HK1254151B - Method for driving electro-optic displays with multiple pixels - Google Patents

Description

Method for driving an electro-optical display having a plurality of pixels

Citation of Related Applications

This application claims priority to U.S. Patent Application Serial No. 15/050,997, filed February 23, 2016, which is hereby incorporated by reference herein in its entirety.

This application is related to U.S. patent application Ser. No. 14/089,610, filed November 25, 2013, now U.S. Patent No. 9,269,311, issued February 23, 2016, which is a divisional of U.S. patent application Ser. No. 13/086,066, filed April 13, 2011, now U.S. Patent No. 8,593,396, issued November 26, 2013, which is itself a divisional of U.S. patent application Ser. No. 13/086,066, filed August 13, 2005, now U.S. Patent No. 8,593,396, issued November 26, 2013. 1/161,715, now a divisional of U.S. Patent Application No. 7,952,557, issued May 31, 2011, 11/161,715 claims the benefit of the following provisional applications: (a) application serial number 60/601,242, filed August 13, 2004; (b) application serial number 60/522,372, filed September 21, 2004; and (c) application serial number 60/522,393, filed September 24, 2004.

The above-mentioned U.S. patent application serial number 11/161,715 is also a part continuation application of U.S. patent application serial number 10/904,707 filed on November 24, 2004 (now U.S. Patent No. 8,558,783 issued on October 15, 2013), and 10/904,707 itself claims the benefit of provisional application serial numbers 60/481,711 and 60/481,713 filed on November 26, 2003.

The above-mentioned U.S. patent application serial number 10/904,707 is a part continuation application of U.S. patent application serial number 10/879,335 filed on June 29, 2004 (now U.S. Patent No. 7,528,822 issued on May 5, 2009), and 10/879,335 claims the benefit of the following provisional applications: serial number 60/481,040 filed on June 30, 2003; serial number 60/481,053 filed on July 2, 2003; and serial number 60/481,405 filed on September 23, 2003.

The above-mentioned U.S. patent application serial number 10/879,335 is also a part continuation application of U.S. patent application serial number 10/814,205 filed on March 31, 2004 (now U.S. Patent No. 7,119,772 issued on October 10, 2006), and 10/814,205 claims the benefit of the following provisional applications: Serial No. 60/320,070 filed on March 31, 2003; Serial No. 60/320,207 filed on May 5, 2003; Serial No. 60/481,669 filed on November 19, 2003; Serial No. 60/481,675 filed on November 20, 2003; and Serial No. 60/557,094 filed on March 26, 2004.

The above-mentioned U.S. patent application serial number 10/814,205 is a part continuation application of U.S. patent application serial number 10/065,795 filed on November 20, 2002 (now U.S. Patent No. 7,012,600 issued on March 14, 2006), which itself claims the benefit of the following provisional applications: Serial No. 60/319,007 filed on November 20, 2001; Serial No. 60/319,010 filed on November 21, 2001; Serial No. 60/319,034 filed on December 18, 2001; Serial No. 60/319,037 filed on December 20, 2001; and Serial No. 60/319,040 filed on December 21, 2001.

This application is also related to U.S. patent application serial number 10/249,973 filed on May 23, 2003, now U.S. Patent No. 7,193,625 issued on March 20, 2007, which claims the benefit of provisional applications serial number 60/319,315 filed on June 13, 2002 and serial number 60/319,321 filed on June 18, 2002.

This application is also related to U.S. patent application serial number 10/063,236 filed on April 2, 2002, now U.S. Patent No. 7,170,670; U.S. patent application serial number 10/064,279 filed on June 28, 2002, now U.S. Patent No. 6,657,772; U.S. patent application serial number 10/064,389 filed on July 9, 2002, now U.S. Patent No. 6,831,769; and U.S. patent application serial number 10/249,957 filed on May 22, 2003, now U.S. Patent No. 6,982,178.

The aforementioned US patent applications serial numbers 10/904,707; 10/879,335; 10/814,205; 10/249,973; and 10/065,795 may be collectively referred to hereinafter as "MEDEOD" (Method for Driving an Electro-Optical Display) applications for convenience.

The entire contents of these co-pending applications, as well as all other US patents and published and co-pending applications described below, are incorporated herein by reference.

Technical Field

The present invention relates to a method for driving an electro-optical display, in particular a bistable electro-optical display, and to an apparatus (controller) for such a method. More particularly, the invention relates to a driving method which is intended to enable more precise control of the grey state of the pixels of an electro-optical display. The invention also relates to a driving method which is intended to enable such a display to be driven in a manner which allows compensation for the "dwell time" during which a pixel remains in a particular optical state before transitioning, whilst still allowing the drive scheme used to drive the display to be DC balanced. The invention is particularly, but not exclusively, of use in particle-based electrophoretic displays, in which one or more types of charged particles are suspended in a liquid and move through the liquid under the influence of an electric field to change the appearance of the display.

Background Art

Electro-optical displays using the methods of the present invention typically comprise an electro-optic material that is solid in the sense that it has a solid outer surface, but the material can (and often does) have internal spaces filled with a liquid or gas. Hereinafter, for convenience, such displays using solid electro-optical materials may be referred to as "solid-state electro-optical displays."

The term "electro-optical," as applied to a material or display, is used herein in its conventional sense in the field of imaging, and refers to a material having first and second display states that differ in at least one optical property, the material being caused to change from its first display state to its second display state by applying an electric field to the material. While the optical property is typically color perceptible to the human eye, it may be another optical property, such as light transmission, reflection, luminescence, or, in the case of displays intended for machine reading, false color in the sense of a change in reflectivity at electromagnetic wavelengths outside the visible range.

The term "gray state" is used herein in its conventional sense in the field of imaging, referring to a state between the two extreme optical states of a pixel, but not necessarily implying a black-white transition between the two extreme states. For example, several of the patents and published applications referenced above describe electrophoretic displays in which the extreme states are white and dark blue, such that the intermediate "gray state" is actually a light blue. In fact, as already mentioned, the transition between the two extreme states may not be a color change at all. The term "gray scale" is used herein to refer to the possible optical states of a pixel, including the two extreme optical states.

The terms "bistable" and "bistability" are used herein in their conventional sense in the art to refer to a display comprising a display element having first and second display states, wherein the first and second display states differ in at least one optical property such that after any given element is driven to assume its first or second display state by an addressing pulse of finite duration, that state persists after termination of the addressing pulse for a time that is at least several times (e.g., at least four times) the minimum duration of the addressing pulse required to change the state of the display element. As shown in published U.S. Patent Application No. 2002/0180687, some particle-based electrophoretic displays that support grayscale can be stable not only in their extreme black and white states but also in intermediate gray states, as can some other types of electro-optical displays. Such displays are properly referred to as "multistable" rather than bistable, but for convenience, the term "bistable" will be used herein to encompass both bistable and multistable displays.

The term "impulse" is used herein in the conventional sense of the time integral of voltage. However, some bistable electro-optical media function as charge converters, and with such media, an alternative definition of impulse may be used, namely the time integral of current (equal to the total applied charge). Depending on whether the medium functions as a voltage-to-time impulse converter or a charge-to-impulse converter, the appropriate definition of impulse should be used.

Much of the following discussion will focus on methods of driving one or more pixels of an electro-optical display through a transition from an initial gray level to a final gray level (which may be the same as or different from the initial gray level). The term "waveform" will be used to refer to the entire voltage versus time curve used to achieve a transition from a particular initial gray level to a particular final gray level. Typically, as shown below, such a waveform will include a plurality of waveform elements; where these elements are substantially rectangular (i.e., a given element includes the application of a constant voltage for a period of time), these elements may be referred to as "voltage pulses" or "drive pulses." The term "drive scheme" refers to a set of waveforms sufficient to achieve all possible transitions between gray levels for a particular display.

Several types of electro-optical displays are known. One type of electro-optical display is a rotating two-color component type, such as described in U.S. Patent Nos. 5,808,783, 5,777,782, 5,760,761, 6,054,071, 6,055,091, 6,097,531, 6,128,124, 6,137,467 and 6,147,791 (although this type of display is commonly referred to as "rotating two-color ball" display, the term "rotating two-color component" is preferably more accurate because in some of the above-mentioned patents, the rotating component is not spherical). This display uses many little bodies (usually spherical or cylindrical) and internal dipoles, and the body includes two or more parts with different optical properties. These bodies are suspended in the cavity filled with liquid in the matrix, and the cavity is filled with liquid so that the body can rotate freely. The appearance of the display is changed by applying an electric field to the display, thereby rotating the body to various positions and changing which part of the body is seen through the viewing surface. This type of electro-optic medium is typically bistable.

Another type of electro-optical display uses an electrochromic medium, for example, an electrochromic medium in the form of a nanoelectrochromic film, which includes an electrode formed at least in part from a semiconducting metal oxide and a plurality of dye molecules attached to the electrode that are capable of reversible color change; see, for example, O'Regan, B. et al., Nature 1991, 353, 737 and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U. et al., Adv. Mater., 2002, 14(11), 845. Nanoelectrochromic 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 2003/0214695. This type of medium is also typically bistable.

Another type of electro-optical display, which has been the subject of intensive research and development for many years, is the particle-based electrophoretic display (EPD), in which multiple charged particles move through a fluid under the influence of an electric field. Compared to liquid crystal displays (LCDs), EPDs can offer good brightness and contrast, wide viewing angles, state bistability, and low power consumption. However, issues with the long-term image quality of these displays have prevented their widespread use. For example, the particles that make up EPDs tend to settle, resulting in a short lifespan for these displays.

As mentioned above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, the fluid is a liquid, but electrophoretic media can be generated using a gaseous fluid; see, for example, Kitamura, T. et al., "Electronic toner movement for electronic paper-like display", IDW Japan, 2001, Paper HCS 1-1, and Yamaguchi, Y. et al., "Toner display using insulative particles charged triboelectrically", IDW Japan, 2001, Paper AMD4-4). See also European Patent Publication Nos. EP1429178; EP1462847; and EP1482354; and International Applications 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. When such gas-based electrophoretic media are used in an orientation that permits particle settling, such as in signs where the media is arranged in a vertical plane, such gas-based electrophoretic media are susceptible to the same problems of particle settling as liquid-based electrophoretic media. In fact, the particle sedimentation problem in gas-based electrophoretic media is more severe than in liquid-based electrophoretic media because the viscosity of gaseous fluids is lower than that of liquids, which causes the electrophoretic particles to sediment faster.

