset pulses are shifted forward in time by 20 milliseconds, correspondingly reducing the zero voltage period at the start of the waveform. The third waveform shown in fig. 17, generally designated 1740, has a BPP1742 inserted between the first and second reset pulses of the waveform; the BPP1742 has the opposite pulse order compared to the BPPs 1702 and 1722, and each pulse is 20 milliseconds long. To accommodate the BPP1742, the first reset pulse is shifted forward in time by 40 milliseconds, correspondingly reducing the zero voltage period at the start of the waveform.

Fig. 18 shows three waveforms generated by modifying the IMP waveform 1440 shown in fig. 14 by removing BPP therefrom. The first waveform shown in fig. 18, generally designated 1760, is generated by removing the BPP 1762 from the waveform 1440 and leaving the remaining waveform elements unchanged, where the BPP 1762 comprises the last 10 ms scan frame of the second reset pulse and the first scan frame of the set pulse. The second waveform shown in fig. 18, generally designated 1780, is similarly generated by removing the BPP 1782 from the waveform 1440 without changing the remaining waveform elements, where the BPP 1782 contains the last two scan frames of the first reset pulse and the first two scan frames of the second reset pulse, such that there is a 40 millisecond period of zero voltage at the point occupied by the removed BPP. Finally, a third waveform (generally designated 1800) shown in fig. 18 is generated by removing a BPP from waveform 1440 (which contains the last scan frame of the first reset pulse and the first scan frame of the second reset pulse), and closing the resulting gap (correspondingly increasing the zero voltage period at the beginning of the waveform) by moving the remaining scan frame of the first reset pulse back in time by 20 milliseconds.

Fig. 19 illustrates another possible modification of the waveform 1720 shown in fig. 17. The upper portion of fig. 19 repeats the basic waveform 1720 of fig. 17, including BPP 1722. Fig. 19 also illustrates a modified waveform (generally designated 1920) containing BPP 1922 similar to BPP 1722, but inserted 40 milliseconds earlier in time before the last four scan frames of the second reset pulse. Fig. 19 also illustrates a second correction waveform (generally designated 1940) that includes a BPP 1942 that is similar to the BPP 1722, but which is inserted 130 milliseconds earlier in time before the thirteen scan frames following the second reset pulse. As already noted, the final gray level achieved by some waveforms as shown in fig. 19 is a function of the balanced pulse versus insertion position, so a modification as shown in fig. 19 can be used to fine tune the final gray level.

Fig. 20 illustrates a modified IMP waveform generated by inserting a zero voltage period (pause) in the basic IMP waveform 1440 shown in fig. 14. The first waveform shown in fig. 20, generally designated 2000, is generated by inserting a 20ms pause, designated 2002, between the second reset pulse and the set pulse of the waveform, both reset pulses being shifted forward in time by 20ms and the zero voltage period being correspondingly reduced at the beginning of the waveform. The second waveform (generally designated 2020) shown in fig. 20 is substantially similar to waveform 2000, except that waveform 2020 has a pause (designated 2022) inserted 40 milliseconds after the first four scan frames of the set pulse than pause 2002. The third waveform shown in FIG. 20, generally designated 2040, is also substantially similar to waveform 2000, but waveform 2040 has a pause (designated 2042) that is inserted 130 milliseconds later than pause 2002 after the first thirteen scan frames of the set pulse. In waveforms 2020 and 2040, the scan frame of the set pulse prior to pause 2022 or 2042, respectively, is shifted forward in time by 20 milliseconds compared to waveform 2000 to accommodate the pause. As already mentioned, the final grey level reached by the waveform is sensitive to the presence and location of the pause, so a modification of the basic waveform like that shown in fig. 20 can be used to fine-tune the final grey level generated by the waveform.

As already indicated, it is desirable that the IMP drive scheme is DC balanced in the sense that for any gray level cycle (i.e. any sequence of gray levels starting and ending at the same gray level), the algebraic sum of the impulses applied to the pixels is zero. Examples of gray scale cycles are:

1-》1

2→3→2

4→4→3-》2-》4

we can define a non-downscaled gray level cycle as a sequence of gray levels that starts at the first gray level, goes through zero or more gray levels to the end of the first gray level, without visiting any gray level more than once except the final gray level (which is the same as the first gray level as already indicated). Clearly, for any gray level, there are a certain number of cycles that are not reducible. In addition, any gray level sequence may be shown, such as a complex sequence:

1→4→3→2-》3→2-》3→2→1→2→1

can become a sequence of non-reducible cycles and non-reducible cycles embedded within the non-reducible cycles. For example, the sequence may be broken down into a finite set of non-reducible cycles, i.e., two consecutive 2 → 3 → 2 cycles are embedded in the 1 → 4 → 3 → 2 → 1 cycle, followed by the 1 → 2 → 1 cycle.

If all the non-reducible cycles are DC balanced, all the possible sequences starting and ending at the same grey level are DC balanced. The preferred embodiment of the IMP drive scheme is that the net voltage impulse for all non-reducible cycles is zero, i.e. the waveform is DC balanced.

