KR20160039107A - Liquid crystal display device and method of reducing power consumption thereof - Google Patents

Abstract

The present invention relates to a liquid crystal display device and a power consumption reducing method thereof, and includes a calculation unit, a gain selection unit, a PWM selection unit, a data modulation unit, and a display panel drive unit. The calculation unit calculates brightness information of the input image using an index (IQBL) that defines the brightness perceived by a human. The gain selector selects a gain based on the index. The PWM selector selects a duty ratio of the PWM based on the index and outputs a PWM signal. The data modulator modulates the pixel data with the gain, and the display panel driver writes the modulated pixel data to the pixels of the display panel to reproduce the input image on the display panel.

Description

TECHNICAL FIELD [0001] The present invention relates to a liquid crystal display device and a method of reducing power consumption of the liquid crystal display device.

The present invention relates to a liquid crystal display device capable of improving power consumption without lowering the image quality and a power consumption reduction method thereof.

A liquid crystal display device (LCD), an organic light emitting diode (OLED) display device, a plasma display panel (PDP), a field emission display device Field Emission Display (FED), and the like.

A liquid crystal display device displays an image by controlling an electric field applied to liquid crystal molecules in accordance with a data voltage. The active matrix type liquid crystal display device has been widely used because it is applied to almost all display devices from small mobile devices to large-sized televisions due to its low price and high performance due to development of process technology and driving technology.

Since the liquid crystal display device is not a self-luminous element, a backlight unit for irradiating light to the liquid crystal display panel is required. In order to extend the battery life of the portable information device, the power consumption of the liquid crystal display device must be reduced. The power consumption of the light sources of the backlight unit and the inverter circuit for driving the light source is almost half of the total power consumption of the liquid crystal display device. Therefore, in order to reduce the power consumption of the liquid crystal display device, a method of reducing the power consumption of the backlight unit is effective.

As a method for reducing the power consumption of a backlight unit, a backlight dimming method is widely used. The backlight dimming method includes a local dimming method in which the display surface is divided into a plurality of blocks and the backlight luminance is individually controlled for each block, and a global dimming method (global dimming method) for collectively lowering the backlight luminance of the entire display surface (Global dimming method).

The local backlight dimming method can improve the static contrast by locally controlling the brightness of the display surface within one frame period, and reduce the power consumption. The local backlight dimming method is disadvantageous in that it can be applied only to a direct-type LED (Light Emitting Diode) backlight unit in which the complexity of the algorithm and hardware is high and the luminance control of each block is easy.

The global backlight dimming method can improve the dynamic contrast and lower the power consumption. The global backlight dimming method has low algorithm and hardware complexity and can be applied to any type of backlight unit.

The backlight dimming method is applied together with the pixel data compensation method. The pixel data compensation method adjusts the gamma curve based on the average picture level (APL). However, this method does not consider the difference in gradation distribution in the same or similar images of the APL, but merely calculates the average luminance. Therefore, gradation accumulation occurs at a high gradation level, and deterioration in image quality that an observer perceives due to a decrease in backlight luminance at a low gradation level do. Therefore, there is a limitation in the backlight dimming method due to the image quality degradation problem, and the power consumption reduction effect is small.

In the example of FIG. 1, the left image and the right image have the same APL = 0.5 but have different gradation distributions, so that the observer feels the difference in brightness. However, since the APL can not distinguish the brightness difference within one screen, if the pixel data and the backlight luminance are adjusted based on the APL, the image quality degraded by the observer is high in a high APL image and a low APL image.

The APL is calculated as shown in Equation 1 below.

Where n is the number of pixels.

1, if the grayscale of all the pixel data is 127 and the number of pixels is 1920 x 1080 in one frame image, APL = 50 (%) as shown below.

The right image of FIG. 2 has grades 127, 255, and 0 distributed as shown below, and includes a bright portion as compared with the right image. The APL is the same as the left image, APL = 50 (%).