A number of patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and Ink have recently been published that describe encapsulated electrophoretic media. Such encapsulated media comprise a plurality of small capsules, each of which itself comprises an inner phase containing electrophoretically mobile particles suspended in a fluid, and a capsule wall surrounding the inner phase. Typically, these capsules are themselves held in a polymeric binder to form a coherent layer positioned between two electrodes. This type of packaging medium is described, for example, in U.S. Patent Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,2 62,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; ,506,438;6,512,354;6,515,649;6,518,949;6,521,489;6,531,997;6,535,197;6,538,801;6,545,291;6,580,545;6,639,578;6,652,075;6,657,772;6,664,944;6,680,725;6,683,333;6,704,133;6,710,540;6,721,08 3; 6,724,519; 6,727,881; 6,738,050; 6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068; 6,825,829; 6,825,970; 6,831,769; 6,839,158; 6,842,167; 6,842,279; 6,842,657; 6,864,875; 6,865,010; 6,866, 760; 6,870,661; 6,900,851; and 6,922,276; and U.S. Patent Application Publication Nos. 2002/0060321; 2002/0063661; 2002/0090980; 2002/0113770; 2002/0130832; 2002/0180687; 2003/0011560; 2003/0020844; 2003/0025855; 2003/0102858; 2003/0132 908;2003/0137521;2003/0214695;2003/0222315;2004/0012839;2004/0014265;2004/0027327;2004/0075634;2004/0094422;2004/0105036;2004/0112750;2004/0119681;2004/0136048;2004/0155857;2004/0180476;20 04/0190114; 2004/0196215; 2004/0226820; 2004/0239614; 2004/0252360; 2004/0257635; 2004/0263947; 2005/0000813; 2005/0001812; 2005/0007336; 2005/0007653; 2005/0012980; 2005/0017944; 2005/0018273; 2005/002 4353; 2005/0035941; 2005/0041004; 2005/0062714; 2005/0067656; 2005/0078099; 2005/0105159; 2005/0122284; 2005/0122306; 2005/0122563; 2005/0122564; 2005/0122565; 2005/0151709; 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 WO 03/107,315.

Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in encapsulated electrophoretic media can be replaced by a continuous phase, thereby creating so-called "polymer-dispersed electrophoretic displays," wherein the electrophoretic medium comprises a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymer material, and wherein the discrete droplets of electrophoretic fluid within such polymer-dispersed electrophoretic displays can be considered capsules or microcapsules, even though no discrete capsule membrane is associated with each individual droplet; see, for example, the aforementioned U.S. Patent No. 6,866,760. Therefore, for the purposes of this application, such polymer-dispersed electrophoretic media are considered a subclass of encapsulated electrophoretic media.

Encapsulated electrophoretic displays are generally not plagued by the aggregation and sedimentation failure modes of conventional electrophoretic devices and offer additional benefits, such as the ability to print or coat the display on a variety of flexible and rigid substrates. (The 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 patch die coating, slot or extrusion coating, slide or laminate coating, curtain coating; roll coating such as blade over roller, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; screen printing processes; electrostatic printing processes; thermal printing processes; inkjet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Additionally, because the display medium can be printed (using a variety of methods), the display itself can be inexpensively manufactured.

A related type of electrophoretic display is the so-called "microcell electrophoretic display." In a microcell electrophoretic display, charged particles and fluids are not encapsulated in capsules, but rather 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-optical 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 visible light from being transmitted through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "shutter mode," in which one display state is substantially opaque and one display state is light-transmissive. See, for example, the aforementioned U.S. Patent Nos. 6,130,774 and 6,172,798, and U.S. Patent Nos. 5,872,552, 6,144,361, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely on changes in electric field strength, can operate in a similar mode; see U.S. Patent No. 4,418,346.

The bi-stable or multi-stable behavior of particle-based electrophoretic displays and other electro-optical displays that exhibit similar behavior (such displays may be conveniently referred to hereinafter as "impulse-driven displays") stands in stark contrast to conventional liquid crystal ("LC") displays. Twisted nematic liquid crystals are not bi-stable or multi-stable, but rather act as voltage converters, such that applying a given electric field to a pixel of such a display produces a specific grayscale level at the pixel, regardless of the grayscale level that previously existed at the pixel. Furthermore, LC displays can be driven in only one direction (from non-transmissive or "dark" to transmissive or "bright"), with the reverse transition from a lighter state to a darker state being achieved by reducing or eliminating the electric field. Finally, the grayscale level of a pixel of an LC display is insensitive to the polarity of the electric field, only its amplitude, and indeed, for technical reasons, commercial LC displays typically reverse the polarity of the drive field at frequent intervals. In contrast, bi-stable electro-optical displays act, to a first approximation, as impulse converters, such that the final state of a pixel depends not only on the applied electric field and the duration of that field application, but also on the state of the pixel before the electric field was applied.

Regardless of whether the electro-optical medium used is bi-stable, to achieve a high-resolution display, the individual pixels of the display must be addressable without interference from neighboring pixels. One way to achieve this is to provide an array of nonlinear elements, such as transistors or diodes, with at least one nonlinear element associated with each pixel, creating an "active matrix" display. The addressing or pixel electrode that addresses a pixel is connected to an appropriate voltage source through the associated nonlinear element. Typically, when the nonlinear element is a transistor, the pixel electrode is connected to the drain of the transistor, and this arrangement will be used in the following description, although it is essentially arbitrary and the pixel electrode can be connected to the source of the transistor. Traditionally, in high-resolution arrays, pixels are arranged in a two-dimensional array of rows and columns, such that any particular pixel is uniquely defined by the intersection of a designated row and a designated column. The sources of all transistors in each column are connected to a single column electrode, while the gates of all transistors in each row are connected to a single row electrode; again, the assignment of sources to rows and gates to columns is conventional but essentially arbitrary and can be reversed if desired. The row electrodes are connected to a row driver, which essentially ensures that only one row is selected at any given moment, i.e., a voltage is applied to the selected row electrode to ensure that all transistors in the selected row are conductive, while a voltage is applied to all other rows to ensure that all transistors in these unselected rows remain non-conductive. The column electrodes are connected to a column driver, which places a voltage on each column electrode that is selected to drive the pixels in the selected row to their desired optical state. (The above voltages are relative to a common front electrode that is typically provided on the side of the electro-optic medium opposite the non-linear array and extends across the entire display.) After a pre-selected interval called the "line address time," the selected row is deselected, the next row is selected, and the voltage on the column driver is changed so that the next row of the display is written. This process is repeated so that the entire display is written in a row-by-row manner.

It might first appear that the ideal method for addressing such an impulse-driven electro-optical display would be a so-called "normal grayscale image flow," in which the controller arranges each writing of an image so that each pixel transitions directly from its initial grayscale level to its final grayscale level. However, some errors are unavoidable when writing images on an impulse-driven display. Some of these errors encountered in practice include:

(a) Previous state dependence; For at least some electro-optical media, the impulse required to switch a pixel to a new optical state depends not only on the current and desired optical states, but also on the previous optical state of the pixel.

(b) Dwell time dependence: For at least some electro-optical media, the impulse required to switch a pixel to a new optical state depends on the time the pixel spends in its various optical states. The precise nature of this dependence is not yet known, but in general, the longer the pixel is in its current optical state, the more impulses are required.

(c) Temperature dependence; the impulse required to switch a pixel to a new optical state depends strongly on the temperature.

(d) Humidity dependence: The impulse required to switch a pixel to a new optical state depends on the ambient humidity for at least some types of electro-optical media.

(e) Mechanical uniformity: The impulse required to switch a pixel to a new optical state can be affected by mechanical variations in the display, such as variations in the thickness of the electro-optic medium or associated laminating adhesive. Other types of mechanical non-uniformity can arise from inevitable variations in the medium between different manufacturing batches, manufacturing tolerances, and material variations.

(f) Voltage Errors: Because of inevitable slight errors in the voltage supplied by the driver, the actual impulse applied to the pixel will inevitably differ slightly from the impulse applied theoretically.

Grayscale image streams generally suffer from the phenomenon of "error accumulation". For example, suppose the temperature dependence results in an error of 0.2L* in the positive direction at each transition (where L* has the usual CIE definition:

L*＝116(R/R ₀ ) ^1/3 -16,

(where R is the reflectance and _R0 is the standard reflectance value). After fifty transitions, this error will accumulate to 10L*. Perhaps more realistically, assume that the average error per transition, expressed as the difference between the theoretical and actual reflectance of the display, is ±0.2L*. After 100 consecutive transitions, the pixel will show an average deviation of 2L* from its expected state; this deviation is noticeable to the average observer on certain types of images.

This error accumulation phenomenon applies not only to errors due to temperature, but to all the types of errors listed above. As described in the aforementioned 2003/0137521, compensating for such errors is possible, but only with limited accuracy. For example, temperature errors can be compensated for using a temperature sensor and a lookup table, but the temperature sensor has limited resolution and may read a temperature slightly different from the temperature of the electro-optical medium. Similarly, prior state dependency can be compensated for by storing prior states and using a multidimensional transition matrix, but 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 such compensation.

Thus, general grayscale image streaming requires very precise control of the applied impulses to produce good results and has been found empirically to be impractical in commercial displays with the current state of electro-optical display technology.

Almost all electro-optical media have a built-in reset (error limiting) mechanism, i.e., their extreme (usually black and white) optical states, which act as "optical tracks". After a certain impulse is applied to a pixel of an electro-optical display, the pixel cannot become whiter (or blacker). For example, in a packaged electrophoretic display, after applying a certain impulse, all the electrophoretic particles are forced against each other or against the capsule wall and cannot move further, thereby generating a limited optical state or optical track. Because of the distribution of electrophoretic particle size and charge in such a medium, some particles hit the track before others, creating a "soft track" phenomenon, whereby the required impulse precision decreases as the final optical state of the transition approaches the extreme black and white states, while the required optical precision increases dramatically for transitions that end near the middle of the optical range of the pixel.

Various types of drive schemes for electro-optical displays are known that utilize optical tracks. For example, Figures 9 and 10 of the aforementioned U.S. Patent Application No. 2003/0137521 and the associated description in paragraphs [0177] to [0180] describe a "slide show" drive scheme in which the entire display is driven to at least one optical track before any new image is written. Clearly, a pure, general grayscale image stream drive scheme cannot rely on the use of optical tracks to prevent grayscale errors because, in such a drive scheme, any given pixel can experience an infinite number of grayscale level changes without contacting any one optical track.