DC balancing the IMP waveform is not absolutely necessary. While large DC imbalances compromise the imaging performance of the display, a small number of DC imbalances are acceptable. When it is not possible to reach full DC balance, the IMP drive scheme is desirably controlled such that the net impulse of any cycle divided by the number of transitions in the non-reducible cycle is less than Q, where Q is one quarter of the smaller of the absolute values of the net impulse for a transition between two extreme optical states of a pixel, where the impulse is determined by the characteristic voltage (charateristicvoltage) of the drive scheme. The net impulse required to drive the imaging film from one extreme optical state to the other is indicative of the characteristic impulse of the medium and the approximate DC imbalance should be measured with respect to this characteristic impulse.

It is also generally desirable that the IMP drive scheme be of the "picket fence" type. As described in the aforementioned "method" application, it is often necessary or desirable to drive an electro-optic display with a drive circuit that can provide only two drive voltages. Since bistable electro-optic media typically need to be driven in two directions between their extreme optical states, it may first of all require at least three drive voltages, namely 0, + V, -V, where V is a substantially arbitrary drive voltage, so that one electrode for a particular pixel, typically the common front electrode in a conventional active matrix display, can be held at 0, while the other electrode, typically the pixel electrode for that pixel, can be held at + V and-V depending on the direction in which the pixel needs to be driven. When a two-voltage drive circuit is used, each waveform of the drive scheme is divided into a plurality of time segments; typically, these time periods are of equal duration, but this is not necessarily the case. In a non-picket fence drive scheme, a positive, zero, or negative drive voltage may be applied for any given pixel for any period of time. For example, in a three drive voltage system, the common front electrode may be held at 0 while the individual pixel electrodes are held at + V, 0, or-V. In the picket fence drive scheme, each time segment is actually split into two; in one of the two generated time periods, only a negative or zero drive voltage may be applied to any given pixel, while in the other generated time period, only a positive or zero drive voltage may be applied to any given pixel. For example, consider a two drive voltage system with drive voltages V and V, where V > V. In the first period of each pair of periods, the common front electrode is set to V and the pixel electrode is set to either V (zero drive voltage) or V (negative drive voltage). In the second of each pair of periods, the common front electrode is set to V and the pixel electrode is set to either V (zero drive voltage) or V (positive drive voltage). The resulting waveform is twice as long as the corresponding non-picket fence waveform.

It is also generally desirable that the IMP drive scheme be capable of local updates. As described in the aforementioned "method" application, it is often desirable to drive an electro-optic display in a manner that allows for local updating of specific areas of the display, where specific areas of the electro-optic display are undergoing change while the rest of the display remains unchanged; for example, it may be desirable to update a dialog box in which the user is entering text without updating the background image of the display. A local update version of any IMP drive scheme may be generated by removing all non-zero voltages from the waveform for a zero transition (i.e. a transition from one grey level to the same grey level). For example, a waveform from gray level 2 to gray level 2 is typically composed of a series of voltage pulses. The non-zero voltage is removed from the waveform and this is done for all other zero transitions, resulting in a local updated version of the IMP waveform. Such a locally updated version is beneficial when it is desired to minimize extraneous flicker during the transition.

The following experiment illustrates the use of the above discussed modification in the fine control of the grey levels generated by the IMP drive scheme.

The encapsulated electrophoretic medium comprises an internal phase comprising polymer coated titanium and polymer coated carbon black particles in a hydrocarbon liquid encapsulated in gel/gum arabic, the encapsulation beingThe loaded electrophoretic medium was prepared and incorporated into experimental single pixel displays, substantially all as described in the aforementioned U.S. patent publication 2002/0180687 [0069 ]]To [0076]As described in the paragraph. The experimental display was then driven with a four gray level IMP drive scheme. It has been found that the display can be driven from grey level 4 (white) to grey level 1 (black) by a +15V, 500 ms pulse, while the opposite transition is effected by a-15V, 500 ms pulse, thus constituting the basic two reset pulse IMP drive scheme. Fig. 21 shows in a highly schematic way all sixteen waveforms of this basic IMP drive scheme, which are labeled with the labels [ Ri, R₂]So that the first number given represents the final grey level. For example, the waveform [14 ] shown in the upper right corner of FIG. 21]A transition from gray level 4 (white) to gray level 1 (black) is made and comprises a first +15V 500 ms reset pulse (which drives the pixel black), a second-15V 500 ms reset pulse (which drives the pixel white), and a +15V 500 ms set pulse (which drives the pixel black).

Driving an experimental display with the basic IMP drive scheme through a sequence of variations in gray level and measuring the reflectivity of the display at the end of each sequence; the results are shown in fig. 22. Each point in fig. 22 represents the reflectivity after a different sequence of grey levels before reaching the final grey level as seen on the abscissa. It will be seen from fig. 22 that the reflectivity variation achieved at the same nominal grey level is significant and such variation is of course undesirable as it negatively impacts the quality of the image produced by the multi-pixel display. In particular, the human eye is very sensitive to small changes in gray levels that occur within a block of pixels that are assumed to be at the same gray level, and FIG. 22 indicates that such changes may be considered as a result of a prior gray level difference for the pixel.

The IMP drive scheme is then modified in the manner described above, namely to insert or remove balanced pulse pairs (closing the resulting gap in the case of removal) and to insert or remove zero voltage periods at the beginning or end of each waveform, so as to achieve a consistent gray level after the various gray level sequences, to produce the modified IMP drive scheme shown in figure 23. Fig. 24 shows the grey levels generated by the modified IMP drive scheme of fig. 23 using the same sequence of grey levels as in fig. 22. It will be seen from fig. 24 that the modified IMP drive scheme of fig. 24 produces much more consistent gray levels than the unmodified drive scheme of fig. 21.

