KR101644189B1 - Liquid crystal display and dimming controlling method of thereof - Google Patents

Abstract

The present invention relates to a liquid crystal display device capable of improving a contrast ratio of a display image and a dimming control method thereof.

The liquid crystal display device includes a liquid crystal display panel in which image data is displayed; A backlight unit for dividing a surface light source irradiated to the liquid crystal display panel, including a plurality of light sources, into blocks of a matrix type; The method includes analyzing the image data in units of blocks and determining a dimming value for each block so as to be proportional to a cumulative average value of the brightness of the image over a plurality of adjacent frames, A backlight control circuit for calculating a change amount of an image by comparing the corresponding neighboring previous frames with representative values for each block and for varying the number of accumulated frames based on the calculation result; And a backlight driving circuit for driving the light sources on a block-by-block basis by a pulse width modulation whose duty ratio is varied according to the dimming value for each block.

Description

TECHNICAL FIELD [0001] The present invention relates to a liquid crystal display (LCD) and a dimming control method thereof.

The present invention relates to a liquid crystal display device capable of improving a contrast ratio of a display image and a dimming control method thereof.

BACKGROUND ART [0002] Liquid crystal display devices are becoming increasingly widespread due to features such as light weight, thinness, and low power consumption driving. This liquid crystal display device is used as a portable computer such as a notebook PC, an office automation device, an audio / video device, and an indoor / outdoor advertisement display device. A liquid crystal display device displays an image using a thin film transistor (hereinafter referred to as "TFT") as a switching element. A transmissive liquid crystal display device that occupies most of the liquid crystal display device displays an image by controlling an electric field applied to the liquid crystal layer to modulate light incident from the backlight unit.

The image quality of the liquid crystal display device depends on the contrast characteristics. The method of modulating the light transmittance of the liquid crystal layer by controlling the data voltage applied to the liquid crystal layer limits the improvement of the contrast characteristic. In order to improve the contrast characteristic, an AFLC (Area Focused Luminance Cortrollable) technique for adjusting the luminance of the backlight unit according to an image has been proposed.

The AFLC technique divides an image displayed on one screen into a predetermined number of logical blocks, and adjusts the brightness of backlights (light sources) corresponding to the blocks according to an average brightness value to be displayed by a data signal corresponding to each block. The AFLC technique analyzes an image of one frame by a predetermined number of blocks to increase the backlight luminance of a block having a large average luminance value and decreases the backlight luminance of a block having a small average luminance value .

This AFLC technique can improve the contrast characteristic and power consumption by locally controlling the backlight luminance of each block in proportion to the brightness of the image outputted on the screen at every moment. However, when the brightness of the image changes frequently, fast backlight change Which causes flickering.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a liquid crystal display device and a dimming control method thereof that can improve a contrast ratio of a display image without causing flicker.

It is another object of the present invention to provide a liquid crystal display device and a dimming control method thereof that can improve a contrast ratio of a display image while improving a backlight response speed.

According to an aspect of the present invention, there is provided a liquid crystal display device including: a liquid crystal display panel displaying image data; A backlight unit for dividing a surface light source irradiated to the liquid crystal display panel, including a plurality of light sources, into blocks of a matrix type; The method includes analyzing the image data in units of blocks and determining a dimming value for each block so as to be proportional to a cumulative average value of the brightness of the image over a plurality of adjacent frames, A backlight control circuit for calculating a change amount of an image by comparing the corresponding neighboring previous frames with representative values for each block and for varying the number of accumulated frames based on the calculation result; And a backlight driving circuit for driving the light sources on a block-by-block basis by a pulse width modulation whose duty ratio is varied according to the dimming value for each block.

The backlight control circuit reduces the cumulative frame number if the amount of change of the image is larger than a predetermined threshold value and increases the cumulative frame number when the amount of change of the image is smaller than the threshold value.