Before proceeding further, it is desirable to define the slideshow drive scheme more precisely. The basic slideshow drive scheme is that the transition from an initial optical state (gray level) to a final (desired) optical state (gray level) is achieved by transitioning to a finite number of intermediate states, where the minimum number of intermediate states is one. Preferably, the intermediate states are at or near the extreme states of the electro-optical medium used. The transitions will differ from pixel to pixel in the display because they depend on the initial and final optical states. The waveform for a particular transition for a given pixel of the display can be expressed as:

There is at least one intermediate or target state between the initial state _R2 and the final state _R1 . The target state is typically a function of the initial and final optical states. The currently preferred number of intermediate states is two, but more or fewer intermediate states may be used. Each individual transition within the overall transition is achieved using a waveform element (typically a voltage pulse) sufficient to drive the pixel from one state in the sequence to the next. For example, in the waveforms indicated by symbols above, the transition from _R2 to the target state, Goal ₁ , is typically achieved using a waveform element or voltage pulse. This waveform element may have a single voltage (i.e., a single voltage pulse) for a finite time, or may include a variety of voltages to achieve a precise Goal ₁ state. This waveform element is followed by a second waveform element to achieve the transition from Goal ₁ to Goal _2. If only two target states are used, the second waveform element is followed by a third waveform element that drives the pixel from the Goal ₂ state to the final optical state _R1 . The target state may be independent of _R2 and _R1 , or may depend on one or both.

The present invention is directed to an improved slideshow drive scheme for electro-optical displays that achieves improved control of grayscale levels. The invention is particularly, but not exclusively, applicable to pulse-width modulation drive schemes, in which the voltage applied to any given pixel of the display at any given moment can only be -V, 0, or +V, where V is an arbitrary voltage. More specifically, the invention relates to two different types of improvements in slideshow drive schemes, namely (a) inserting certain modifying elements into the basic waveforms used for such drive schemes; and (b) arranging the drive scheme so that at least some grayscale levels are closer to the desired grayscale level from the optical track.

In another aspect, the present invention relates to dwell time compensation in a drive scheme for an electro-optical display. As discussed in the MEDEOD application, it has been found that, at least in the case of many particle-based electro-optical displays, the impulse required to change a given pixel by an equal change in grayscale level (as judged by the eye or a standard optical instrument) is not necessarily constant, nor is it necessarily interchangeable. For example, consider a display in which each pixel can display a grayscale level of 0 (white), 1, 2, or 3 (black) that is advantageously spaced apart. (The spacing between the levels can be linear in percent reflectance, as measured by the eye or an instrument, but other spacings can also be used. For example, the spacing can be linear in L*, or can be selected to provide a particular gamma; monitors typically employ a gamma of 2.2, and when an electro-optical display is used as a monitor replacement, it may be desirable to use a similar gamma.) It has been found that the impulse required to change a pixel from level 0 to level 1 (hereinafter referred to as a "0-1 transition" for convenience) is generally different from the impulse required for a 1-2 or 2-3 transition. Furthermore, the impulse required for a 1-0 transition is not necessarily the opposite of the impulse required for a 0-1 transition. Furthermore, some systems appear to exhibit a "memory" effect, such that the impulse required for, for example, a 0-1 transition varies to some extent depending on whether a particular pixel undergoes a 0-0-1, 1-0-1, or 3-0-1 transition. (Where the notation "x-y-z," where x, y, and z are all optical states 0, 1, 2, or 3, denotes a sequence of optical states that are accessed sequentially in time.) While these problems can be reduced or overcome by driving all pixels of the display to one of the extreme states for a period of time before driving the desired pixel to the other state, the resulting "flicker" of a solid color is generally unacceptable; for example, a reader of an e-book may want the text of the book to scroll down the screen, and if the display is required to flash solid black or white at frequent intervals, the reader may be distracted or forget where he last read. Furthermore, such flickering of the display increases its energy consumption and may shorten the operating life of the display. Finally, it has been found that, at least in some cases, the impulse required for a particular transition is affected by the temperature and total operating time of the display, and compensation for these factors is required to ensure accurate grayscale reproduction.

As briefly mentioned above, it has been found that, at least in some cases, the impulse required for a given transition in a bi-stable electro-optical display varies with the dwell time that a pixel is in its optical state, a phenomenon hereinafter referred to as "dwell time dependence" or "DTD," although the term "dwell time sensitivity" is used in the aforementioned application Ser. No. 60/320,070. Thus, it may be desirable, or even practically necessary in some cases, to vary the impulse applied for a given transition depending on the dwell time of a pixel in its initial optical state.

The phenomenon of dwell time dependence will now be explained in more detail with reference to FIG1 of the accompanying drawings, which shows the reflectivity of a pixel as a function of time for a sequence of transitions denoted as R ₃ →R ₂ →R ₁ , where (generalizing the nomenclature used above) each R _k term represents a gray level in the gray level sequence, where R with a larger index appears before R with a smaller index. The transitions between R ₃ and R ₂ , and between R ₂ and R _1, are also indicated. DTD is the change in the final optical state R ₁ caused by a change in the time spent in optical state R ₂ (referred to as the dwell time). DTD can be compensated for by selecting different waveforms for different dwell times or different ranges of dwell times in the previous optical state. This compensation method is called "dwell time compensation,""DTC," or simply "time compensation."

However, this DTC may conflict with other desired properties of the drive scheme. In particular, due to the reasons discussed in detail in the MEDEOD application, for many electro-optical displays, it is highly desirable to ensure that the drive scheme used is DC balanced, in the sense that for any series of transitions that start and end in the same optical state, the impulse applied (that is, the integral of the applied voltage relative to time) is zero. This ensures that the net impulse (also referred to as "DC imbalance") experienced by any pixel of the display is limited to a known value, regardless of the exact series of transitions experienced by the pixel. For example, a 15V, 300 millisecond pulse can be used to drive a pixel from a white state to a black state. After this transition, the pixel experiences a 4.5V second DC imbalance impulse. If a -15V, 300 millisecond pulse is used to drive the pixel back to white, the pixel is DC balanced for the entire process from white to black and back to white. This DC balance should be maintained for all possible processes from an original optical state to a series of optical states identical or different from the original optical state and then back to the original optical state.

The drive scheme can be compensated for dwell time by adding or removing voltage features from the basic 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

change Waveform Black to black 0V for 420 milliseconds Black to white -15V for 400ms, then 0V for 20ms White to black +15V for 400ms, then 0V for 20ms White to white 0V for 420 milliseconds

The drive scheme is DC balanced in that any series of transitions that returns the pixel to its initial optical state is DC balanced, ie the net area under the voltage distribution of the entire series of transitions is zero.

Optical errors can come from the display's DTD. For example, a pixel may start in the white state, be driven to the black state, dwell 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-optical display, then according to one aspect of the invention it is desirable to compensate for the DTD by selecting different waveforms for different ranges of dwell times in the previous optical state. For example, it may be found that the final white state in the example just given is brighter after a short dwell time in the previous black state than after a long dwell time in the previous black state. One dwell time compensation scheme is to modify the duration of the pulse that causes the pixel layer to transition from black to white to offset this DTD of the final optical state. For example, when the dwell time in the previous black state is short, the pulse length in the black to white transition may be shortened, and for long dwell times in the previous black state, the pulse is kept longer. This tends to produce a darker white state for shorter previous state dwell times, which offsets the effect of the DTD. For example, according to Table 2 below, a black to white waveform may be selected that varies with the dwell time in the black state.

Table 2

Dwell time Waveform 0 to 0.3 seconds -15V for 280ms, 0V for 140ms 0.3 seconds to 1 second -15V for 340ms, 0V for 80ms 1 second to 3 seconds -15V for 380ms, 0V for 40ms 3 seconds or more -15V for 400ms, 0V for 20ms

The problem with this approach to DTC for a drive scheme is that the entire drive scheme is no longer DC balanced. Because the impulse of the black to white transition is a function of the time spent in the black state, and similarly, the impulse of the white to black transition can be a function of the dwell time in the white state, the net impulse over the black to white to black sequence is generally not DC balanced. For example, suppose the sequence is performed as follows: a black to white transition is performed after a short dwell time in black using a voltage pulse of -15V = a -4.2V second impulse lasting 280 milliseconds, followed by a long dwell time in the white state by a white to black transition using a voltage pulse of 15V lasting 400 milliseconds (6V second impulse). The net impulse in this sequence (black-white-black cycle) is -4.2V seconds + 6V seconds = 1.8V seconds. Repeating this cycle results in a buildup of DC imbalance, which can be detrimental to the performance of the display.

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

Another aspect of the present invention relates to methods and apparatus for driving an electro-optical display that allows for rapid response to user input. The aforementioned MEDEOD application describes several methods and controllers for driving an electro-optical display. Most of these methods and controllers use a memory having two image buffers, the first storing a first or initial image (present on the display at the beginning of a transition or rewrite of the display) and the second storing a final image, which is desired to be placed on the display after the rewrite. The controller compares the initial and final images and, if they differ, applies a drive voltage to each pixel of the display that causes the pixel to undergo a change in optical state so that, at the end of the rewrite (alternatively referred to as an update), the final image is formed on the display.

However, in most of the aforementioned methods and controllers, the update operation is "atomic," meaning that once an update is initiated, the memory cannot accept any new image data until the update is complete. This presents difficulties when the display is intended to accept user input, for example, via a keyboard or similar data input device, because the controller does not respond to user input while the update is in progress. For electrophoretic media, where the transition between two extreme optical states can take hundreds of milliseconds, this unresponsive period can vary from approximately 800 to approximately 1800 milliseconds, much of which can be attributed to the required update cycle of the electro-optical material. While the duration of the unresponsive period can be reduced by removing some performance artifacts that increase the update time and by increasing the response speed of the electro-optical material, such techniques alone cannot reduce the unresponsive period to less than approximately 500 milliseconds. This is still longer than desired for interactive applications, such as electronic dictionaries, where users expect a rapid response to user input. Therefore, there is a need for an image update method and controller with a reduced unresponsive period.

This aspect of the invention utilizes the known concept of asynchronous image updating to significantly reduce the duration of the unresponsive period. Using a structure that has been developed for grayscale image displays has been shown to reduce the unresponsive period by up to 65% compared to prior art methods and controllers, with only a moderate increase in controller complexity and memory requirements.

