KR101633114B1 - Liquid crystal display and picture quality controlling method thereof - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device capable of improving moving picture response characteristics and a picture quality control method thereof.

The liquid crystal display device includes a liquid crystal display panel on which modulation data is displayed; A timing controller for dividing one frame period into a first sub frame period and a second sub frame period; A backlight including at least one light source for irradiating light to the liquid crystal display panel; And the input data is analyzed to derive a frame representative value, the on-time duty of the light sources is adjusted based on the frame representative value, and the input data is converted into the modulation data And an image quality control circuit for adjusting the image quality.

Description

TECHNICAL FIELD [0001] The present invention relates to a liquid crystal display (LCD)

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a liquid crystal display device capable of improving a moving picture response time (hereinafter referred to as "MPRT") performance and a picture quality control method thereof.

A liquid crystal display device of an active matrix driving type displays a moving picture by using a thin film transistor (hereinafter referred to as "TFT") as a switching element. This liquid crystal display device can be thinner and finer than a cathode ray tube (hereinafter, referred to as "CRT ") and is applied to a display device in a portable information device, office equipment, .

In the liquid crystal display device, a blurring phenomenon in which a screen is not clear and blurry appears in a moving image due to the retention characteristics of a liquid crystal. The CRT emits phosphors only for a very short time as shown in FIG. 1 to display data on a cell, and then displays an image in impulse driving without light emission in the cell. 2, the liquid crystal display device displays an image in a hold driving mode in which data charged in the liquid crystal cell is held for the remaining field period (or frame period) after data is supplied to the liquid crystal cell during the scanning period .

Since the moving picture displayed on the CRT is displayed by impulse driving, the perceived image of the viewer is sharpened as shown in FIG. On the other hand, in the liquid crystal display device, due to the retention characteristic of the liquid crystal in the moving image, the contrast of the perception image felt by the spectator is blurred as shown in Fig. The difference of these perceptual images is due to the integration effect of the images which are temporally continuous in the eye following the movement. Therefore, even if the response speed of the liquid crystal display device is high, the MPRT performance is degraded due to mismatch between the eye movement and the static image of each frame. In order to improve the motion blur phenomenon, there is known a black data insertion technique in which video data is displayed on a screen and then black data is supplied to the screen to drive the liquid crystal display device by sub-impulse driving.

The black data insertion technique is a technique in which one frame period of the screen is time-divided into a first sub frame period (SF1) and a second sub frame period (SF2) as shown in Fig. 5, and digital video data RGB ) Is sequentially displayed on the screen and then black data BD is inserted during the second sub frame period SF2 and sequentially displayed on the screen.

However, in the black data insertion technique, as the black data (BD) is inserted, the brightness is lowered and the recall rate of the image is greatly reduced. Further, since the bright image is displayed during the first sub frame period SF1 and the dark image is displayed during the second sub frame period SF2, the bright image and the dark image are periodically repeated and the flicker is severe.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a liquid crystal display and its image quality control method capable of improving MPRT performance while minimizing luminance or flicker.

According to an aspect of the present invention, there is provided a liquid crystal display device including: a liquid crystal display panel displaying modulation data; A timing controller for dividing one frame period into a first sub frame period and a second sub frame period; A backlight including at least one light source for irradiating light to the liquid crystal display panel; And the input data is analyzed to derive a frame representative value, the on-time duty of the light sources is adjusted based on the frame representative value, and the input data is converted into the modulation data And an image quality control circuit for adjusting the image quality.

Wherein the image quality control circuit comprises: an image analysis unit for analyzing the input data on a frame-by-frame basis for an analysis area of a predetermined size; A representative value deriving unit for deriving the maximum gray level value in the analysis area as the frame representative value; A duty controller for adjusting the duty of the light sources to a maximum during the second sub frame period and adjusting the duty of the light sources according to the maximum gray level value during the first sub frame period; And a data modulator for adjusting the input data to the modulated data in consideration of the maximum gray level value and the adjusted duty.