And part C: balance pulse pair residence time compensation method and device

As already mentioned, in a third aspect, the present invention provides a method for driving an electro-optic display having at least one pixel capable of achieving at least two different gray levels. In this method, at least two different waveforms are used for the same transition between specific gray levels, depending on the duration of the dwell time of the pixel in the state where the transition starts; the two waveforms differ from each other by at least one insertion and/or removal of at least one balanced pulse pair, or insertion of at least one period of zero voltage, wherein a "balanced pulse pair" has the meaning defined above. It is highly preferred that the driving scheme in this method is DC balanced (the term being defined above).

In this Balanced Pulse Pair Dwell Time Compensation (BPPDTC) method (as in the BPPSS and IMP methods already described), the insertion and/or removal of balanced pulse pairs and/or zero voltage periods (pauses) may be implemented within a single waveform element or between two consecutive waveform elements. Two waveforms for the same transition after different dwell times in the initial state where the transition begins may be referred to hereinafter as "alternative dwell times" or "ADT" waveforms.

It should be noted that ADT waveforms differ from one another by the location and/or duration of the BPP or pause within the waveform (see, e.g., the discussion of fig. 25B-25E below), as such movement of the BPP or pause may be generally considered as a combination of removing the BPP or pause at one location and inserting the BPP or pause at a different location, or (in the case of a change in duration at the same location) as removing the BPP or pause at the location and inserting a different BPP or pause at the same location.

In the BPPDTC driving scheme, the insertion or removal of BPP and/or pauses causes the same problems and can be handled in the same manner as in the BPPSS and modified IMP driving schemes described earlier in parts a and B. Thus, where the difference between ADT waveforms according to the BPPDTC aspect of the invention comprises the removal of at least one BPP, the period previously occupied by the or each removed BPP may become a zero voltage period. Alternatively, this period may be "closed" by moving some or all of the later waveform elements forward in time, typically inserting a zero voltage period at some later stage of the waveform, typically at the end of the waveform, to ensure that the entire length of the waveform is maintained. (in any practical display, which will typically have at least thousands of pixels, in any transition at least one pixel will typically undergo every possible transition, and if the waveform length is different for all pixels, the controller logic becomes extremely complex.) alternatively, of course, the period may be "closed" by moving some or all of the earlier waveform elements backwards in time, inserting a zero voltage period at the earlier stage of the waveform (typically at the beginning of the waveform).

Similarly, inserting BPPs increases the total duration of the waveform unless the existing zero voltage period is removed at the same time. Since it is highly desirable that all waveforms of a drive scheme have the same total length, when one waveform of a drive scheme has an inserted BPP, all other waveforms of the drive scheme should have a period of zero voltage added thereto, or some other modification made to compensate for the increase in total waveform length caused by the inserted BPP. For example, if a 40 millisecond BPP is inserted into the black-to-white waveforms shown in table 1 above (which have a waveform length of 420 milliseconds), then 40 milliseconds may be added to the remaining three waveforms shown in table 1, such that all waveforms have a length of 460 milliseconds. Clearly, BPP may be added to the other three waveforms instead of pauses, if appropriate, or some combination of BPP and pauses of a total length of 40 milliseconds may be used.

The preferred drive scheme and waveforms of the BPPDTC aspect of the invention will now be described by way of illustration only. The balanced pulse pairs used in such drive schemes and waveforms may be of any of the types described above; for example, a BPP of the type shown in FIGS. 4A-4C may be used.

Fig. 25A-25E illustrate alternative residence time waveforms that may be used for a single transition in accordance with the BPPDTC aspect of the invention. Fig. 25A illustrates the black-to-white waveforms mentioned in the third row of table 1 above and the last row of table 2. Since this is a waveform suitable for a black-to-white transition after a long dwell time in the black state, it may be considered a black-to-white waveform modified in accordance with the BPPDTC aspect of the invention to generate a waveform suitable for a black-to-white transition after a short dwell time in the black state. As already noted, the basic waveform of FIG. 25A includes a-15V, 400 millisecond pulse followed by a 0V duration of 20 milliseconds.

Fig. 25B illustrates a modification of the basic waveform of fig. 25A, which has been found to be effective in reducing the reflectivity of the final white state when the black-to-white transition is performed after the initial black state has only resided for a short time of no more than 0.3 seconds. The waveform of fig. 25B is generated by inserting a BPP similar to BPP 300 shown in fig. 4A at the end of a-15V, 400ms pulse of the waveform of fig. 25A, so that the waveform of fig. 25B contains a-15V, 420 ms pulse followed by a +15, 20ms pulse and 0V for 20 ms.

Fig. 25C and 25D illustrate two additional ADT waveforms for the same black-to-white transition as the waveforms of fig. 25A and 25B. It has been found that the waveforms of fig. 25C and 25D are effective to normalize the reflectivity of the final white state when the black-to-white transition is performed after the black state resides for 0.3 to 1 second and 1 to 3 seconds, respectively. The waveforms of fig. 25C and 25D are generated by inserting the same BPP as in fig. 25B in the waveform of fig. 25A, but the position of insertion is different from that of fig. 25B. As noted above, it has been invented that the position at which the BPP is inserted into (or removed from) the base waveform has a significant effect on the final optical state after the transition, and therefore moving the insertion position of the BPP in the base waveform is an effective means of compensating the waveform for variations in the dwell time of the pixel in the initial optical state.