Finally, the present invention relates to a method and apparatus for driving an electro-optical display in which the data defining the drive scheme is compressed in a particular manner. The aforementioned MEDEOD application describes a method and apparatus for driving an electro-optical display in which the data defining the drive scheme (or multiple drive schemes) used is stored in one or more look-up tables ("LUTs"). Such a LUT must of course contain data defining the waveform of each waveform of the or each drive scheme, and a single waveform typically requires multiple bytes. As described in the MEDEOD application, the LUT may have to take into account more than two optical states, as well as adjustments for factors such as the temperature, humidity, operating time, etc. of the medium. Therefore, the amount of storage required to maintain the waveform information may be large. It is desirable to reduce the amount of storage allocated to the waveform information in order to reduce the cost of the display controller. A simple compression scheme that can be practically accommodated in the display controller or host computer would help reduce the cost of the display controller. The present invention relates to a simple compression scheme that is particularly advantageous for electro-optical displays.

Summary of the Invention

The present invention provides a method for improving the performance of an electro-optical display (e.g., an electrophoretic display) over a range of temperatures by adjusting the display's frame rate to accommodate temperature-induced changes in the electrophoretic medium. The method involves storing a base waveform that defines a sequence of voltages to be applied to a pixel during a specific transition between grayscale levels at a first temperature and a base frame rate, and also storing a temperature-dependent multiplication factor n, where n is a positive number. The specific transition is then achieved by applying the base waveform to the pixel at a frame rate that is n times the base frame rate. The new frame rate can be faster or slower than the base frame rate; for example, higher temperatures will allow operation at a faster frame rate. The temperature-dependent multiplication factor n can be stored in a lookup table (LUT), whereby a temperature measurement is obtained and the value of n matching the temperature is obtained from the LUT. In some embodiments, the method further includes adjusting the amplitude of the base waveform by a second temperature-dependent factor p, which can also be stored in the LUT. By adjusting the frame rate, the overall performance of the electro-optical medium is improved, for example, as indicated by a reduction in the intensity of a residual image (a phenomenon known as "ghosting") after a pixel changes from a first image to a second image.

BRIEF DESCRIPTION OF THE DRAWINGS

As already mentioned, Figure 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.

2A and 2B show waveforms for two different transitions in a prior art three-reset pulse slideshow drive scheme of the type described in the aforementioned MEDEOD application.

2C and 2D illustrate the change in reflectivity over time for two pixels of an electro-optic display to which the waveforms of FIGs. 2A and 2B, respectively, are applied.

3A and 3B show waveforms for two different transitions in a prior art double reset pulse slide show drive scheme of the type described in the aforementioned MEDEOD application.

4A, 4B, and 4C illustrate balanced pulse pairs that may be used to modify prior art slide show waveforms, such as those shown in FIGs. 2A, 2B, 3A, and 3B, according to the BPPSS method of the present invention.

FIG. 5A shows a set of dwell time compensation waveforms used in the first dwell time compensated balanced pulse pair driving scheme of the present invention.

FIG. 5B shows a set of dwell time compensation waveforms used in the first dwell time compensated balanced pulse pair driving scheme of the present invention.

FIG. 5C shows a set of dwell time compensation waveforms used in the first dwell time compensated balanced pulse pair driving scheme of the present invention.

FIG. 5D shows a set of dwell time compensation waveforms used in the first dwell time compensated balanced pulse pair driving scheme of the present invention.

FIG. 5E shows a set of dwell time compensation waveforms used in the first dwell time compensated balanced pulse pair driving scheme of the present invention.

FIG. 6A shows a set of dwell time compensation waveforms used in the second dwell time compensated balanced pulse pair driving scheme of the present invention.

FIG. 6B shows a set of dwell time compensation waveforms used in the second dwell time compensated balanced pulse pair driving scheme of the present invention.

FIG. 6C shows a set of dwell time compensation waveforms used in the second dwell time compensated balanced pulse pair driving scheme of the present invention.

FIG7 shows a comparison of ghosting in gray tone transitions between a standard frame rate (solid line) and a temperature-adjusted frame rate (dashed line) at several temperatures.

DETAILED DESCRIPTION

The present invention provides a method for adjusting the drive waveform of an electrophoretic display to improve performance over a temperature range. Specifically, a basic waveform, including a voltage sequence and a basic frame rate, can be stored for a specific transition along with a temperature-dependent multiplication factor. Thus, a specific transition at a specific transition can be driven by applying the basic waveform at a frame rate equal to the basic frame rate adjusted by the temperature-dependent multiplication factor.

Balanced pulse-to-slideshow method and apparatus

A balanced pulse pair slideshow (BPPSS) method is a method for driving an electro-optical display having at least one pixel capable of achieving at least three different gray levels including two extreme optical states. The method comprises applying a base waveform to the pixel, the base waveform comprising at least one reset pulse sufficient to drive the pixel to or near one of the extreme optical states, followed by at least one set pulse sufficient to drive the pixel to a gray level different from the one extreme optical state, the base waveform being modified by at least one of the following:

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

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

(c) Inserting at least one zero voltage period into the basic waveform.

Furthermore, the term balanced pulse pair ("BPP") denotes a sequence of two pulses of opposite polarity such that the total impulse 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 of constant voltage but of opposite polarity and of equal length. The term "basic waveform element" or "BWE" may be used hereinafter to refer to any reset or set pulse of a basic waveform. The insertion of a balanced pulse pair and/or a zero voltage time period (hereinafter referred to as a "gap") can be implemented within a single basic waveform element or between two consecutive waveform elements. All of these modifications have the characteristic of not affecting the net impulse of the waveform; net impulse refers to the integral of the waveform voltage curve integrated over the duration of the waveform. Balanced pulse pairs and zero voltage pauses, of course, have zero net impulse. Although typically the pulses of a BPP will be inserted adjacent to each other, this is not required, and the two pulses can be inserted at different locations.

Where modifying the base waveform according to the BPPSS method includes removing at least one BPP, the time period previously occupied by the or each removed BPP may be retained as a zero voltage period. Alternatively, the time period may be "closed" by shifting some or all subsequent waveform elements to be earlier in time, but in this case, it is typically necessary to insert the zero voltage period at some later stage of the waveform, typically at its end, to ensure that the overall length of the waveform is maintained, as it is typically necessary to ensure that all pixels of the display are driven with waveforms of equal length. Alternatively, of course, the time period may be "closed" by shifting some or all earlier waveform elements to be later in time, with the zero voltage period being inserted at some earlier stage of the waveform, typically at its beginning.

As already noted, the BPPSS waveform of the present invention is a modification of the basic slideshow waveform described in the aforementioned MEDEOD application. As described above, the slideshow waveform includes one or more reset pulses that cause the pixel to move to, or at least approach, one extreme optical state (optical track); if the waveform includes two or more reset pulses, then each reset pulse after the first will cause the pixel to move to the opposite extreme optical state, thereby traversing substantially its entire optical range. (For example, if the display uses an electro-optical medium having a reflectivity range of, say, 4% to 40%, then each reset pulse after the first might cause the pixel to traverse a reflectivity range of 8% to 35%.) If more than one reset pulse is used, then successive reset pulses must, of course, have alternating polarities.

The slideshow waveform also includes a set pulse that drives the pixel from the extreme optical state left by the last reset pulse to the desired final gray level of the pixel. Note that when the desired final gray level is one of the extreme optical states and the last reset pulse leaves the pixel in that desired extreme optical state, the set pulse can have a zero duration. Similarly, if the initial state of the pixel before applying the slideshow waveform is in one of the extreme optical states, the first reset pulse can have a zero duration.

A preferred BPPSS waveform of the present invention will now be described, by way of illustration only, with reference to the accompanying drawings.

Figures 2A and 2B of the accompanying drawings illustrate t waveforms for two different transitions in a prior art (basic) slideshow drive scheme of the type described in the aforementioned MEDEOD application. This slideshow drive scheme uses three reset pulses for each transition. Figures 2C and 2D illustrate the corresponding changes in the optical state (reflectivity) of the pixel with the waveforms of Figures 2A and 2B applied, respectively, over time. Following the convention used in the aforementioned co-pending applications, serial numbers 10/065,795 and 10/879,335, Figures 2C and 2D are plotted so that the bottom horizontal line represents the extreme black optical state, the top horizontal line represents the extreme white optical state, and intermediate levels represent gray states. The start and end of the reset and set pulses of the waveforms are indicated by vertical dashed lines in Figures 2A and 2B, and various BWEs (i.e., reset and set pulses) are shown as consisting of ten or fewer pulses of equal length, but in general, BWEs can have any length longer, and if composed of a series of equal-length pulses, more than ten such pulses are typically used for the maximum-length BWE.

The basic waveform shown in Figures 2A and 2C (generally labeled 100) implements a white-to-white transition (i.e., a "transition" in which the initial and final states of the pixel are both the white extreme optical state). Waveform 100 includes a first negative (i.e., black-directed) reset pulse 102 that drives the pixel to its black extreme optical state, a second positive (white-directed) reset pulse 104 that drives the pixel to its white extreme optical state, a third negative (black-directed) reset pulse 106 that drives the pixel to its black extreme optical state, and a set pulse 108 that drives the pixel to its white extreme optical state. Each of the four pulses 102, 104, 106, and 108 has a maximum duration of ten units. (To avoid tedious reference to "duration units," these units may be referred to hereinafter as "time units" or "TUs.")

2B and 2D illustrate waveforms (generally designated 150) for a dark gray to light gray transition using the same three-reset pulse drive scheme as in FIGs. 2A and 2C. Waveform 150 includes a first reset pulse 152 that is negative and black-oriented, like first reset pulse 102 of waveform 100. However, because the transition using waveform 150 begins at a dark gray level, the duration of first reset pulse 152 (shown as four TUs) is shorter than that of reset pulse 102 because a shorter first reset pulse is required to place the pixel in its black extreme optical state at the end of the first reset pulse. For the remaining six TUs of first reset pulse 152, zero voltage is applied to the pixel. (FIGS. 2B and 2D illustrate first reset pulse 152 having a negative voltage for four TUs at the end of the relevant time period, but this is arbitrary, and periods of negative and zero voltage can be arranged as desired.)

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

As can be seen from the foregoing, in the prior art slide show drive scheme shown in Figures 2A-2D, the duration of the first reset pulse and 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 have zero duration. For example, in the drive scheme of Figures 2A-2D, a 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) and 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).