Wherein the duty controller controls the duty of the light sources to be 0% so that the light sources are extinguished during the first sub frame period if the maximum gradation value is less than a predetermined halftone value, Controlling the duty of the light sources to be 50%; And controlling the duty of the light sources to be 50% so that the light sources are fully turned on during the second sub frame period if the maximum tone value exceeds the halftone value, The duty of the light sources is controlled to be C (C? 50)% so as to be lit up for a period corresponding to the luminance difference between the light sources.

The duty (C%) of the light sources is controlled to increase in proportion to the difference in luminance between the halftone value and the maximum gradation value.

Wherein the data modulator modulates data input to the analysis area into black data and outputs the modulated data during the first sub frame period if the maximum gray value is less than the halftone value, % With the data adjusted up to exhibit the same luminance value as the original luminance value (luminance value at duty% 100%) and outputs during the second sub frame period; (50 + C) < / RTI >% if the maximum grayscale value exceeds a halftone value, and outputs data during the second sub frame period by modulating data input to the analysis area into white data, Modulated with the data adjusted up to exhibit the same luminance value as the original luminance value (luminance value at duty% 100%) and output during the first sub frame period.

The analysis area is set to the entire screen or to each block; The duty of the light sources is adjusted in units of the analysis area.

A timing controller for dividing one frame period into a first sub frame period and a second sub frame period, and a timing controller including one or more light sources to control the liquid crystal display panel A method of controlling image quality of a liquid crystal display device having a backlight for irradiating light includes the steps of: (A) analyzing the input data on a frame-by-frame basis for an analysis area of a predetermined size; (B) deriving a maximum gradation value in the analysis area; (C) adjusting the duty of the light sources to a maximum during the second sub-frame period and adjusting the duty of the light sources according to the maximum gray-scale value during the first sub-frame period; And adjusting (D) adjusting the input data to the modulation data in consideration of the maximum tone value and the adjusted duty.

The liquid crystal display device and the image quality control method thereof according to the present invention improve the MPRT performance by alternately outputting the black data and the gray data because the luminance lowering and the flicker are not conspicuously visually observed in the low gray-scale image below the halftone value. In this low-grayscale image, the backlight power consumption is reduced to less than half of the conventional one because the light sources are controlled with a duty of 50%.

Further, in the liquid crystal display device and its image quality control method according to the present invention, the gray level data and the white data are alternately outputted because the brightness decrease and the flicker can be conspicuously conspicuous in the high gray level image exceeding the intermediate gray level value. In addition, by controlling the light sources with a duty of (50 + C)%, the backlight is extinguished in a part of the period in which gray data is displayed, thereby greatly improving MPRT performance while reducing brightness and flicker, and reducing backlight power consumption.

Hereinafter, preferred embodiments of the present invention will be described with reference to FIGS. 6 to 11. FIG.

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

6, a liquid crystal display according to an embodiment of the present invention includes a liquid crystal display panel 10, a timing controller 11, a data driving circuit 12, a gate driving circuit 13, an image quality control circuit 14, A light source driving circuit 15, and a backlight 16. The data driving circuit 12 and the gate driving circuit 13 constitute a panel driving circuit.

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 data, a latch for storing data for one line 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 the digital data value from the latch and generating a data voltage of positive / negative polarity using the selected gamma voltage, A multiplexer for selecting a data line to which the polarity data voltage is supplied, and an output buffer connected between the multiplexer and the data line. The data driving circuit 12 latches the modulated data R'G'B 'under the control of the timing controller 11 and outputs the latched data R'G'B' to the positive / negative gamma compensation voltage To analog data voltages of positive polarity / negative polarity and supplies them to the data lines DL. The operation of the data driving circuit 12 is performed twice for each frame.

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 scan pulses (or gate pulses) under the control of the timing controller 11 and supplies the scan pulses (or gate pulses) to the gate lines GL. The operation of the gate drive circuit 13 is performed twice for each frame.