Fig. 25E is a preferred alternative to the waveform of fig. 25A for performing a black-to-white transition after a long dwell time (3 seconds or longer) in the black state. The waveform of fig. 25E is generally similar to the waveforms of fig. 25B-25D in that it is generated by inserting the same BPP in the waveform of fig. 25A. However, in fig. 25E, the BPP is inserted at the start of the waveform; it has also been found desirable to have the pulse of BPP last 40 milliseconds instead of 20 milliseconds, as this makes the total duration of the waveform 500 milliseconds, and when the waveform of fig. 25E is used in conjunction with the waveforms of fig. 25B-25D, it is necessary to "insert" an additional 40 milliseconds of 0V at the end of the waveform. Thus, a preferred set of ADT waveforms for black-to-white transitions is shown in table 3 below:

TABLE 3

Note that the impact for the black-to-white transition is either-15V x 400ms, or 6V seconds, for all ADT waveforms in table 3, and hence for all initial state dwell times, so that the driving scheme is DC balanced.

As already mentioned, DTCs can also be implemented by removing BPP from the base waveform. For example, consider the following drive scheme shown in table 4:

TABLE 4

Transformation of	Wave form
		Black to black	0V，820ms
Black to white	+15V, 400 ms; 15V, 400 ms; then 0V for 20ms
		White to black	15V, 400 ms; +15V, 400 ms; then 0V for 20ms
White to white	0V，820ms

Note that in this drive scheme, not only the entire drive scheme but also all waveforms are "internally" DC balanced; the desirability of such internal DC balancing is discussed in detail in the aforementioned WO 2004/090857. Also, the method for DTC will be discussed with reference to black-to-white transitions, although it should be understood that DTC for white-to-black transitions can be implemented in a similar manner.

In this example, the DTC for the black-to-white transition is achieved by removing BPP, i.e., by removing a portion of one voltage pulse having one polarity and duration while removing a similar portion of one voltage pulse having the opposite polarity and equal duration. The removed pulse portions may either be replaced with zero voltage segments or the remainder of the waveform may be shifted in time to occupy the segments previously occupied by the removed pulse pairs and, to maintain the total update time, the zero voltage segments matching the duration of the removed pulse pairs may be added anywhere else, typically at the beginning or end of the waveform.

Fig. 26A, 26B, and 26C schematically illustrate the procedure of correction of the black-to-white waveform listed in the third row in table 4 above for a DTC for a short time residing in the black state for less than 0.3 seconds. Fig. 26A illustrates the basic waveforms of table 4. Fig. 26B schematically shows the BPP formed by removing the last 80ms portion of the positive voltage pulse and the first 80ms portion of the negative voltage pulse from the waveform of fig. 26A, the resulting gap being eliminated by moving the negative pulse forward in time, as shown by the arrows in fig. 26B. The resulting dwell time compensation waveform is shown in fig. 26C, which includes 320ms positive pulses, 320ms negative pulses, and 180ms zero voltage periods.

In this case, it has been found that DTCs for all dwell times can be simply achieved by varying the length of BPP removed, and that the basic waveform of fig. 26A is found to be satisfactory for a long dwell time of 3 seconds or more in the black state. The full list of ADT waveforms for the black-to-white transition in this case is therefore shown in table 5 below:

TABLE 5

Dwell time	Wave form
		0 to 0.3 second	+15V, 320 ms; -15V, 320 ms; then 0V, 180ms
0.3 second to 1 second	+15V, 360 ms; -15V, 360 ms; then 0V, 100ms
		1 second to 3 seconds	+15V, 380 ms; 15V, 380 ms; then 0V for 60ms
3 seconds or longer	+15V, 400 ms; 15V, 400 ms; then 0V for 20ms

As already mentioned, when the BPP is removed from the basic waveform in the manner shown in fig. 26B, it is not necessary to move the remaining portion in time; the removed BPP may be simply replaced by a zero voltage period. Table 6 below shows a similar set of modified ADT waveforms as in table 5, but with the removed BPP replaced with a zero voltage period:

TABLE 6

Dwell time	Wave form
		0 to 0.3 second	+15V, 320 ms; 0V, 160 ms; -15V, 320 ms; then 0V for 20ms
0.3 second to 1 second	+15V, 360 ms; 0V, 80 ms; -15V, 360 ms; then 0V for 20ms
		1 second to 3 seconds	+15V, 380 ms; 0V, 40 ms; 15V, 380 ms; then 0V for 60ms
3 seconds or longer	+15V，400ms；15V, 400 ms; then 0V for 20ms

Although the BPPDTC aspect of the invention has been described initially above with reference to a display having only two grey levels, it is not so limited and may be applied to displays having a greater number of grey levels. Also, although in the particular waveforms shown in the figures, two elements of inserting or removing BPP are implemented at a single point within the waveform, the invention is not limited to waveforms in which BPP is inserted or removed at a single point; the two elements of the BPP may be inserted or removed at different points, i.e. the two pulses forming the BPP do not have to be exactly consecutive but may be separated by a time interval. In addition, one or both BPP pulses may be subdivided into several portions, which may then be inserted into or removed from the waveform for the DTC. For example, the BPP may consist of +15V, 60 millisecond pulses and-15V, 60 millisecond pulses. The BPP may be divided into two portions, for example, a +15V, 60ms pulse followed by a-15V, 20ms pulse, and a-15V, 40ms pulse, and both portions are inserted into or removed from the waveform at the same time to implement the DTC.