Generally, 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 display new images quickly. Because each reset pulse occupies a relatively long period of time, it is desirable to reduce the number of reset pulses to a minimum consistent with acceptable grayscale performance of the display, and a one- or two-reset-pulse slideshow drive scheme is generally preferred. Figures 3A and 3B of the accompanying drawings illustrate waveforms for two different transitions in a prior art slideshow drive scheme using a dual-reset pulse, of the type described in the aforementioned MEDEOD application.

FIG3A shows a single reset pulse waveform (generally designated 200) for white to light gray, which includes a reset pulse 202 that drives the pixel from its initial white state to black, and a set pulse 208 (same as pulse 158 in FIG2B ) that drives the pixel from black to light gray. Although waveform 200 uses only a single reset pulse, it should be understood that it is actually part of a dual reset pulse slideshow drive scheme with a first reset pulse of zero duration, as shown by the zero voltage period on the left-hand side of FIG3A .

3B shows a black to light gray dual reset pulse waveform (generally labeled 250) that includes a first reset pulse 252 that drives the pixel from its initial black state to white, a second reset pulse 254 that drives the pixel from white to black, and a set pulse 208 that is identical to the reset pulse in FIG. 3A that drives the pixel from black to light gray.

As mentioned above, the BPPSS waveform of the present invention is derived from the basic slideshow waveform shown in Figures 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 zero voltage period into the basic waveform. In the case of removing the BPP, the resulting gap can be closed or maintained as a zero voltage period. Combinations of these modifications can also be used.

Figures 4A-4C illustrate balanced pulse pairs for BPPSS waveforms. The BPP shown in Figure 4A (generally designated 300) comprises a constant voltage negative pulse 302, followed by a positive pulse 304 of the same duration and voltage as pulse 302 but with opposite polarity. Clearly, BPP 300 applies a zero net impulse to the pixel. The BPP shown in Figure 4B (generally designated 310) is identical to BPP 300, except that the order of the pulses is reversed. The BPP shown in Figure 4C (generally designated 320) is derived from BPP 310 by introducing a zero voltage period 322 between positive pulse 304 and negative pulse 302, respectively.

It should be noted that during the rewriting of the display from an initial image to a desired final image, not all pixels of the display have to reach a given target state (e.g., a reverse monochrome projection target state) at the same point in time. The points in time in the transition to the target state are a function of the initial and desired final gray levels R2 and R1, respectively. Ideally (and as generally shown herein), the points in time of R2 and R1 match, the entire display is driven through the various target states, and all pixels reach these target states simultaneously. However, it is often desirable to change the relative timing of the various waveforms of the drive scheme. Time shifting of the waveforms can be performed for aesthetic reasons, for example, to improve the appearance of the transition or the appearance of the resulting image. Moreover, modifications such as those discussed below can offset the relative temporal positions of the target states so that for various combinations of R1 and R2, the target states are reached at different times during the transition.

It is important to recognize that this waveform modification affects not only the reflectivity of the final optical state (i.e., final grayscale), but also the intermediate target states. While the target state of the basic waveform is typically close to one of the extreme optical states (optical tracks), and by definition, near the optical tracks of the last target state or the last two target states in the preferred form of the drive scheme, the above modifications can cause the reflectivity in the target state to deviate from the optical track. It is the change in the degree of drive toward the optical track that makes a small adjustment in the final optical state (grayscale).

It has been found that it is desirable to keep the impulse of each voltage pulse comprising the BPP relatively small. The amplitude of the BPP can be defined by a parameter d, whose absolute value describes the length of each of the two voltage pulses of the BPP, and whose sign indicates the sign of the second of the two pulses. For example, the BPPs shown in Figures 4A and 4B can be assigned d values of +1 and -1, respectively (while the BPP of Figure 4C is assigned a d value of -1 in a consistent scheme, where a gap modification is inserted between the two pulses). In some embodiments, all BPPs used have d values whose amplitude is less than PL, and preferably less than PL/2, where PL (in the same units used to measure BPP) is defined as the length of the voltage pulse required to drive a pixel from one extreme optical state to the other under the drive voltage characteristics of the drive scheme, or the average of such voltage pulses if the transition lengths in the two directions are different. In the example just given, d is expressed in display scan frames, and the BPPs of Figures 4A and 4B each have voltage pulses that are one scan frame long. In this case, PL would also be defined in scan frames. Of course, all quantities can alternatively be expressed in time units, such as seconds or milliseconds.

As described in the aforementioned MEDEOD application, it is often necessary or desirable to drive an electro-optical display using a drive circuit that is capable of providing only two drive voltages (also known as "fence" drive). Since a bistable electro-optic medium typically needs to be driven in two directions between its extreme optical states, it might first appear that at least three drive voltages are required, namely 0, +V, and -V, where V is an essentially arbitrary drive voltage such that one electrode of 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 either +V or -V, depending on the direction in which the pixel needs to be driven. When a dual voltage drive circuit is used, each waveform of the drive scheme is divided into time segments; typically these time segments are of equal duration, but this is not necessarily the case. In a non-fence drive scheme, a positive, zero, or negative drive voltage can be applied to any particular pixel in any time segment. For example, in a three drive voltage system, the common front electrode can be held at 0, while the individual pixel electrodes are held at +V, 0, or -V. In a fence drive scheme, each time segment is effectively divided into two; in one of the two resulting segments, only negative or zero drive voltage can be applied to any particular pixel, while in the other resulting segment, only positive or zero drive voltage can be applied to any particular pixel. For example, consider a dual drive voltage system with drive voltages V and v, where V>v. In the first segment of each pair of segments, the common front electrode is set to V, and the pixel electrode is set to V (zero drive voltage) or v (negative drive voltage). In the second segment of each pair of segments, the common front electrode is set to v, and the pixel electrode is set to v (zero drive voltage) or V (positive drive voltage). The resulting waveform is twice the size of the corresponding non-fence waveform.

It is also generally desirable that IMP drive schemes be capable of local updates. As described in the aforementioned MEDEOD application, it is generally desirable to drive an electro-optical display in a manner that allows local updates to specific areas of the display that are undergoing changes while the rest of the display remains unchanged; for example, it may be desirable to update a dialog box in which a user is entering text without updating the background image on the display. Locally updated versions of some drive schemes can be created by removing all non-zero voltages from the waveform used for zero transitions (i.e., transitions from one gray level to the same gray level). For example, the waveform from gray level 2 to gray level 2 typically consists of a series of voltage pulses. Removing the non-zero voltages from this waveform, and performing this operation for all other zero transitions, results in a locally updated version of the waveform. This locally updated version may be advantageous when it is desirable to minimize extraneous flicker during transitions.

Balanced pulse pair dwell time compensation method and device

In some embodiments, at least two different waveforms can be used for the same transition between specific gray levels of a pixel of an electro-optical display, depending on the duration of the pixel's dwell time in the state at which the transition begins. The two waveforms can differ from each other by at least one insertion and/or removal of at least one balanced pulse pair, or the insertion of at least one zero voltage period, where "balanced pulse pair" has the meaning previously defined. It is highly preferred that, in this approach, the drive scheme be DC balanced, as that term is defined above.

In this balanced pulse pair dwell time compensation (BPPDTC) method (as in the BPPSS method already described), balanced pulse pairs and/or zero voltage time periods (pauses) can be inserted or removed within a single waveform element or between two consecutive waveform elements. Two waveforms for the same transition after different dwell times in an initial state at the start of the transition may be referred to hereinafter as "alternative dwell time" or "ADT" waveforms.

It should be noted that ADT waveforms can differ from one another by the location and/or duration of the BPP or pause within the waveform (see, for example, the discussion of Figures 5B-5E below), as such movement of the BPP or pause can be formally viewed as a combination of the removal of a BPP or pause at one location and the insertion of a BPP or pause at a different location, or (in the case of a variation in duration at the same location) the removal of a BPP or pause at one location and the insertion of a different BPP or pause at the same location.

In the BPPDTC drive scheme, the insertion of BPPs and/or pauses raises the same issues and can be handled in the same manner as for the BPPSS drive scheme described above. Thus, where the difference between the BPPDTC waveforms includes the removal of at least one BPP, the time period previously occupied by the or each removed BPP can be retained as a zero voltage period. Alternatively, the time period can be "closed" by shifting some or all later waveform elements to be earlier in time, typically inserting a zero voltage period at some later stage in the waveform (typically at its end) to ensure that the overall length of the waveform is maintained. (In any practical display, which typically has at least several thousand pixels, at any transition there will typically be at least one pixel undergoing every possible transition, and if the waveforms for all pixels were of different lengths, the controller logic would become extremely complex.) Alternatively, of course, the time period can be "closed" by shifting some or all earlier waveform elements to be later in time, inserting a zero voltage period at some earlier stage in the waveform (typically at its beginning).

Similarly, inserting a BPP increases the total duration of a waveform unless the existing zero voltage period can be removed at the same time. Since it is highly desirable for all waveforms of a drive scheme to have the same total length, when one waveform of a drive scheme has a BPP inserted, all other waveforms of the drive scheme should have zero voltage periods added to them, or some other modification should be made to compensate for the increase in overall waveform length caused by the insertion of the BPP. For example, if a 40 millisecond BPP is inserted into the black to white waveform shown in Table 1 above (waveform length of 420 milliseconds), a 40 millisecond pause can be added to the remaining three waveforms shown in Table 1 so that all waveforms have a length of 460 milliseconds. Obviously, if appropriate, BPPs could be added to the other three waveforms instead of pauses, or some combination of BPPs and pauses totaling 40 milliseconds could be used.

A preferred drive scheme and waveform for the BPPDTC aspect of the present 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 type described above; for example, the BPP type shown in Figures 4A-4C may be used.

Figures 5A-5E illustrate alternative dwell-time waveforms that can be used for a single transition according to aspects of the BPPDTC of the present invention. Figure 5A illustrates the black-to-white waveform referenced in the third row of Table 1 and the last row of Table 2 above. Since this is a waveform suitable for a black-to-white transition after a long dwell time in the black state, it can be considered a basic black-to-white waveform that is modified according to aspects of the BPPDTC of the present invention to produce a waveform suitable for a black-to-white transition after a shorter dwell time in the black state. As already noted, the basic waveform of Figure 5A comprises a -15V, 400 millisecond pulse followed by 0V for 20 milliseconds.