The timing controller 11 generates control signals for controlling operation timings of the data driving circuit 12 and the gate driving circuit 13 based on timing signals (Vsync, Hsync, DE, DCLK) from an external system board (DDC, GDC). The timing controller 11 multiplies the data control signal DDC and the gate control signal GDC and multiplies the data driving circuit 12 with a frame frequency of 120 x N (where N is a positive integer of 2 or more) Hz, ) And the gate drive circuit (13). The timing controller 11 time-divides one frame period into a first sub-frame period and a second sub-frame period, and outputs the modulated data R'G'B 'input from the image quality control circuit 14 to the multiplied frame frequency And supplies them to the data driving circuit 12 in synchronization.

The backlight 16 includes one or more light sources to illuminate the liquid crystal display panel 10 with light. The backlight 16 may be implemented as either a direct type or an edge type. The direct-type backlight 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 edge type backlight 16 has a structure in which a plurality of optical sheets and a light guide plate are stacked under the liquid crystal display panel 10 and a plurality of light sources are disposed on a side surface of the light guide plate. The light sources may be implemented as light sources such as a cold cathode fluorescent lamp (CCFL) and an external electrode fluorescent lamp (EEFL), and may also be implemented as a light emitting diode (LED) Point light sources.

The light source driving circuit 15 drives the light sources in response to the light source control signal LCS input from the picture quality control circuit 14. [

The image quality control circuit 14 derives the frame representative value based on the analysis result of the input data RGB and generates the light source control signal LCS based on the frame representative value to control the on time duty of the light sources And modulates the input data (RGB) in consideration of the frame representative value and the adjusted duty.

The picture quality control circuit 14 of Fig. 7 is shown.

7, the picture quality control circuit 14 includes an image analyzing unit 141, a representative value deriving unit 142, a duty adjusting unit 143, and a data modulating unit 144.

The image analysis unit 141 analyzes the input data RGB on a frame basis with respect to the analysis region (s) having a predetermined size and outputs the analysis result to the representative value derivation unit 142. Here, the analysis area may be set to the entire screen (when implementing global dimming) or may be set for each block (when implementing local dimming).

The representative value derivation unit 142 derives a frame representative value based on the analysis result of the input data (RGB). The frame representative value indicates the maximum gradation value (B) in the analysis area. The threshold value derivation unit 142 derives the highest gray level value among the gray level values satisfying the specific threshold value in the analysis area as the maximum gray level value (B). The specific threshold value can be determined differently considering the number of pixel data having each gradation value and / or the distribution density of each gradation value.

The duty adjusting unit 143 compares the maximum gradation value B input from the representative value deriving unit 142 with a preset intermediate gradation value A. The halftone value A indicates a tone value that exhibits half the luminance of the peak white gradation value while driving the light sources with an on-time duty of 100%. For example, when the peak white gradation value is 255 gray, the halftone value A may be set to 160 gray.

If the maximum gradation value B is less than or equal to the intermediate gradation value A, the duty controller 143 determines that the light sources corresponding to the analysis region are off during the first sub frame period SF1 as shown in FIG. ) To 0%, and controls their duty to be 50% so as to be turned on (ON) during the second sub frame period SF2. Thus, the total duty of the light sources during one frame period is adjusted to 50%. For example, when the maximum gray scale value B of 80 gray is input corresponding to the gray scale value A of 160 gray as shown in FIG. 9, the duty controller 143 sets the duty of the light sources in the first sub frame period SF1, 0%, and 50% in the second sub frame period SF2.

If the maximum gray-level value B exceeds the intermediate gray-level value A, the duty controller 143 controls the light sources corresponding to the analysis area as shown in FIG. 8 to be turned on and off during the second sub-frame period SF2 (ON) for a period corresponding to the luminance difference between the intermediate gradation value A and the maximum gradation value B during the first sub-frame period SF1 so that their duty is controlled to 50% The duty is controlled to be C (C? 50)%. Thus, the total duty of the light sources during one frame period is adjusted to (50 + C)%. The duty (C%) of the light sources in the first sub frame period SF1 increases as the luminance difference between the intermediate gradation value A and the maximum gradation value B increases. For example, when the maximum gray scale value B of 200 gray is input corresponding to the gray scale value A of 160 gray as shown in FIG. 10A, the duty controller 143 sets the duty of the light sources to the second sub frame period SF2, And 20% in the first sub frame period SF1. The duty of 20% is the minimum value to realize the luminance difference between 200 gray and 160 gray. 10B, when the maximum gray scale value B of 240 gray is input corresponding to the gray scale value A of 160 gray, the duty controller 143 sets the duty of the light sources to the second sub frame period SF2, And 40% in the first sub frame period SF1. The duty of 40% is the minimum value to realize the luminance difference between 240 gray and 160 gray.