As shown in FIG. 2, the grayscale cluster phenomenon in which the grayscales in the high grayscale range are the same as the grayscale in the high image of the APL appears, and the brightness difference in all the pixels can be felt due to the backlight luminance drop in the low grayscale as shown in FIG. FIG. 2 shows tonality clustering (red box in the right image) at a high gray level when applying the conventional backlight dimming method in the original image (left image) of APL = 63.1%. FIG. 3 shows a phenomenon in which the backlight dimming method according to the related art is applied to a dark image of APL = 11.7%, resulting in an overall darkness compared to the original image due to a decrease in backlight luminance.

The present invention provides a liquid crystal display device capable of reducing power consumption without deteriorating image quality and a power consumption reducing method thereof.

The liquid crystal display of the present invention includes a calculation unit, a gain selection unit, a PWM selection unit, a data modulation unit, and a display panel drive unit.

The calculation unit calculates brightness information of the input image using an index (IQBL) that defines the brightness perceived by a human. The gain selector selects a gain based on the index. The PWM selector selects a duty ratio of the PWM based on the index and outputs a PWM signal. The data modulator modulates the pixel data with the gain, and the display panel driver writes the modulated pixel data to the pixels of the display panel to reproduce the input image on the display panel.

The PWM selection unit controls the light source of the backlight unit using the PWM signal to control the backlight luminance of the display panel according to the brightness distribution of the input image.

A method of reducing power consumption of a liquid crystal display device includes calculating brightness information of an input image using an index that defines a brightness perceived by a human being; Selecting a gain based on the indicator; A PWM signal is generated by selecting a duty ratio of PWM (Pulse Width Modulation) based on the index, and the light source of the backlight unit is controlled by the PWM signal to control the backlight luminance of the display panel according to the brightness distribution of the input image step; And reproducing the input image on the display panel by modulating the pixel data with the gain and writing the modulated pixel data to pixels of the display panel.

The present invention can reduce the power consumption without deteriorating picture quality in a liquid crystal display device by selecting a gain of a pixel based on a new indicator (IQBL) reflecting human brightness perception and adjusting the backlight luminance.

1 is a diagram showing an example of an image having a different gradation distribution and the same APL.
2 is a diagram illustrating an example in which a backlight dimming method is applied to an image having a high APL.
3 is a diagram illustrating an example in which a backlight dimming method is applied to an image having a low APL.
4 is a flowchart illustrating a power consumption reduction method according to an embodiment of the present invention.
FIG. 5 is a graph that defines the gradation k of the pixel data.
6 is a graph showing the duty of the PWM for controlling the backlight luminance.
FIG. 7 is a diagram showing APL and IQBL for the image shown in FIG.
8 is a diagram illustrating an example in which the duty ratio of PWM is differently applied in one block in the local dimming method according to the embodiment of the present invention.
9 is a view illustrating a liquid crystal display device according to an embodiment of the present invention.
10 is a block diagram showing the power consumption reduction apparatus shown in FIG.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Like reference numerals throughout the specification denote substantially identical constituent elements. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

Referring to FIG. 2, the power consumption reduction method of the present invention receives input image data and calculates IQBL (Image Quality Brightness Level) for the image data (S1). IQBL is a new indicator that reflects the brightness perception of human beings, not simply the average brightness of one frame image, but the brightness of the viewer. IQBL has a value between 0 and 1, and expressed as a percentage, as shown in Equation 2 below. The IQBL is included in a lot of high gradation in images And has a high value . Therefore, the IQBL of an image with higher gradation levels in two images with the same APL is calculated to be a relatively higher value .

Where Gray is the gray scale and n is the number of pixels.

The power consumption reduction method of the present invention derives an optimal pixel gain based on IQBL and modulates pixel data by pixel gain (S3). The pixel gain can be calculated as IQBL and the k curve curve defined in Fig. The pixel gain is Gray _out in equation (3). k is the brightness compensation adjustment parameter of the entire image. k is optimized to a value that varies depending on the gradation of the pixel to reflect the human perception characteristic, and has a value between 0 and 1. The curve slope of the k-th curve is defined as follows.

k is defined along a nonlinear curve (k curve) that decreases as the gradation increases. k curve is the optimal k value in which the brightness of the image is increased in the cognitive image quality test and no other artifacts are generated. k curve may be implemented as a look-up table (LUT). On the other hand, k is not limited to Fig. The k curve can be changed depending on the type of display device, the driving characteristics, and the manufacturer.