FIG5B shows a modification of the basic waveform of FIG5A that has been found to be effective in reducing the reflectivity of the final white state when the black-to-white transition is achieved only after a short dwell time of no more than 0.3 seconds in the initial black state. The waveform of FIG5B is generated by inserting a BPP similar to BPP 300 shown in FIG4A at the end of the −15 V, 400 msec pulse of the waveform of FIG5A, such that the waveform of FIG5B includes a −15 V, 420 msec pulse, followed by a +15 V, 20 msec pulse, and 0 volts for 20 msec.

Figures 5C and 5D show two additional ADT waveforms for the same black-to-white transition as the waveforms of Figures 5A and 5B. The waveforms of Figures 5C and 5D have been found to effectively normalize the reflectivity of the final white state when the black-to-white transition is achieved after dwell times of 0.3 to 1 second and 1 to 3 seconds, respectively, in the black state. The waveforms of Figures 5C and 5D are generated by inserting the same BPP as in Figure 5B into the waveform of Figure 5A, but at different positions than those used in Figure 5B. As described above, it has been found 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, so shifting the insertion position of the BPP along with the base waveform is an effective means for compensating the waveform for variations in the dwell time of the pixel in the initial optical state.

FIG5E is a preferred alternative to the waveform of FIG5A for achieving a black to white transition after a long dwell time (3 seconds or more) in the black state. The waveform of FIG5E is generally similar to the waveform of FIG5B-5D in that it is generated by inserting the same BPP into the waveform of FIG5A. However, in FIG2E, the BPP is inserted at the beginning of the waveform; it is also found that it is desirable to make the pulse of the BPP last 40 milliseconds instead of 20 milliseconds. Since this makes the total duration of the waveform 500 milliseconds, when the waveform of FIG5E is used in conjunction with the waveform of FIG5B-5D, it is necessary to "fill" the waveform of FIG5B-5D with an additional 40 milliseconds of 0V at the end of the waveform. Therefore, a set of preferred ADT waveforms for black to white transitions is shown in Table 3 below:

Table 3

Note that for all ADT waveforms in Table 3, the impulse for the black to white transition is -15V*400 milliseconds or 6V seconds, thus making the drive scheme DC balanced for all initial state dwell times.

As already mentioned, DTC can also be achieved by removing the BPP from the basic waveform. For example, consider the drive scheme shown in Table 4 below:

Table 4

Note that in this drive scheme, not only the entire drive scheme, but all waveforms are "internally" DC balanced; the desirability of such internal DC balance is discussed in detail in the aforementioned co-pending application Ser. No. 10/814,205. Likewise, the method for DTC will be discussed with reference to black-to-white transitions, but it should be understood that DTC for white-to-black transitions can be implemented in a similar manner.

In this case, DTC for black-to-white transitions is achieved by removing the BPP, i.e., by removing a portion of a voltage pulse of one polarity and one duration while simultaneously removing a similar portion of a voltage pulse of the opposite polarity and the same duration. The removed pulse portion can be replaced with a zero voltage period, or the remainder of the waveform can be shifted in time to occupy the period previously occupied by the removed pulse pair, and to maintain the overall update time, a zero voltage segment matching the duration of the removed pair can be added elsewhere (typically at the beginning or end of the waveform).

Figures 6A, 6B, and 6C schematically illustrate the process of modifying the black-to-white waveform listed in the third row of Table 4 above for DTC in the black state with a short dwell time of less than 0.3 seconds. Figure 6A shows the base waveform from Table 4. Figure 6B schematically shows the removal of the BPP formed by the last 80 millisecond portion of the positive voltage pulse and the first 80 millisecond portion of the negative voltage pulse from the waveform of Figure 6A, where the resulting gap is eliminated by shifting the negative pulse forward in time, as indicated by the arrow in Figure 6B. The resulting dwell time compensation waveform is shown in Figure 6C, which includes a 320 millisecond positive pulse, a 320 millisecond negative pulse, and a 180 millisecond zero voltage period.

In this case, it was found that DTC for all dwell times could be achieved simply by varying the length of the removed BPP, and that the basic waveform of FIG6A was satisfactory for long dwell times of 3 seconds or more in the black state. Therefore, the complete list of ADT waveforms for the black to white transition in this case is shown in Table 5 below:

Table 5

Dwell time Waveform 0 to 0.3s +15V for 320ms, -15V for 320ms, then 0V for 180ms 0.3s to 1s +15V for 360ms, -15V for 360ms, then 0V for 100ms 1s to 3s +15V for 380ms, -15V for 380ms, then 0V for 60ms 3s or larger +15V for 400ms, -15V for 400ms, then 0V for 20ms

As already mentioned, when the BPP is removed from the basic waveform in the manner shown in FIG6B , it is not necessary for the remaining components to be offset in time; the removed BPP can simply be replaced by a zero voltage period. Table 6 below shows a set of modified ADT waveforms that are similar to those in Table 5, but with the removed BPP replaced by a zero voltage period:

Table 6

Dwell time Waveform 0 to 0.3s +15V for 320ms, 0V for 160ms, -15V for 320ms, then 0V for 20ms 0.3s to 1s +15V for 360ms, 0V for 80ms, -15V for 360ms, then 0V for 20ms 1s to 3s +15V for 380ms, 0V for 40ms, -15V for 380ms, then 0V for 60ms 3s or larger +15V for 400ms, -15V for 400ms, then 0V for 20ms

While the BPPDTC aspects of the present invention have been described primarily with reference to displays having only two grayscale levels, they are not limited thereto and can be applied to displays having a greater number of grayscale levels. Furthermore, while in the specific waveforms illustrated in the figures, the insertion or removal of the two elements of the BPP is achieved at a single point within the waveform, the present invention is not limited to waveforms in which the BPP is inserted or removed at a single point; the two elements of the BPP can be inserted or removed at different points, meaning that the two pulses comprising the BPP do not need to be immediately consecutive but can be separated by a time interval. Furthermore, one or both pulses of the BPP can be subdivided into multiple components, which can then be inserted into or removed from the DTC waveform. For example, a BPP can consist of a +15V, 60ms pulse and a -15V, 60ms pulse. This BPP can be divided into two components, such as a +15V, 60ms pulse followed by a -15V, 20ms pulse, and a -15V, 40ms pulse, with both components being simultaneously inserted into or removed from the waveform to achieve DTC.

It has also been found that inserting or removing zero voltage segments from the waveform affects the final grayscale level after the transition, so this insertion or removal of zero voltage segments provides a second method of adjusting the final grayscale level to achieve DTC. This insertion or removal of zero voltage segments can be used alone or in combination with the insertion or removal of BPP.

While the BPPDTC aspects of the present invention have been described above primarily with reference to pulse-width modulated waveforms, where the voltage applied to a pixel at any given time can only be -V, 0, or +V, the present invention is not limited to use with such pulse-width modulated waveforms and can be used with voltage-modulated waveforms, or waveforms that utilize both pulse and voltage modulation. The definition of a balanced pulse pair described above can be satisfied by two pulses of opposite polarity with zero net impulse, and there is no requirement that the two pulses have the same voltage or duration. For example, in a voltage-modulated drive scheme, a BPP may consist of a +15V, 20 millisecond pulse followed by a -5V, 60 millisecond pulse.

From the above it can be seen that the BPPDTC aspect of the present invention allows dwell time compensation of a drive scheme while maintaining the DC balance of the drive scheme.Such DTC can reduce ghosting levels in electro-optical displays.

Target buffer method and apparatus

A target buffer can be used to drive an electro-optical display having pixels capable of achieving at least two different grayscale levels. The first of the two methods, a non-polar target buffer method, includes providing initial, final, and target data buffers; determining when data in the initial data buffer and the final data buffer differ, and when such a difference is detected, updating the value in the target data buffer in such a manner that: (i) when the initial data buffer and the final data buffer contain the same value for a particular pixel, the target data buffer is set to that value; (ii) when the initial data buffer contains a greater value for the particular pixel than the final data buffer, the target data buffer is set to the value of the initial data buffer plus an increment; and (iii) when the initial data buffer contains a smaller value for the particular pixel than the final data buffer, the target data buffer is set to the value of the initial data buffer minus the increment; updating an image on the display using the data in the initial data buffer and the target data buffer as the initial and final states for each pixel, respectively; next, copying 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), final, initial, and target data buffers are again provided, along with a polarity bit array arranged to store a polarity bit for each pixel of the display. Again, the data in the initial and final data buffers are compared, and when they differ, the values in the polarity bit array and the target data buffer are updated in such a manner that: (i) when the values of a particular pixel in the initial and final data buffers differ, and the value in the initial data buffer represents an extreme optical state for the pixel, the polarity bit for the pixel is set to a value representing a transition toward the opposite extreme optical state; and depending on the associated value in the polarity bit array, the target data buffer is set to the value of the initial data buffer plus or minus an increment. The image on the display is then updated in the same manner as in the first method, and the data from the target data buffer is then copied into the initial data buffer. These steps are repeated until the initial and final data buffers contain identical data.

Prior art controllers for bi-stable electro-optic displays typically use logic similar to that shown in Listing 1 below (all listings here are pseudo-code):

List 1

With a controller operating in this manner, the display waits to receive new image information, and then, when such new image information is received, performs a full update before allowing the new information to be sent to the display, i.e., once one new image has been accepted by the display, the display cannot accept a second new image until the rewrite of the display required to display the first new image has completed, and in some cases, this rewrite procedure can take hundreds of milliseconds, see some of the drive schemes listed in Sections A-C above. Thus, when a user scrolls or types, the display appears insensitive to user input for this full update (rewrite) time.

In contrast, a controller implementing the non-polar target buffer method of the present invention operates through the following logic illustrated in List 2 (hereafter, for convenience, this type of controller may be referred to as a "List 2 controller"):

List 2

In this modified controller logic for the NPTB method, there are three image buffers. The initial and final buffers are the same as in the prior art controller, and the new third buffer is a "target" buffer. The display controller can accept new image data into the final buffer at any time. When the controller discovers that the data in the final buffer is no longer equal to the data in the initial buffer (i.e., the image needs to be rewritten), it constructs a new target data set by incrementing or decrementing the value in the initial buffer by 1 (or leaving it unchanged) based on the difference between the relevant values in the initial and final buffers. The controller then performs a display update in the usual way using the values from the initial and target buffers. When the update is complete, the controller copies the value in the target buffer to the initial buffer and then repeats the difference operation between the initial and final buffers to generate a new target buffer. When the initial and final buffers have the same data set, the overall update is complete.