Wherein the backlight control circuit comprises: an APL calculation unit for analyzing the image data on a block-by-block basis and calculating an APL value for each block for the corresponding frame; An APL storage unit for storing APL values for each block for each frame, together with APL values for each block of previous frames neighboring the corresponding frame; A variation amount calculation unit for comparing the APL value of each block with respect to the corresponding frame with APL values of previous frames corresponding to each position and calculating a variation amount of each block based on the comparison result; A tap length adjuster for adjusting the tap length of the MAF indicating the number of the accumulated frames to be applied to each block based on the variation amount of the block-by-block image; And a controller for referring to the adjusted tap length and the APL values of the previous frame adjacent to the current frame and accumulating the number of frames corresponding to the adjusted tap length, And a dimming value determining unit for determining the dimming value.

The tap length adjuster reduces the tap length if the change amount of the image is larger than a predetermined threshold value and increases the tap length if the change amount of the image is smaller than the threshold value.

The threshold value is set to a larger value than the fluctuation of the APL caused by the flicker.

The maximum value for the tap length is preset to the smallest value among the values determined by the multiplier of 2 within the range capable of effectively removing the flicker.

A liquid crystal display panel in which image data is displayed according to an embodiment of the present invention; a liquid crystal display panel including a plurality of light sources and having a backlight unit for dividing a surface light source irradiated on the liquid crystal display panel into a matrix- The control method may include analyzing the image data on a block-by-block basis and calculating an APL value for each block for each frame; Storing APL values for each block for each frame, together with APL values for each block of previous frames neighboring the corresponding frame; Comparing the APL value for each frame with the APL values of previous frames corresponding to each position, and calculating a change amount of each block based on the comparison result; Adjusting a tap length of the MAF indicating the number of accumulated frames to be applied to each block based on the variation amount of the image per block; And a controller for referring to the adjusted tap length and the APL values of the previous frame adjacent to the current frame and accumulating the number of frames corresponding to the adjusted tap length, As a dimming value.

A liquid crystal display device and a dimming control method thereof according to the present invention analyze input image data block by block and determine a dimming value for each block so as to be proportional to a cumulative average value of brightnesses of blocks over adjacent frames, The representative value of each frame of the frame based on the comparison result is compared with representative values of neighboring previous frames corresponding to each position to calculate a change amount of the image and the number of accumulated frames is varied based on the calculation result. That is, if the amount of change in the image is large, the number of accumulated frames is reduced to improve the backlight response speed, and if the amount of change in the image is small, the number of frames accumulated for flicker prevention is increased. As a result, the liquid crystal display device and the dimming control method thereof according to the present invention can improve the contrast ratio of the display image while speeding up the backlight response speed without causing flicker.

Hereinafter, preferred embodiments of the present invention will be described with reference to FIGS. 1 to 11. FIG.

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

1, a liquid crystal display according to an exemplary embodiment of the present invention includes a liquid crystal display panel 10, a data driving circuit 12 for driving data lines DL of the liquid crystal display panel 10, A gate driving circuit 13 for driving the gate lines GL of the panel 10, a timing controller 11 for controlling the data driving circuit 12 and the gate driving circuit 13, a liquid crystal display panel 10, A backlight driving circuit 15 for driving the light sources of the backlight unit 16 and a backlight unit 15 for analyzing the input image and controlling the backlight driving circuit 15 according to the analysis result, And a control circuit (14).

The liquid crystal display panel 10 includes two glass substrates and a liquid crystal layer formed therebetween. A plurality of data lines DL and a plurality of gate lines GL are intersected with each other on a lower glass substrate of the liquid crystal display panel 10. [ The liquid crystal cells Clc are arranged in a matrix form in the liquid crystal display panel 10 by the intersection structure of the data lines DL and the gate lines GL. A TFT, a pixel electrode 1 of a liquid crystal cell Clc connected to the TFT, and a storage capacitor Cst are formed on the lower glass substrate of the liquid crystal display panel 10.

On the upper glass substrate of the liquid crystal display panel 10, a black matrix, a color filter, and a common electrode 2 are formed. The common electrode 2 is formed on an upper glass substrate in a vertical electric field driving mode such as a TN (Twisted Nematic) mode and a VA (Vertical Alignment) mode. The common electrode 2 is formed of an IPS (In Plane Switching) mode, an FFS (Fringe Field Switching) Is formed on the lower glass substrate together with the pixel electrode 1 in the same horizontal electric field driving system. On the upper glass substrate and the lower glass substrate of the liquid crystal display panel 10, a polarizing plate is attached and an alignment film for forming a pre-tilt angle of the liquid crystal is formed on the inner surface in contact with the liquid crystal.