5, the parameters of the k curve are defined as shown in Table 1 below. NP _LOW and NP _HIGH are the low and high gradation inflection points of the curve.

The power consumption reduction method of the present invention adjusts the backlight luminance based on IQBL (S4). The backlight luminance can be controlled by a PWM (Pulse Width Modulation) signal that defines the lighting ratio of the light source. The PWM signal has a duty ratio (%) that varies according to IQBL. The higher the duty ratio of the PWM signal is, the higher the lighting ratio of the backlight light source becomes, and the backlight luminance becomes higher. The brightness of the pixels is proportional to the brightness of the backlight source.

The duty ratio of the PWM signal is defined by a PWM curve as shown in FIG. The duty ratio of the PWM signal is determined according to the PWM curve that defines the duty ratio of the PWM and is selected to be higher as the IQBL increases. The minimum duty ratio of the PWM signal is at least 30% according to the offset value and increases along the concave curve at the low IQBL and increases along the convex curve at the high IQBL, reaching the maximum value of 100% at over 80% .

In FIG. 6, the parameters for controlling the backlight brightness of the IQBL in the PWM curve are defined as shown in Table 2.

The offset value is experimentally set to the minimum value of the backlight luminance which does not seriously deteriorate the image quality in the low gradation recognized by the observer. The offset value may vary depending on the driving characteristics of the liquid crystal display device and the light source characteristics of the backlight unit.

It should be noted that the present invention varies the duty ratio of the pixel gain and the PWM signal based on the IQBL reflecting the brightness information in the image in consideration of the brightness perception of the human being, rather than adjusting the backlight brightness based on the APL. APL can not be used as an index to show the difference in brightness that an observer perceives in an image, only calculating the average gradation of one frame image. On the other hand, IQBL is an index reflecting human's brightness perception because the brightness perceived by observers in two images with the same APL is higher in the stronger image.

Adjusting the backlight brightness based on the APL allows the backlight brightness to be adjusted with the same PWM signal for the two images having the same APL but different brightness distribution, so that the luminance difference experienced by the observer greatly degrades the image quality and power consumption can not be lowered significantly. On the contrary, the invention is based on the IQBL that is, to reflect the human brightness characteristic is that the gray level of the pixel The lower the pixel gain is, the lower the luminance of the backlight is reduced by the offset value so that the observer does not feel the decrease in luminance significantly, and the backlight luminance is minimized, thereby maximizing the power consumption reduction effect. This can be seen from the fact that the increasing and decreasing trends of the k curve of FIG. 5 and the PWM curve of FIG. 6 are opposite to each other. IQBL is higher for images with high gradation . The pixel gain is proportional to k, as shown in FIG. 5, where k is lower as IQBL is higher. On the other hand, the duty ratio of the PWM signal is proportional to IQBL along the nonlinear PWM curve . Therefore, since the backlight luminance is proportional to IQBL , the lower the pixel gain is, the lower the brightness is .

The power consumption reduction method of the present invention multiplies the pixel data of the input image by the pixel gain (Gray _out ), modulates the pixel data based on the IQBL, and writes the modulated pixel data to the pixels of the display panel to reproduce the input image on the display panel (S5). At the same time, the power consumption reduction method of the present invention adjusts the backlight luminance with the IQBL-based modulated PWM signal.

As shown in FIG. 1, the IQBL of the left image is 0.5 and the right image is 0.81, as shown in FIG. 7, when APL = 0.5 and the IQBL of the left image and the right image having the same gray level distribution are calculated. IQBL is calculated based on Equation (2) as follows.