It has also been found that the insertion or removal of a zero voltage segment in or from the waveform affects the final gray level after the transition, and thus this insertion or removal of a zero voltage segment provides a second method for adjusting the final gray level to achieve a DTC. Such insertion or removal of the zero voltage segment may be used alone or in combination with the insertion or removal of the BPP.

Although the BPPDTC aspect of the invention is described above primarily with reference to a pulse width modulated waveform (where the voltage applied to a pixel at any given time can only be-V, 0 or + V), the invention is not limited to the use of such a pulse width modulated waveform and may use a voltage modulated waveform, or a pulse and voltage modulated waveform. The foregoing definition of balanced pulse pairs may be satisfied by two pulses of opposite polarity with zero net impact, and it is not required that the two pulses be of the same voltage or duration. For example, in a voltage modulated drive scheme, the BPP may consist of a +15V, 20ms pulse followed by a-5V, 60ms pulse.

From the foregoing description, it will be seen that the BPPDTC aspect of the present invention allows for the dwell time compensation of the drive scheme while maintaining the DC balance of the drive scheme. Such DTCs can reduce ghosting in electro-optic displays.

And part D: target buffer method and device

As already mentioned, the present invention provides two different methods of driving an electro-optic display having a plurality of pixels using a target buffer, wherein each pixel is capable of achieving at least two different gray levels. The first of these two methods, the non-polar target buffer method, includes: providing initial, final and target data buffers; determining when the data in the initial and final data buffers differs, and when such a difference is found, updating the value in the target data buffer by: (i) setting the target data buffer to the value when the initial and final data buffers contain the same value for the particular pixel; (ii) setting the target data buffer to the value of the initial data buffer plus an increment when the initial data buffer contains a value for the particular pixel that is greater than the value of the final data buffer; and (iii) setting the target data buffer to the value of the initial data buffer minus the increment when the initial data buffer contains a value for the particular pixel that is less than the value of the final data buffer; updating an image on the display using data in the initial data buffer and the target data buffer as initial and final states, respectively, for each pixel; then, copying the data from the target data buffer to the initial data buffer; and repeating these steps until the initial and final data buffers contain the same data.

In the second of these two methods, i.e., the polarity target buffer method, the final, initial and target data buffers are again provided, along with a polarity bit array that is set to store a polarity bit for each pixel of the display. Again, the values in the initial and final data buffers are compared, and when their values in the array of polarity bits are different, the target data buffer is updated by (i) setting the polarity bit for a particular pixel to a value representing a transition to the opposite extreme optical state when the values in the initial and final data buffers are different for that pixel and the value in the initial data buffer represents the extreme optical state of that pixel; and the target data buffer is set to the value of the initial data buffer plus or minus an increment, depending on the associated value in the polarity bit array. The image on the display is then updated in the same way as in the first method, after which the data from the target data buffer is copied into the initial data buffer. These steps are repeated until the initial and final buffers contain the same data.

Prior art controllers for bistable electro-optic displays typically use logic similar to that shown in Table 1 below (where all lists are in pseudo-code):

list 1

A controller, display or the like operating in this way waits to receive new image information, and then, when such new image information is received, a full update is made before having new information sent to the display, i.e. once a new image has been accepted by the display, the display cannot accept a second new image until the rewriting of the display required to display the first new image has been completed, and in some cases this rewriting process may take hundreds of milliseconds, see the driving scheme proposed in the preceding sections a-C. Thus, the display appears insensitive to user input at the full update (rewrite) time when the user is scrolling or typing.

In contrast, a controller implementing the non-polar target buffer method of the present invention is run by the following illustrated logic of Table 2 (for convenience, this type of controller may be referred to hereinafter as a "List 2 controller"):

list 2

In the modified controller logic for the NPTB method, there are three picture buffers. The initial and final buffers are the same as the prior art controller and the new third buffer is the "target" buffer. The display controller may accept new image data into the final buffer at any time. When the controller finds that the data in the final buffer is no longer equal to the data in the initial buffer (i.e. it is required to overwrite the image), a new target data set is constructed by adding 1 or subtracting 1 from the values in the initial buffer (or leaving them unchanged) depending on the difference between the relevant values in the initial and final buffers. The controller then uses the values from the initial and target buffers to perform display updates in the usual manner. When this update is complete, the controller copies the value from the destination buffer into the initial buffer, and then repeats the differencing operation between the initial and final buffers. The entire update is done when the initial and final buffers have the same data set.

Thus, in the NPTB method, the entire update is implemented as a series of sub-update operations, one such sub-update operation occurring when the image is updated using the initial and target buffers. The term "intermediate frame" (meso-frame) will be used later on during the period required for each of these sub-update operations; of course such intermediate frames represent the period between the frame required for a single scan frame of the display (see the aforementioned "method" application) and the super-frame, or the period required to complete a complete update.