The data driving circuit 12 includes a plurality of data drive ICs. The data drive IC includes a shift register for sampling a clock signal, a register for temporarily storing the digital image data, a data register for storing one line of data in response to a clock signal from the shift register, A digital / analog converter for selecting a positive / negative gamma voltage under reference to a gamma reference voltage corresponding to a digital data value from a latch and a latch for supplying a positive / negative gamma voltage, And an output buffer connected between the multiplexer and the data line DL, and the like. The data driving circuit 12 latches the digital image data RGB under the control of the timing controller 11 and outputs the latched digital image data RGB to the positive polarity / And then supplies them to the data lines DL.

The gate drive circuit 13 includes a plurality of gate drive integrated circuits. The gate drive integrated circuit includes a shift register, a level shifter for converting an output signal of the shift register into a swing width suitable for TFT driving of the liquid crystal cell, and an output buffer. The gate drive circuit 13 sequentially outputs a scan pulse (or a gate pulse) under the control of the timing controller 11 and supplies it to the gate lines GL to select a horizontal line to which the data voltage is to be applied.

The timing controller 11 rearranges the digital image data RGB input from the system board on which the external video source is mounted according to the resolution of the liquid crystal display panel 10 and supplies it to the backlight control circuit 14. [ Then, digital image data (RGB) fed back from the backlight control circuit 14 is supplied to the data driving circuit 12.

The timing controller 11 generates timing control signals for controlling the operation timings of the data driving circuit 12 and the gate driving circuit 13 based on the timing signals Vsync, Hsync, DE, DCLK from the system board DDC, GDC). The timing controller 11 inserts an interpolation frame between the frames of the input video signal input at a frame frequency of 60 Hz and multiplies the data timing control signal DDC and the gate timing control signal GDC to obtain 60 × N The operation of the data driving circuit 12 and the gate driving circuit 13 can be controlled with a frame frequency of Hz or more.

The backlight unit 16 includes a plurality of light sources, and divides the surface light source irradiated on the liquid crystal display panel 10 into blocks in the form of a matrix. The backlight unit 16 may be implemented as either a direct type or an edge type. Although the backlight unit shown in Fig. 1 exemplifies a direct-type backlight unit, the backlight unit of the present invention is not limited to the direct-type backlight unit but may include any known backlight unit. The direct-type backlight unit 16 has a structure in which a plurality of optical sheets and a diffusion plate are stacked under the liquid crystal display panel 10, and a plurality of light sources are disposed under the diffusion plate. The light sources may be realized by point light sources such as a light emitting diode (LED). The optical sheets include at least one prism sheet and at least one diffusing sheet to diffuse the light incident from the diffusing plate and refract the path of light at an angle substantially perpendicular to the light incident surface of the liquid crystal display panel.

The backlight control circuit 14 outputs digital image data RGB of one frame to a predetermined number of logical units such as a plurality of blocks BLK [1,1] to BLK [ n, m]). The backlight control circuit 14 analyzes the input image data RGB on a block basis and determines the dimming value BLdim for each block so as to be proportional to the cumulative average value of the brightness of the image over the adjacent several frames. In particular, the backlight control circuit 14 compares the block-by-block representative value of the frame based on the analysis result with the block-by-block representative values of neighboring previous frames corresponding to each position in determining the dimming value (BLdim) And calculates a variation amount of the image, and varies the number of frames accumulated based on the calculation result. That is, if the amount of change in the image is large, the number of frames accumulated for improving the backlight response speed is reduced, and if the amount of change in the image is small, the number of frames accumulated for preventing flickering is increased.

The backlight driving circuit 15 converts the light sources of the backlight unit 16 into a block unit by pulse width modulation (PWM) in which the duty ratio is varied in accordance with the block-dependent dimming value BLdim input from the backlight control circuit 14. [ . The lighting time of the light sources is controlled according to the PWM duty ratio.

3 shows the backlight control circuit 14 in detail.