The higher the IQBL, the greater the number of high gradations that human beings feel brightly in the image. Therefore, the IQBL can distinguish the presence and the level of the partially bright image from the image having the same APL. In the present invention, the backlight luminance is controlled based on the IQBL, so that the PWM duty in one image can be made different according to the brightness distribution of the image as shown in FIG.

In the local dimming method, the screen is virtually divided into blocks of matrix form, and the backlight luminance is controlled on a block-by-block basis according to the dimming value in each block. In the conventional local dimming method, since only one dimming value is calculated in one block, the backlight luminance within one block can not be changed. On the other hand, according to the present invention, since the PWM is selected based on the IQBL, the PWM duty ratio of the bright portion can be relatively increased even in one block as shown in FIG. 8, so that the brightness of the image can be expressed more finely And the power consumption can be further reduced.

As can be seen from the above, the IQBL can distinguish the presence and the level of the partially bright image in the image having the same APL. The power consumption reduction method of the present invention can increase the power consumption improvement effect of the liquid crystal display device without degrading the image quality by selecting the duty ratio of the pixel gain and the PWM based on the new concept index (IQBL) considering human brightness perception .

9 shows a liquid crystal display device according to an embodiment of the present invention.

9, the liquid crystal display device of the present invention includes a display panel 100, a display panel driver for writing pixel data of an input image to a pixel array of the display panel 100, And a backlight unit.

The display panel driver includes a data driver 102, a gate driver 104, a timing controller (TCON) 106, and the like.

A plurality of data lines (S1 to Sm) and a plurality of gate lines (G1 to Gn) are intersected in the pixel array of the display panel (100). The pixel array of the display panel 100 includes pixels arranged in a matrix form to display an input image. Each of the pixels includes an R subpixel, a G subpixel, and a B subpixel. Each of the pixels may further include a white (W) sub-pixel.

Each of the pixels includes a liquid crystal cell Clc, a storage capacitor Cst, a thin film transistor (TFT), and the like. The liquid crystal cell Clc delays the phase of light by using liquid crystal molecules driven by the electric field between the pixel electrode 1 to which the data voltage is applied through the TFT and the common electrode 2 to which the common voltage Vcom is applied Adjust the transmittance according to the data. The storage capacitor Cst holds the voltage of the liquid crystal cell Clc for one frame period. The TFT is turned on in response to the gate pulse? From the gate line 102 to supply the data voltage from the data lines S1 to Sm to the pixel electrode 1 of the liquid crystal cell Clc .

The liquid crystal display device may be implemented in any known liquid crystal mode such as TN (Twisted Nematic) mode, VA (Vertical Alignment) mode, IPS (In Plane Switching) mode, FFS (Fringe Field Switching) Further, the liquid crystal display device can be implemented in various forms such as a transmissive liquid crystal display device, a transflective liquid crystal display device, and a reflective liquid crystal display device. The transmissive liquid crystal display device or the transflective liquid crystal display device includes a backlight unit 200 and a light source driving unit 201.

The data driver 102 converts the pixel data received from the timing controller 110 into an analog gamma compensation voltage to generate a data voltage and outputs the data voltage to the data lines S1 to Sm. The pixel data input to the data driver 102 is digital video data of the input video.

The gate driver 104 supplies the scan lines 12 with a scan pulse (or gate pulse) synchronized with the output voltage of the data driver 102 under the control of the timing controller 110. The gate driver 104 sequentially shifts the scan pulses to sequentially select pixels to which data is written, in units of lines.

The timing controller 106 receives the pixel data of the input image from the host system 110 and the timing signals synchronized with the pixel data. The timing controller 106 selects the pixel gain and the PWM based on the index IQBL indicating the brightness perception characteristic of the human using the power consumption reduction apparatus shown in Fig.