The NPTB method of the present invention improves interactive performance in two ways. First, in the prior art method, the controller uses the final data buffer during the update process so that no new data can be written into the final data buffer while updating, and therefore the display cannot respond to new inputs during the entire period required for the update. In the NPTB method of the invention, the final data buffer is used only for the calculation of the data set in the target data buffer, and the calculation is a simple computer calculation that can be performed much faster than the update operation, which requires a physical response from the electro-optical material. Once the computation of the data set in the target data buffer is complete, the update does not require that the final data buffer be accessed, so that the final data buffer can be used to accept new data.

For the reasons discussed in the above-mentioned "method" application and for the reasons discussed later in relation to the waveforms, it is generally desirable to drive a pixel in a cyclic manner, in the sense that once a pixel has been driven away from one extreme optical state by a voltage pulse of one polarity, a voltage pulse of the opposite polarity is not applied to the pixel until it reaches its other extreme optical state; see, for example, fig. 11A and 11B and the associated description of 2003/0137521 above. This limitation is met by the PTB method of the present invention which may use a controller operating with the logic illustrated in table 3 below (hereinafter, for convenience, this type of controller may be referred to as a "table 3 controller"; this table assumes a four gray scale system with gray scales numbered 1 (black) to 4 (white), although the pseudo-code for operation with gray scales having different numbers may be easily modified by those skilled in the art):

list 3

The PTB method requires four image buffers, the fourth being a "polarity" buffer with a single bit for each pixel of the display that indicates the current transition direction of the associated pixel, i.e., whether the pixel is currently transitioning from white to black (0) or black to white (1). If the associated pixel is not currently undergoing a transition, the polarity bit holds its value from the previous transition; for example, a pixel that is now in a light gray state and was previously white will have a polarity bit of 0.

In the PTB approach, the polar bit array is considered when building a new target buffer data set. If the pixel is currently black or white and a transition to the opposite state is required, the polarity bit is set accordingly and the target value is set to the gray level closest to black or white, respectively. Alternatively, if the initial state for the pixel is an intermediate (gray) state, the target value is calculated by adding or subtracting 1 to the state (+ 1 if the polarity is 1; or-1 if the polarity is 0) depending on the value of the polarity bit.

It should be noted that in this driving scheme the behavior of a pixel in the intermediate state is independent of the current value of the final state for that pixel. The pixel will continue in the same direction since it started to transition from black to white or from white to black until it reaches the opposite optical rail (the extreme optical state, usually black or white). If the desired image, and thus the object state, changes during the transition, the pixel will return in the opposite direction, and so on.

Preferred waveforms for use in the TB method of the present invention will now be discussed. Table 7 below shows one possible transition matrix that can be used for one-bit (monochrome) operations using the NPTB and PTB methods of the present invention, using two intermediate states.

TABLE 7

The structure of the transition matrix, with black, white, and two intermediate grey states, looks very similar to the problems used in prior art two-bit drive schemes, such as those described in the "methods" application. However, in the TB method of the present invention, these intermediate states do not correspond to stable gray states, but only to transition states, which exist only between the completion of one intermediate frame and the start of the next intermediate frame. There is no limitation on the uniformity of the reflectance in these intermediate states.

It should be noted that in the transition matrix shown in table 7, many elements (indicated by dashed lines) are not allowed. The controller only allows the grey level to be changed by one unit per transition in either direction, so that transitions involving multiple changes in grey level (e.g. direct 1-4 black to white transitions) are inhibited. The elements on the main diagonal of the transition matrix (corresponding to zero transitions) are disabled for intermediate states; such major diagonal elements are not recommended for white and black states, but are not strictly prohibited, as indicated by the asterisks in table 7.

In the monochrome NPTB method, the update sequence appears as a series of states, starting and ending at extreme optical states (optical guides), with intermediate gray level sequences separated by zero dwell time. For example, a simple transition from black to white would appear as:

1→2→3→4

on the other hand, if the final state of the display changes during an update, the transition may become:

1→2→3→2→1

multiple changes in the final state may produce such transitions as:

1→2→3→2→3→4

more generally, there are four possible transition types between the extreme black and white optical states.

1→2→3(→2→3)-s-4

1→2(→3→2)→1

4→3-→2(→3→2)->1

4→3(→2→3)→4

Where the parenthesis indicates zero or more repetitions of the sequence within the parenthesis.

Optimization ("tuning") of such NPTB drive schemes requires tuning the non-zero elements of the transition matrix to ensure consistent reflectivity values for the 1 (black) and 4 (white) states, regardless of the number of repetitions of the bracketed sequence. The waveform must operate in the black and white extreme optical states for any dwell time, but the dwell time in the intermediate states is always zero, so that, as described above, the reflectivity of the transition states is unimportant.

Typically, the time required for any single inter frame update is equal to the length of the longest element in the transition matrix. Thus, the time for the entire update is three times the length of the longest element. In the best case, the black-to-white-to-black (1 → 4 and 4 → 1, respectively) waveform may be divided into three equal length segments; this approach will reduce the delay of the update to one third of the total update time while maintaining the same duration for the full update. As the length of the inter-frame update becomes longer (which may be the result of optimizing the waveform), the beneficial effects become less. For example, if an element becomes twice as long, the latency will increase to two-thirds of the simple update time, and a full transition will require the first twice as long. It is possible to test to find the longest element present in a given intermediate frame and dynamically adjust the update time to that length, but the effect of this additional calculation is unlikely to be significant.