Thus, in the NPTB approach, the overall 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" will be used hereinafter for the time period required for each of these sub-update operations; such an intermediate frame, of course, specifies the time period required for a single scan frame (see the aforementioned MEDEOD application) and superframe of the display, or the time period required to complete the entire 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 new data cannot be written to the final data buffer while the update is in progress, and therefore the display cannot respond to new inputs during the entire time period required for the update. In the NPTB method of the present invention, the final data buffer is only used to calculate the data set in the target data buffer, and this calculation is purely computer calculation and can be performed more quickly than the update operation, which requires a physical response from the electro-optical material. Once the calculation of the data set in the target data buffer is completed, the update does not require further access to the final data buffer, making the final data buffer available to accept new data.

For reasons discussed in the aforementioned MEDEOD application and discussed further below with respect to waveforms, it is generally desirable that pixels be driven in a cyclical manner, in the sense that once a pixel is driven away from one extreme optical state by a voltage pulse of one polarity, no voltage pulse of the opposite polarity is applied to the pixel until the pixel reaches its other extreme optical state. This constraint is satisfied by the PTB method of the present invention, which may use a controller operating with the logic illustrated in Listing 3 below (hereafter, for convenience, this type of controller may be referred to as a "List 3 controller"; the listing assumes a four-grayscale system with gray levels numbered from 1 for black to 4 for white, but one skilled in the art may readily modify the pseudocode for operation with a different number of gray levels):

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 direction of the associated pixel's transition, i.e., whether the pixel is currently transitioning from white to black (0) or from black to white (1). If the associated pixel is not currently undergoing a transition, the polarity bit retains its value from the previous transition; for example, a pixel that was stationary in a light gray state and was previously white would have a polarity bit of 0.

In the PTB method, the polarity bit array is taken into account when constructing a new target buffer data set. If the pixel is currently black or white and needs to be transitioned to the opposite state, the value of the polarity bit is set accordingly, and the target value is set to the grayscale closest to black or white, respectively. Alternatively, if the initial state of the pixel is an intermediate (gray) state, the target value is calculated by incrementing or decrementing the state by 1 (+1 if polarity = 1; -1 if polarity = 0) depending on the value of the polarity bit.

It should be noted that in this drive scheme, the behavior of a pixel in an intermediate state is independent of the current value of the pixel's final state. When starting a transition from black to white or from white to black, the pixel will continue in the same direction until it reaches the opposite optical trajectory (an extreme optical state, typically black or white). If the desired image, and therefore the target state, changes during the transition, the pixel will return in the opposite direction, and so on.

The preferred waveforms for 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) operation using the NPTB and PTB methods of the present invention, using two intermediate states.

Table 7

The structure of this transition matrix, with black, white, and two intermediate gray states, appears very similar to those used in prior art two-bit drive schemes, such as those described in the MEDEOD application. However, in the TB method of the present invention, these intermediate states do not correspond to stable gray states, but are merely transition states that exist only between the completion of one intermediate frame and the beginning of the next. Furthermore, there are no restrictions on the uniformity of the reflectivity of these intermediate states.

It should be noted that in the transition matrix shown in Table 7, many elements are not allowed (indicated by the dashed lines). The controller only allows each transition to change the gray level by one unit in either direction, thereby prohibiting transitions that involve multiple changes in gray level (such as a direct 1-4 black to white transition). For intermediate states, elements on the main diagonal of the transition matrix (corresponding to zero transitions) are prohibited; such main diagonal elements are not recommended for the white and black states, but are not strictly prohibited, as indicated by the asterisks in Table 7.

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

On the other hand, if during the update the final state of the display changes, the transition might become:

Multiple changes of the final state may produce transitions, for example:

More generally, there are four possible types of transitions between the extreme black and white optical states:

The brackets indicate zero or more repetitions of the enclosed sequence.

Optimization ("tuning") of this type of NPTB drive scheme requires adjusting the nonzero elements of the transition matrix to ensure consistent reflectivity values for states 1 (black) and 4 (white), regardless of the number of repetitions of the bracketed sequence. The waveform must function for arbitrary dwell times in the extreme black and white optical states, but the dwell time in the intermediate states is always zero, so the reflectivity of the transition states is unimportant, as discussed above.

Typically, the time required for any single intermediate frame update is equal to the length of the longest element in the transition matrix. Therefore, the time for the total update is three times the length of that longest element. In the best case, the black to white and white to black (and respectively) waveforms can be divided into three equal-length parts; this approach reduces the update delay to one-third of the full update time while maintaining the same duration of the full update. As the length of the intermediate frame updates becomes longer (which may be a result of optimizing the waveforms), the benefit becomes less significant. For example, if one element becomes twice as long, the delay increases to two-thirds the time of a simple update, and the full transition will take twice as long as before. 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 benefit of this extra computation is unlikely to be significant.

Consideration should be given to 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 dwell time dependence of the medium should be zero (ideally, or at least very low), since the waveform combines a series of near-zero dwell times between intermediate frames with potentially longer dwell times between transitions. Second, the medium should have little or no sensitivity to the optical state that precedes the initial state of a particular transition, since the direction of the transition can change in the intermediate flow; for example, a transition may be preceded by an or transition. Finally, the response of the electro-optic medium should be symmetrical, especially near the black and white states; it is difficult to produce a DC-balanced waveform that can perform an or transition to the same black or white state, respectively.

For the reasons mentioned above, the “intermediate inversion” in the NPTB drive scheme makes developing optimized waveforms very difficult. In contrast, the PTB drive scheme greatly reduces the requirements on the electro-optical medium and should therefore alleviate many of the difficulties in optimizing the NTPB drive scheme 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 only allows two black-to-white and white-to-black transitions, namely:

as well as

In practice, these two transitions can be the same as normal and transitions, with the transition divided into three equal parts. Some slight re-tuning may be required to account for any delay between intermediate frames, but the adjustment is straightforward. For simple typing input, this drive scheme should reduce latency by two-thirds.

There are some disadvantages to the PTB approach. The polarity bit array requires additional memory and a more complex controller to operate this simpler drive scheme because the direction of the transition allowed at each pixel requires consideration of additional data (the polarity bit) in addition to the initial and final states of the transition. Furthermore, while the PTB approach does reduce the latency to start an update, the controller must wait until the update is complete before reversing the transition. This limitation is noticeable if the user types a character and then immediately erases it; the delay before erasing the character is equal to the full update time. This limits the usefulness of the PTB approach for cursor tracking or scrolling.

Although the NPTB and PTB methods have been described above primarily for monochrome driving schemes, they are also compatible with grayscale driving schemes. The NPTB method is inherently fully grayscale compatible; the grayscale compatibility of the PTB method is discussed below.

From the perspective of the drive scheme, it is obviously more difficult to produce a feasible grayscale drive scheme for the NPTB method than the corresponding monochrome drive scheme, because in the grayscale drive scheme, the intermediate states now correspond to actual gray levels, and the optical values of these intermediate states are thus limited. It is also very difficult to produce a grayscale drive scheme for the PTB method. In order to reduce delays, the intermediate frame transitions must be significantly shortened. For example, the transition can be an independent transition, the last stage of a transition, or the first stage of a transition. Therefore, there are competing demands to make this transition short (to achieve a shorter overall update) and accurate (in the case where the transition stops at gray level 3).

The grayscale PTB method can be modified by introducing multiple grayscale steps (i.e., by allowing the grayscale level to change by more than one unit during each intermediate frame, corresponding to reinserting elements of more than one step removed from the main diagonal of the associated transition matrix, such as shown in Table 7 above), thereby eliminating the degeneracy of the intermediate frame steps described in the previous paragraph. This modification can be achieved by replacing the polarity bit matrix with a counter array that contains more than one bit for each pixel of the display, up to the number of bits required for full grayscale image representation. The waveform will then contain up to the full N×N transition matrix, each waveform evenly divided into four (or other essentially arbitrary number of intermediate frames).

Although the specific TB method discussed above is a two-bit grayscale method with two intermediate gray levels, the TB method can certainly be used with any number of gray levels. However, as the number of gray levels grows, the incremental benefits of reducing the delay will tend to diminish.

Thus, the present invention provides two types of TB methods that significantly reduce update delays 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 crucial.

Waveform compression method and device

The amount of waveform data that needs to be stored to drive a bi-stable electro-optical display can be reduced by certain compression methods described below. This "waveform compression" or "WC" method can be used to drive an electro-optical display having multiple pixels, each capable of achieving at least two different gray levels. In one embodiment, the method includes: storing a base waveform that defines a sequence of voltages to be applied during a particular transition of the pixel between gray levels; storing a multiplication factor for the particular transition; and achieving the particular transition by applying the sequence of voltages to the pixel for a time period that depends on the multiplication factor.

When an impulse-driven electro-optical display is being driven, each pixel of the display receives a voltage pulse (i.e., the voltage difference between the two electrodes associated with the pixel) or a temporal sequence of voltage pulses (i.e., a waveform) to effect a transition from one optical state of the pixel to another, typically a transition between grayscale levels. The data required to define the set of waveforms for each transition (forming a complete drive scheme) is stored in a memory, typically on a display controller, although the data may alternatively be stored on a host computer or other auxiliary device. A drive scheme can include a large number of waveforms, and (as described in the aforementioned MEDEOD application) it may be necessary to store multiple sets of waveform data to account for changes in environmental parameters (e.g., temperature and humidity) as well as non-environmental changes (e.g., the service life of the electro-optical medium). Therefore, the amount of storage required to maintain the waveform data can be significant. It would be desirable to reduce this storage to reduce the cost of the display controller. A simple compression scheme that can be practically accommodated in a display controller or host computer would help reduce the amount of storage 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 data for a particular transition is typically stored as a series of bits, each bit group specifying a specific voltage to be applied at a specific point in the waveform. For example, consider a three-level voltage drive scheme, where a positive voltage (in this example, +10V) is used to drive a pixel toward black, a negative voltage (-10V) is used to drive a pixel toward white, and zero voltage is used to maintain the pixel in its current optical state. The voltage for a given time element (a scan frame for an active matrix display) can be encoded using two bits, for example, as shown in Table 8 below:

Table 8

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

Using this binary representation, a waveform for an active matrix driver consisting of a +10V pulse lasting 5 scan frames, followed by two zero voltage scan frames, would be represented as:

01 01 01 01 01 00 00.

Waveforms containing a large number of time segments require a large number of bytes to store the waveform data.