The timing controller 106 transmits the pixel data (R'G'B ') modulated by the power consumption reduction device to the data driver 102 and transmits the PWM signal selected based on the IQBL to the light source driver 201. The power consumption reduction device may be incorporated in the timing controller 106. [

The timing controller 106 can synchronize the operation timings of the data driver 102, the gate driver 104, and the light source driver 201 based on the timing signals input in synchronization with the pixel data of the input image. The timing signals include a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, a data enable signal DE, and the like. The timing controller 106 generates a data timing control signal, a gate timing control signal, and a PWM signal based on the timing signals to synchronize the data driver 102, the gate driver 104, and the light source driver 201. The data timing control signal defines the operation timing and the output timing of the data composition unit 102. The gate timing control signal defines the operation timing and the output timing of the gate driver 104.

The host system 110 may be implemented as any one of a TV system, a set top box, a navigation system, a DVD player, a Blu-ray player, a personal computer (PC), a home theater system, and a phone system.

The power consumption reduction method shown in FIG. 4 can be implemented by hardware as shown in FIG.

10, the power consumption reduction apparatus includes a data receiving unit 72, an IQBL calculating unit 74, a gain selecting unit 76, a PWM selecting unit 78, a data modulating unit 80, and a data transmitting unit 82, .

The data receiving unit 72 receives the pixel data R, G, B of the input image and the timing signals Vsync, Hsync, DE from the host system 110. The pixel data is delayed by the calculation time of the IQBL calculating section 74 and the gain selecting section 76. [ The data receiver 72 delays the pixel data and the timing signals input to the data modulator 80 to synchronize the IQBL calculator 74, the gain selector 76 and the data modulator 80.

The IQBL calculator 74 calculates the IQBL of the pixel data based on Equation (2). The gain selector 76 selects a pixel gain based on IQBL as shown in Equation (3).

The PWM selector 78 selects a PWM duty ratio based on the IQBL received from the IQBL calculator 74 and outputs a PWM signal. The PWM selection unit 78 controls the backlight luminance of the display panel 100 according to the brightness distribution of the input image by controlling the light source of the backlight unit 200 with the PWM signal selected based on the IQBL.

The data modulator 80 multiplies the pixel data by the pixel gain to output the modulated pixel data R'G'B '. The data transmission unit 82 transmits the pixel data R'G'B 'and the data timing control signals Vsync_delay, Hsync_delay and DE_delay to the data driver 102.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification, but should be defined by the claims.

72: Data receiving unit 74: IQBL calculating unit
76: Gain selection unit 78: PWM selection unit
80: Data modulator 82: Data transmitter
100: display panel 102: data driver
104: Gate driver 106: Timing controller
110: host system 200: backlight unit
201: Light source driver

Claims (6)

A calculation unit for calculating brightness information of an input image as an index that defines brightness perceived by a human;
A gain selector for selecting a gain based on the index;
A PWM selection unit for selecting a duty ratio of PWM (Pulse Width Modulation) based on the index and outputting a PWM signal;
A data modulator for modulating the pixel data with the gain; And
And a display panel driver for writing the modulated pixel data to the pixels of the display panel to reproduce the input image on the display panel,
And the PWM selection unit controls the light source of the backlight unit with the PWM signal to control the backlight luminance of the display panel differently according to the brightness distribution of the input image. The method according to claim 1,
Wherein the index is IQBL defined by the following equation.

Here, Gray is a gray level and n is the number of pixels in one screen. 3. The method of claim 2,
Wherein the gain is Gray _out calculated by the following equation.

Here, k is a compensation parameter defined by a nonlinear curve that becomes lower as the gradation becomes higher. The method of claim 3,
Wherein the duty ratio of the PWM signal is proportional to the index, while k is lower as the index is higher. 5. The method of claim 4,
Wherein the backlight luminance is lowered as the gain is increased. Calculating brightness information of the input image as an index that defines brightness perceived by the human being;
Selecting a gain based on the indicator;
A PWM signal is generated by selecting a duty ratio of PWM (Pulse Width Modulation) based on the index, and the light source of the backlight unit is controlled by the PWM signal to control the backlight luminance of the display panel according to the brightness distribution of the input image step; And
And reproducing the input image on the display panel by modulating the pixel data with the gain and writing the modulated pixel data to the pixels of the display panel.

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