It should also be considered what electro-optical properties of the medium make a display using the medium suitable for use with this type of NPTB drive scheme. First, the residence time dependence of the medium will be zero (ideally, orAt least very low) because the waveform combines a series of near-zero dwells between intermediate frames with a potentially much longer dwell time between transitions. Second, the media should be less sensitive or insensitive to the optical state prior to the initial state of a particular transition, since the direction of the transition may change in the intermediate stream; for example, the 2 → 1 transition may be preceded by a 1- "2 or 3 → 2 transition. Finally, the electro-optic medium should be symmetrical in its response, especially around the black and white states; it is difficult to generate a DC balance waveform that can perform to reach the same black or white state, respectivelyOr 4 → 3- → 4 transition.

For the foregoing reasons, "intermediate inversion (intermediate inversion)" in the NPTB driving scheme makes it very difficult to form an optimized waveform. In contrast, PTB drive schemes greatly reduce the need for electro-optic media, thereby reducing many of the difficulties in optimizing NTPB drive schemes while still providing improved performance.

Although the structure of the transition matrix for the PTB driving scheme is the same as that for the NPTB driving scheme, the PTB driving scheme allows only two black-to-white and white-to-black transitions, i.e.:

1 → 2 → 3 → 4; and

4→3→2→1。

in practice, these two transitions may be identical to the normal 1 → 4 and 4 → 1 transitions, which are divided into three equal parts. Some slight readjustment may be required to account for any delay between intermediate frames, but the adjustment is straightforward. For simple typing, this driving scheme results in a two-thirds reduction in latency.

The PTB method has some disadvantages. Additional memory is required for the polarity bit array and a more complex controller to run this simpler driving scheme, since allowing the direction of the transition at each pixel requires consideration of additional data (polarity bits) in addition to the initial and final states of the transition. Also, while the PTB approach does reduce the delay for starting the update, the controller must wait until the update is complete to reverse the transition. This limitation is evident in the case where the user types a character and then deletes it immediately; the delay before the character is deleted is equal to the full update time. This limits the usefulness of the PTB method for cursor tracking or scrolling.

Although the NPTB and PTB methods are described above primarily with respect to monochrome drive schemes, they are also compatible with many grayscale drive schemes. The NPTB method is inherently fully gray scale compatible; the grayscale compatibility of the PTB method is discussed below.

From a driving scheme point of view it is clear that it is more difficult to generate a usable grey scale driving scheme for the NPTB method than a corresponding monochrome driving scheme, since in a grey scale driving scheme the intermediate states now correspond to the actual grey levels, whereby the optical values of these intermediate states are limited. It is also difficult to generate a gray scale driving scheme for the PTB method. To reduce the delay, the inter frame transition must be slightly shortened. For example, the 2 → 3 transition may be a separate transition, which may be the last segment of 1 → 2 → 3, or may be the beginning segment of 2 → 3 → 4. Therefore, there is a strong need to make the transition shorter (to achieve a shorter total update) and accurate (if the transition stops at gray level 3).

The grayscale PTB method may eliminate the degeneracy of the inter-frame step described in the preceding paragraph by introducing multiple grayscale steps (i.e., by allowing the grayscale to change by more than one unit during each inter-frame, which corresponds to reinserting multiple elements removed from the main diagonal of the associated transition matrix with more than one step, as shown in table 7 above). This correction may be achieved by replacing the polarity bit matrix with a counter array comprising more than one bit per pixel of the display, up to the number of bits required for full gray scale image representation. The waveforms will then contain up to a full N × N transition matrix, with each waveform being divided equally into four (or other substantially arbitrary number of intermediate frames).

Although the particular TB method discussed above is a two-bit gray scale method with two intermediate gray levels, the TB method can of course be used for any number of gray levels. However, the beneficial effect of increasing the delay reduction will tend to decrease as the number of gray levels increases.

Thus, the present invention provides two types of TB methods that significantly reduce the update delay in monochrome mode while minimizing the complexity of the controller algorithm. These methods may prove particularly useful in interactive one-bit (monochrome) applications, such as personal digital assistants and electronic dictionaries, where fast response to user input is of paramount importance.

Part E: waveform compression method and device

As already mentioned, a final main aspect of the invention relates to a method for reducing the amount of waveform data that has to be stored for driving a bistable electro-optic display. More particularly, this aspect of the invention provides a "waveform compression" or "WC" for driving an electro-optic display having a plurality of pixels, wherein each pixel is capable of achieving at least two different gray levels, said method comprising: storing a base waveform defining a sequence of voltages to be applied by the pixel during a particular transition between grey levels; storing a multiplication factor (multiplicationaffector) for the particular transition; and effecting said particular transition by applying said sequence of voltages to said pixel for a period of time dependent on the multiplication factor.