The data modulating section 144 compares the maximum gradation value B inputted from the representative value deriving section 142 with the intermediate gradation value A and considers the comparison result and the duty adjusted by the duty adjusting section 143 To modulate input data (RGB).

If the maximum gradation value B is less than or equal to the intermediate gradation value A, the data modulator 144 multiplies the data (RGB) input to the corresponding analysis region by the peak black gradation value 0 gray And outputs data (RGB) input to the analysis area in the first sub frame period SF1 with the luminance value (luminance value at the duty 100%) at the duty 50% And outputs the modulated data during the second sub frame period SF2. For example, when the maximum gray-level value B of 80 gray is input corresponding to the gray-scale value A of 160 gray as shown in FIG. 9, the data modulator 144 converts the black data of 0 gray into the modulation data R'G 'B') for the first sub frame period (SF1), and modulated data (R'G'B ') that is adjusted upward to exhibit the same luminance value as the original luminance value at a duty ratio of 50% And outputs during the second sub frame period SF2.

8, the data modulation section 144 converts the data (RGB) input to the analysis region into a peak white gradation value (255 gray), and outputs the peak gradation value (255 gray) (50 + C)% and the original luminance value (luminance value at a duty of 100%) at the duty (50 + C)%, ), And outputs the modulated data during the first sub frame period SF1. 10A, when the maximum gray scale value B of 200 gray is input corresponding to the gray scale value A of 160 gray, the data modulator 144 converts the white data of 255 gray to the modulation data R'G 'B') for the second sub frame period (SF2), and modulated data (R'G'B ') that is adjusted upward to exhibit the same luminance value as the original luminance value at duty ratio 70% And outputs it during the first sub frame period SF1. 10B, when the maximum gray-level value B of 240 gray is input in correspondence with the gray-level value A of 160 gray, the data modulator 144 converts the white data of 255 gray to the modulated data R'G 'B') for the second sub frame period (SF2), and modulated data (R'G'B ') that is adjusted upward to exhibit the same luminance value as the original luminance value at duty ratio 90% And outputs it during the first sub frame period SF1.

As described above, the image quality control circuit 14 outputs the black data and the gray data alternately so that the MPRT performance is greatly improved because the luminance reduction and the flicker are not conspicuously recognized in the low gradation image of the intermediate tone value A or less. Further, since the picture quality control circuit 14 controls the light sources with a duty of 50%, the backlight power consumption is reduced to 1/2 or less of the conventional value.

The image quality control circuit 14 alternately outputs gray data and white data because a luminance decrease and a flicker can be conspicuously conspicuous in a high gradation image exceeding the intermediate gradation value (A). The image quality control circuit 14 controls the light sources with a duty of (50 + C)% to turn off the backlight in a part of the period in which the gray data is displayed. Therefore, the MPRT performance is greatly improved while reducing the brightness and minimizing the flicker, Reduce power consumption compared to existing models.

11 illustrates a method of controlling image quality of a liquid crystal display according to an embodiment of the present invention.

11, the image quality control method analyzes the input data RGB on a frame-by-frame basis with respect to the analysis region (s) having a predetermined size (S10, S20). Here, Dimming implementation), or may be set on a block-by-block basis (in a local dimming implementation).

The image quality control method derives the maximum gradation value B in the analysis area based on the analysis result of the input data RGB. In step S30, the highest gradation value And derives the tone value as the maximum tone value (B). The specific threshold value can be determined differently considering the number of pixel data having each gradation value and / or the distribution density of each gradation value.