3, the backlight control circuit 14 includes an APL calculating unit 141, an APL storing unit 142, a variation amount calculating unit 143, a tap length adjusting unit 144, and a dimming value determining unit 145. [ Respectively.

The APL calculating unit 141 receives the digital image data RGB in units of frames and divides the digital image data RGB of one frame into a matrix form on the display screen of the liquid crystal display panel 10 as shown in FIG. And calculates a representative value for each block by analyzing in units of virtual blocks BLK [1,1] to BLK [n, m]. The representative value for each block may be taken as an APL (Average Picture Level) value obtained by deriving the maximum gradation value among the RGB values of the pixels included in each block and dividing the sum of the maximum values by the number of pixels included in the block.

The APL storage unit 142 stores the APL values for each block of the corresponding frame input from the APL calculation unit 141 together with the APL values of the previous frames neighboring the corresponding frame.

The variation calculating unit 143 compares the APL value of the corresponding frame for each block input from the APL calculating unit 141 with the APL values of previous frames inputted from the APL storing unit 142, Quot;

The tap length adjusting unit 144 adjusts the tap length of the moving average filtering (MAF) to be applied to each block of the frame based on the amount of change of the image for each block inputted from the change amount calculating unit 143. [ The MAF is a technique for adjusting the brightness of the backlight in proportion to the cumulative average value of image brightness determined by the image data signal output from a plurality of adjacent frames. For example, assuming that the brightness of the image of the X position block of the Tth frame is P (T, X) and the brightness of the backlight of the X position block of the Tth frame is L (T, X) (T, X) is determined by accumulating K + 1 frames of T (T, X), P (T-1, X), ..., P K + 1 is the MAF technique. According to the MAF technique, since the luminance of the backlight is adjusted based on the accumulation of image brightness of a plurality of adjacent frames, even though the image data is shaken due to frequent changes of the image, only the luminance of the backlight And no change is seen. As a result, the flicker caused by frequent changes of the brightness of the backlight is eliminated. The tap length means the cumulative number of frames. The tap length adjuster 144 eliminates the flicker by increasing the tap length, that is, the cumulative number of frames, if the amount of change of the image per block is smaller than a predetermined threshold value. On the other hand, if the amount of change of the image per block is larger than a predetermined threshold value, the tap length, i.e., the cumulative number of frames is reduced, thereby improving the response speed of the backlight in response to a sudden change in the image. Here, the threshold value is set to a larger value than the fluctuation of the APL caused by the flicker. According to the measurement results of various kinds of images, the flicker caused by the flicker did not exceed 5 in the range of 0 to 255 gradations in most cases. Also, in almost all screen switching (rapid change of image), APL changed to a value larger than 5. Therefore, in the following examples, the threshold is set to 5. In the adaptive operation of the MAF, the tap length of the filter can not be increased indefinitely due to the capacity of the memory, and the maximum value must be set in advance. The maximum value for the tap length is sufficient to effectively remove the flicker, for example, a value of about 10 is sufficient. However, in the actual implementation, since the memory designated for the maximum value is most efficiently determined by the multiplier of 2, in the following examples, the maximum value of 16, which is the smallest power of 2, is set. Hereinafter, the operation of the tap length adjusting unit 144 according to an image will be described.

The tap length adjusting unit 144 adjusts the tap length of the MAF to be applied to each block of the corresponding frame in response to a case where most of the change amount of each block image is smaller than a predetermined threshold value 5 By adjusting the tab length to be near the predetermined maximum value 16 as shown in Fig. 6, flicker is prevented by preventing frequent changes in luminance of the backlight.

The tap length adjusting unit 144 adjusts the tap length of the MAF to be applied to each block of the corresponding frame in correspondence with the portion where the amount of change of the image per block in FIG. 7 is smaller than a predetermined threshold value 5 As shown in FIG. 8, to a predetermined maximum value 16, thereby preventing frequent changes in brightness of the backlight, thereby eliminating flicker. 7, the tap length adjuster 144 adjusts the tap length of the corresponding frame (before and after the fortieth frame) in response to a portion (a sharp change in the image) that is much larger than the predetermined threshold value 5 The tap length of the MAF to be applied to each block is adjusted to a level (2 to 3) much lower than a predetermined maximum value 16 as shown in FIG. 8, thereby improving the response speed of the backlight in response to a sudden change in the image.