When a shock-driven electro-optic display is driven, each pixel of the display receives a voltage pulse (i.e. the voltage difference between two electrodes associated with that pixel) or a plurality of time series (i.e. waveforms) of voltage pulses to effect a transition from one optical state of the pixel to another, typically a transition between grey levels. The data required to define the set of waveforms (forming a complete drive scheme) for each transition is stored in a memory, typically on the display controller, although alternatively the data may be stored on the host or other auxiliary device. The drive scheme may contain a large number of waveforms and (as described in the aforementioned "method" application) may require the storage of multiple waveform data sets to allow for variations in environmental parameters (such as temperature and humidity) as well as variations in non-environmental parameters (such as the operational lifetime of the electro-optic medium). Therefore, the memory capacity required to hold the waveform data may be considerable. It is desirable to reduce this capacity of the memory to reduce the cost of the display controller. A simple compression scheme that can be accommodated in the display controller or host in a practical situation is helpful in reducing the memory capacity required for the waveform data, thereby reducing the cost of the display controller. The waveform compression method of the present invention provides a simple compression scheme that is particularly advantageous for electrophoretic displays and other known bi-stable displays.

The uncompressed waveform for a particular transition is typically stored as a series of bit sets (bitsets), each specifying a particular voltage to be applied at a particular point of the waveform. As an example, consider a three-level voltage drive scheme in which a pixel is driven to black (in this example, +10V) with a positive voltage, to white with a negative voltage (-10V), and is held at its current optical state at zero voltage. The voltage for a given time element (for a scan frame of an active matrix display) may be encoded in two bits, for example as shown in table 8 below:

TABLE 8

Desired voltage (V)	Binary representation
		+10	01
-10	10
		0	00

Using this binary representation, the waveform of a +10V pulse for an active matrix drive and containing five scan frames of zero voltage followed by two scan frames would be represented as:

01010101010000。

a waveform containing a large number of time segments requires a large number of bit sets of waveform data to be stored.

According to the WC method of the present invention, waveform data is stored as a basic waveform (binary representation as described above) and a multiplication factor. For a number of periods depending on the multiplication factor, the display controller (or other suitable hardware) applies a sequence of voltages defined by the basic waveform to the pixel. In a preferred form of this WC method, sets of bits (as given above) are used to represent the basic waveform, and the voltages defined by each set of bits are applied to the pixel over n time periods, where n is the multiplication factor associated with the waveform. The multiplication factor must be a natural number. For the multiplication factor of 1, the applied waveform is unchanged from the basic waveform. For multiplication factors greater than 1, the representation of the voltage sequence is compressed for at least some waveforms, i.e. fewer bits are required to represent these waveforms than if the data were stored in uncompressed form.

As an example, using the three voltage level binary representation of table 8, consider such a waveform: it requires 12 +10V scan frames, followed by 9-10V scan frames, followed by 6 +10V scan frames, followed by 3 0V scan frames. The waveform is represented in uncompressed form as:

010101010101010101010101 10 10 10 10 10 10 10 10 10 01 01 01 01 01 01 00 00 00

and expressed in a compressed manner as:

multiplication factor: 3

Basic waveform 01010101101010010100.

The length of the voltage sequence that must be allocated to each waveform is determined by the longest waveform. For encapsulated electrophoretic and many other electro-optic displays, the longest waveform is typically required at the lowest temperature, and the electro-optic medium responds slowly to the applied electric field at the lowest temperature. At the same time, the resolution necessary to achieve a successful transition is reduced when the response is slow, so by grouping successive scan frames via the WC method of the invention, there is little loss in the accuracy of the optical state. Using this compression method, each waveform can be assigned the number of scan frames (or generally time periods) suitable for updating waveforms at medium and high temperatures with short time. At low temperatures (where the number of scan frames required may exceed the memory allocation), a multiplication factor greater than 1 may be used to generate the long waveform. This ultimately leads to reduced memory requirements and reduced cost.

The WC approach of the present invention is in principle equivalent to simply changing the frame time of an active matrix display at various temperatures. For example, the display may be driven at 50Hz at room temperature and 25Hz at 0 deg.C to extend the allowable waveform time. However, the WC approach is better because the backplane is designed to minimize the effect of capacitive and resistive voltage artifacts at a given scan rate. At least one type of artifact increases when there is a significant deviation from the preferred scan rate in either direction. Therefore, it is better to keep the actual scan rate unchanged, and group the scan frames with the WC method, which actually provides a way to achieve a virtual change in scan rate without actually changing the physical scan rate.

Claims

1. A method for driving an electro-optic display having a plurality of pixels, each pixel capable of implementing at least two different gray levels, comprising:

(d) determining when the data in the initial and final data buffers differs, and upon finding such a difference, updating the value in the target data buffer by: (i) setting the target data buffer to the value when the initial and final data buffers contain the same value for the particular pixel; (ii) setting the target data buffer to the value of the initial data buffer minus an increment when the initial data buffer contains a greater value for the particular pixel than the final data buffer; and (iii) setting the target data buffer to increase the value of the initial data buffer by the increment when the initial data buffer contains a smaller value for the particular pixel than the final data buffer;

2. A method for driving an electro-optic display having a plurality of pixels, each pixel capable of achieving at least three different gray levels, said method comprising:

(e) determining when the data in the initial and final data buffers differs, and upon finding such a difference, updating the polarity bit array and the value in the target data buffer by: (i) setting the polarity bit for a particular pixel to a value representing a transition to the opposite extreme optical state when the values in the initial and final data buffers are different for the pixel and the values in the initial data buffer represent the extreme optical state of the pixel; (ii) setting the target data buffer to be the value of the initial data buffer plus or minus an increment according to the correlation value in the polarity bit array when the values for the specific pixels are different in the initial and final data buffers;