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. The display controller (or other appropriate hardware) applies a voltage sequence defined by the basic waveform to the pixel, the duration of which depends on the multiplication factor. In a preferred form of this WC method, bit groups (such as given above) are used to represent the basic waveform, but the voltage defined by each bit group is applied to the pixel in n time segments, where n is the multiplication factor associated with the waveform. The multiplication factor must be a natural number. For a 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, that is, the bits required to represent these waveforms are less than the bits required if the data is stored in uncompressed form.

By way of example, using the three voltage level binary representation of Table 8, consider the following waveform, which requires 12 scanning frames of +10 V, followed by 9 scanning frames of -10 V, followed by 6 scanning frames of +10 V, followed by 3 scanning frames of 0 V. This waveform is represented in uncompressed form as:

01 01 01 01 01 01 01 01 01 01 01 01 01 10 10 10 10 10 10 10 10 01 0101 01 01 01 01 00 00 00

And expressed in compressed form as:

Multiplication factor: 3

The basic waveform is 01 01 01 01 10 10 10 01 01 00.

The length of the voltage sequence that must be assigned to each waveform is determined by the longest waveform. For packaged electrophoresis and many other electro-optical displays, the longest waveforms are typically required at the lowest temperatures, where the electro-optical medium responds slowly to the applied field. At the same time, when the response is slow, the resolution required to achieve a successful transition is reduced, so by grouping consecutive scan frames through the WC method of the present invention, there is almost no loss of accuracy in the optical state. By using this compression method, each waveform can be assigned multiple scan frames (or typically time segments) suitable for waveforms at medium and high temperatures with short update times. At low temperatures, the number of scan frames required may exceed the memory allocation, and a multiplication factor greater than 1 can be used to generate long waveforms. This ultimately leads to reduced memory requirements and costs.

The WC method of the present invention is in principle equivalent to simply changing the frame time of an active matrix display at various temperatures (as described below). For example, the display can be driven at 50 Hz at room temperature and at 25 Hz at 0°C to extend the allowed waveform time (as described below). In some embodiments, the WC method is superior to changing the frame rate because the backplane is designed to minimize the effects of capacitance and resistance voltage artifacts at a given frame rate. When there is a significant deviation from this optimal frame rate in either direction, at least one type of artifact appears. Therefore, in some cases, it is best to keep the actual frame rate constant while grouping the scan frames using the WC method, which in effect provides a way to achieve a virtual change in frame rate without actually changing the physical frame rate.

Scan rate compression method

The present invention provides a method for improving the performance of an electro-optical display (e.g., a bistable electrophoretic display) over a range of temperatures by adjusting the display's frame rate to accommodate temperature-induced changes in the electro-optic medium. For example, in an electrophoretic display, decreasing temperature results in a decrease in electrophoretic mobility because the viscosity of the internal phase increases. Consequently, when the display is driven using a waveform optimized for a temperature different from the current operating temperature, temperature fluctuations can result in slow updates and/or image artifacts. To overcome this problem, some display controllers include a complete set of waveforms (gray _m(T) -> gray _n(T) ) for a selected set of temperatures (T1, T2, T3, ...). For a given operating temperature, the set of grayscale transitions (gray _m(T) -> gray _n(T) ) closest to the measured temperature is used to implement the grayscale transitions. Nevertheless, at intermediate temperatures, such as between _T1 and _T2 , the display's performance may be unacceptable due to higher-order effects of temperature variations.

The claimed method can significantly reduce the amount of memory required to store waveforms for a given grayscale transition within a certain temperature range. The method includes storing a base waveform that defines a voltage sequence to be applied to a pixel during a specific transition between grayscale levels at a first temperature and a base frame rate, and also storing a temperature-dependent multiplication factor n, where n is a positive number. The temperature-dependent multiplication factor n can be between 0.1 and 100, such as between 0.5 and 10, or between 0.8 and 3. In some embodiments, n is approximately 0.9, approximately 0.95, approximately 1.05, approximately 1.1, approximately 1.15, approximately 1.2, approximately 1.25, or approximately 2. The specific transition is then achieved by applying the base waveform to the pixel at a frame rate that is n times the base frame rate. The new frame rate can be faster or slower than the base frame rate; for example, higher temperatures will require operation at a faster frame rate. The temperature-dependent multiplication factor n can be stored in a lookup table (LUT), whereby a temperature measurement is obtained and the n value matching that temperature is obtained from the LUT. In some embodiments, the method further includes adjusting the amplitude of the base waveform by a second temperature-dependent factor p, which may also be stored in the LUT. By adjusting the frame rate, the overall performance of the electro-optical medium is improved, for example, as indicated by a reduction in the intensity of a residual image after a pixel changes from a first image to a second image, a phenomenon known as "ghosting." The frame rate can be adjusted using techniques known in the art and described in the many patents and patent applications listed in the Background section.

Because each row of the active matrix needs to be individually selected during each frame, the basic frame rate does in practice exceed about 50 to 100 Hz. In some cases, frames of this length make it difficult to finely control grayscale with many fast-switching electro-optical media. For example, some packaged electrophoretic media essentially complete switching between their extreme optical states in about 100 ms (a transition of about 30 L* units), and with such media, a 20 ms frame corresponds to a grayscale shift of about 6 L* units. This shift is too large to accurately control grayscale; the human eye is sensitive to differences in grayscale levels of about 1 L* unit, and controlling impulses only on a scale equivalent to about 6 L* units can produce visible artifacts. Such artifacts include "ghosting" due to the electro-optical medium's previous state dependence, that is, if the transition is underdriven or not completely cleared, the second image will have a remnant of the first image, i.e., a "ghost image." The basic frame rate is typically of the order of 50 Hz, however, in theory the basic frame rate may be in any reasonable range, eg between 1 Hz and 200 Hz, eg between 40 Hz and 80 Hz.

FIG7 illustrates the variation in ghosting due to temperature, and the ability to correct it using the method of the present invention. A standard waveform optimized for 26°C was evaluated for ghosting by driving the electrophoresis test plate between the first and second gray states multiple times, and then measuring the amount of residual reflectivity in the second, darker state using a standardized optical bench with a calibrated light source and photodiode. However, when this standard waveform is applied to the electrophoresis test plate at the same frame rate at temperatures other than 26°C, the ghosting worsens because the transitions are underdriven (lower temperatures) or overdriven (higher temperatures). See the solid line in FIG7 . In contrast, using the technique of the present invention, the frame rate is modified by a temperature-dependent factor n, and the ghosting is significantly improved using the same standard waveform. See the dashed line in FIG7 . (Note that the solid and dashed lines intersect at 26°C because they both use the same, i.e., 26°C optimized, frame rate.) Therefore, it is not necessary to store a complete set of transitions at 22°C, 26°C, and 30°C. Instead, the same 26°C base waveform can be used, with slightly different frame rates at 22°C and 30°C.

The temperature-dependent multiplication factor n can be stored in a lookup table (LUT), stored, for example, in flash memory. The display can include a temperature sensor to allow the display to monitor the display's temperature in real time. Once the temperature is known, the corresponding factor n can be matched from the lookup table. In principle, n can be measured for every unit of °C within the operating range, or even for every tenth of °C within the operating range. Overall, this accumulation of n takes up very little memory compared to storing a complete set of waveforms for each temperature.

In some embodiments, it is also beneficial to modify the amplitude of the waveform based on temperature. In such embodiments, the amplitude of the basic waveform can be changed by a second temperature-dependent factor p. The second temperature-dependent multiplication factor p can be between 0.1 and 100, such as between 0.5 and 10, such as between 0.8 and 3. In some embodiments, p is about 0.75, about 0.8, about 0.9, about 1.1, about 1.5, about 2, about 3, about 4, or about 5. Thus, the present invention allows for simultaneous adjustment of the frame rate and amplitude of the basic waveform to offset performance variations due to environmental conditions (e.g., temperature). It should be understood that "amplitude" refers to the magnitude of the voltage of the waveform compared to ground or some other floating voltage. For example, many of the waveforms shown in the figure will have an amplitude of 15 volts, even if the waveform includes a square wave from 0 to 15V and 0 to -15V. By varying the frame rate and amplitude of the waveform, the total energy consumption of the electro-optical display over time can be maintained (or reduced) without sacrificing performance. A second temperature-dependent factor p may also be stored in the same or a different LUT so the display controller can adjust the amplitude of the base waveform to optimize performance.

It will be apparent to those skilled in the art that many changes may be made to the specific embodiments of the invention that have been described without departing from the spirit and scope of the invention.Therefore, the entire foregoing description should be interpreted as illustrative and not restrictive.

Claims (15)

1. A method for driving an electro-optic display having multiple pixels, each pixel capable of achieving at least two different gray levels, the method comprising: Store a basic waveform that defines a sequence of voltages to be applied to a pixel during a specific transition between gray levels at a first temperature and a basic frame rate. Storage temperature-dependent multiplication factor n, where n is a positive number; and The specific transition is achieved by applying the basic waveform to the pixel at a frame rate that is n times the basic frame rate; The specific transition refers to the transition from the initial optical state to the final optical state by transitioning to a finite number of intermediate states. 2. The method of claim 1, wherein the basic frame rate is between 1 Hz and 200 Hz. 3. The method of claim 2, wherein the basic frame rate is between 40Hz and 80Hz. 4. The method of claim 3, wherein the basic frame rate is 50Hz. 5. The method of claim 1, wherein the basic waveform comprises a set of bits. 6. The method of claim 1, wherein the basic waveform is DC balanced. 7. The method of claim 1, wherein the temperature-related multiplication factor n is stored in a lookup table. 8. The method of claim 7, further comprising obtaining a temperature measurement and matching the n value with the measured temperature. 9. The method of claim 1, further comprising: Storage temperature-dependent multiplication factor p, where p is a positive number; and The amplitude of the basic waveform is adjusted by a factor p. 10. The method of claim 9, wherein the amplitude of the basic waveform is between 2 volts and 60 volts. 11. The method of claim 10, wherein the amplitude of the basic waveform is between 4 volts and 21 volts. 12. The method of claim 11, wherein the amplitude of the basic waveform is 15 volts. 13. The method of claim 9, wherein the temperature-related multiplication factor p is stored in a lookup table. 14. The method of claim 13, further comprising obtaining a temperature measurement and matching a p-value with the measured temperature. 15. The method of any one of claims 1-14, wherein the electro-optic display comprises an electrophoretic medium.