The tap length adjusting unit 144 adjusts the tap length of the corresponding frame (before and after the 20 < th > frame and the 80 < th > frame 10) by adjusting the tap length of the MAF to be applied to each block of the backlight (for example, front and rear) to a predetermined maximum value 16 as shown in FIG. 10, thereby preventing frequent changes in brightness of the backlight and eliminating flicker. 9, when the variation amount of an image per block includes a portion larger than a predetermined threshold value 5 (in the case where a fast motion of an object exists in the image), the tap length adjusting unit 144 adjusts (4 to 12) lower than a predetermined maximum value 16 as shown in FIG. 10 by adjusting the tap length of the MAF to be applied to each block of the first to fourth (40th to 60th) frames, Improve response speed.

The dimming value determining unit 145 refers to the tap length determined by the tap length adjusting unit 144 and the APL values of the corresponding frame of the previous frame neighboring the corresponding frame stored in the APL storing unit 142, A value obtained by accumulating the corresponding number of frames and taking an average is determined as a dimming value (BLdim) for each block of the corresponding frame.

FIG. 11 shows a dimming control method of a liquid crystal display according to an embodiment of the present invention.

Referring to FIG. 11, the dimming control method receives digital image data on a frame basis, analyzes digital image data of one frame on a display screen of a liquid crystal display panel in units of virtual blocks divided into a matrix form, And calculates a star APL value (S10, S20)

The dimming control method stores the APL values of each frame for each frame together with the APL values of the previous frames neighboring the corresponding frame (S30).

In this dimming control method, the APL value of the corresponding frame for each block is compared with the APL values of the previous frames corresponding to each position, and the amount of change of the image for each block is calculated based on the comparison result (S40)

The dimming control method compares the variation amount of each block image with a predetermined threshold value at step S50 and adjusts the tap length of MAF to be applied to each block according to the comparison result. The tap length means the cumulative number of frames. As a result of the comparison, if the change amount of the image per block is larger than the predetermined threshold (Yes), the dimming control method improves the response speed of the backlight in response to the abrupt change of the image by reducing the tap length, that is, the cumulative number of frames. (S60). On the other hand, if the variation amount of the image per block is smaller than the predetermined threshold (No), the dimming control method eliminates the flicker by increasing the tap length, that is, the frame accumulation number (S70) Can be predetermined to be greater than the flicker of APL due to flicker. Further, the maximum value for the tap length can be preset to the smallest value among the values determined by the multiplier of 2 within the range in which the flicker can be effectively removed.

The dimming control method refers to the determined tap length and the APL values of the previous frames adjacent to the stored corresponding frame and accumulates the number of frames corresponding to the determined tap length so as to obtain an average Value as the dimming value for each block of the frame (S80, S90)

As described above, in the liquid crystal display and dimming control method thereof according to the present invention, the input image data is analyzed on a block-by-block basis, and a dimming value for each block is determined so as to be proportional to a cumulative average value of brightnesses of blocks over adjacent frames A representative value of each block of the frame based on the analysis result is compared with representative values of neighboring previous frames corresponding to each position to calculate a change amount of the image and the number of frames accumulated based on the calculation result . That is, if the amount of change in the image is large, the number of accumulated frames is reduced to improve the backlight response speed, and if the amount of change in the image is small, the number of frames accumulated for flicker prevention is increased. As a result, the liquid crystal display device and the dimming control method thereof according to the present invention can improve the contrast ratio of the display image while speeding up the backlight response speed without causing flicker.

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.

1 is a view showing a liquid crystal display device according to an embodiment of the present invention.

2 is a diagram showing an example in which a predetermined number of image data of one frame is divided in accordance with the number of division of a surface light source.

3 is a block diagram showing a backlight control circuit in detail;

Figure 4 shows a MAF technique.

FIGS. 5 and 6 are diagrams illustrating the results of applying the MAF technique of the present invention in the case where there is no abrupt change in the image.

FIGS. 7 and 8 are diagrams showing a result of applying the MAF technique of the present invention in the case of a sudden change of an image. FIG.

FIGS. 9 and 10 are diagrams showing a result of applying the MAF technique of the present invention when fast motion of an object exists in an image. FIG.

11 is a diagram illustrating a dimming control method of a liquid crystal display according to an embodiment of the present invention.

Description of the Related Art

10: liquid crystal display panel 11: timing controller

12: data driving circuit 13: gate driving circuit

14: light source control circuit 15: light source driving circuit

16: backlight unit 141: APL calculation unit

142: APL storage unit 143:

144: tap length adjusting unit 145: dimming value determining unit

Claims (10)

A liquid crystal display panel displaying image data; A backlight unit for dividing a surface light source irradiated to the liquid crystal display panel, including a plurality of light sources, into blocks of a matrix type; The method includes analyzing the image data in units of blocks and determining a dimming value for each block so as to be proportional to a cumulative average value of the brightness of the image over a plurality of adjacent frames, A backlight control circuit for calculating a change amount of an image by comparing the corresponding neighboring previous frames with representative values for each block and for varying the number of accumulated frames based on the calculation result; And And a backlight driving circuit for driving the light sources on a block-by-block basis by a pulse width modulation whose duty ratio is varied according to the dimming value for each block. The method according to claim 1, Wherein the backlight control circuit reduces the cumulative frame number if the variation amount of the image is larger than a predetermined threshold value and increases the cumulative frame number when the variation amount of the image is smaller than the threshold value. The method according to claim 1, The backlight control circuit comprising: An APL calculation unit for analyzing the image data on a block-by-block basis and calculating an APL (Average Picture Level) value for each frame; An APL storage unit for storing APL values for each block for each frame, together with APL values for each block of previous frames neighboring the corresponding frame; A variation amount calculation unit for comparing the APL value of each block with respect to the corresponding frame with APL values of previous frames corresponding to each position and calculating a variation amount of each block based on the comparison result; A tap length adjuster for adjusting a tap length of MAF (Moving Average Filtering) to be applied to each block based on a change amount of the block-by-block image; And And a value obtained by averaging the number of frames corresponding to the adjusted tap length by referring to the adjusted tap length and the APL values of the previous frame neighboring to the current frame, And a dimming value deciding section for deciding the dimming value as a value. The method of claim 3, Wherein the tap length adjusting unit comprises: Wherein the controller decreases the tap length if the change amount of the image is larger than a predetermined threshold value and increases the tap length if the change amount of the image is smaller than the threshold value. 5. The method of claim 4, Wherein the threshold value is set to a larger value than a fluctuation of the APL caused by the flicker. 5. The method of claim 4, Wherein the maximum value of the tap length is preset to a smallest value among the values determined by the multiplier of 2 within a range capable of effectively removing the flicker. 1. A dimming control method for a liquid crystal display device having a liquid crystal display panel in which image data is displayed and a backlight unit for dividing a surface light source irradiating the liquid crystal display panel including a plurality of light sources into blocks of a matrix type, Analyzing the image data on a block-by-block basis and calculating an APL (Average Picture Level) value for each block; Storing APL values for each block for each frame, together with APL values for each block of previous frames neighboring the corresponding frame; Comparing the APL value for each frame with the APL values of previous frames corresponding to each position, and calculating a change amount of each block based on the comparison result; Adjusting a tap length of MAF (Moving Average Filtering) to be applied to each block based on the variation amount of the block-by-block image; And And a value obtained by averaging the number of frames corresponding to the adjusted tap length by referring to the adjusted tap length and the APL values of the previous frame neighboring to the current frame, And a step of determining a value of the second threshold value. 8. The method of claim 7, Wherein the adjusting the tap length comprises: And decreasing the tap length if the amount of change of the image is greater than a predetermined threshold value and increasing the tap length if the amount of change of the image is less than the threshold value. 9. The method of claim 8, Wherein the threshold value is set to a larger value than a shake of the APL caused by the flicker. 8. The method of claim 7, Wherein the maximum value of the tap length is preset to a smallest value among values determined as a multiplier of 2 within a range capable of effectively removing flicker.

Images