The image quality control method compares the derived maximum gradation value B with a preset intermediate gradation value A. (S40) Here, the intermediate gradation value A is obtained by driving the light sources at an on-time duty of 100% Means a tone value that exhibits half the luminance of the peak white tone value. For example, when the peak white gradation value is 255 gray, the halftone value A may be set to 160 gray.

If the maximum gradation value B is less than the intermediate gradation value A (Yes), the image quality control method determines that the light sources corresponding to the analysis region are turned off during the first sub frame period SF1 The duty is controlled to 0%, and their duty is controlled to 50% so as to be turned on (ON) during the second sub frame period SF2. The image quality control method modulates data (RGB) input to the analysis area into black data having a peak black gradation value (0 gray), outputs the data during the first sub frame period (SF1), inputs (RGB) with the data adjusted up to exhibit the same luminance value as the original luminance value (luminance value at duty% 100) at duty 50% and outputs during the second sub frame period SF2 S50)

If the maximum gray-level value B exceeds the intermediate gray-level value A (No), the image quality control method turns on the light sources corresponding to the analysis area in the second sub-frame period SF2, The duty is controlled to be 50% and the duty thereof is set to C% so as to be ON for a period corresponding to the luminance difference between the halftone value A and the maximum gradation value B during the first sub frame period SF1, . Thus, the total duty of the light sources during one frame period is adjusted to (50 + C)%. The duty (C%) of the light sources in the first sub frame period SF1 is adjusted in proportion to the luminance difference between the intermediate tone value A and the maximum tone value B. The image quality control method modulates data (RGB) input to the analysis region into white data having a peak white gradation value (255 gray), outputs the data during the second sub frame period (SF2), inputs (RGB) is modulated with the data adjusted up to exhibit the same luminance value as the original luminance value (luminance value at duty ratio 100%) at the duty (50 + C)% and is supplied during the first sub frame period SF1 (S60)

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.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a characteristic diagram showing a light emission characteristic of a cathode ray tube. FIG.

2 is a characteristic diagram showing the luminescence characteristics of a liquid crystal display device.

3 is a view showing a perception image of a cathode ray tube felt by a spectator.

4 is a view showing a perception image of a liquid crystal display device felt by a spectator.

5 is a diagram for explaining a conventional black data insertion technique.

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

7 is a detailed view of the picture quality control circuit of Fig.

8 is a diagram for explaining duty control and data modulation in an image quality control circuit;

9 is a diagram showing an example of the operation of the image quality control circuit when the maximum gradation value is lower than the intermediate gradation value.

10A and 10B are diagrams showing an example of the operation of the picture quality control circuit when the maximum gradation value exceeds the intermediate gradation value.

11 is a view illustrating a method of controlling image quality 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: picture quality control circuit 15: light source driving circuit

16: backlight 141: image analysis unit

142: Representative value derivation unit 143: Duty adjustment unit

144: Data modulation section

Claims (11)

A liquid crystal display panel displaying modulation data; A timing controller for dividing one frame period into a first sub frame period and a second sub frame period; A backlight including at least one light source for irradiating light to the liquid crystal display panel; And The input data is analyzed to derive a frame representative value, the on-time duty of the light sources is adjusted based on the frame representative value, and the input data is converted into the modulation data in consideration of the frame representative value and the adjusted duty And an image quality control circuit for adjusting the image quality, Wherein the image quality control circuit derives a maximum gradation value in an analysis area of a predetermined size with respect to the input data as the frame representative value, and when the maximum gradation value exceeds a preset intermediate gradation value, The duty of the light sources is controlled to 50% so as to be turned on during the frame period, the data input to the analysis area is modulated into white data, and the light is output during the second sub frame period, (50 < = C)% < / RTI > by controlling the duty of the light sources to be C (C50%) so as to be turned on for a period corresponding to the luminance difference between the halftone value and the maximum gradation value, Modulated with gray data upwardly adjusted so as to exhibit the same luminance value as the original luminance value (luminance value at duty 100%), A liquid crystal display device for outputting probe frame period. The method according to claim 1, Wherein the picture quality control circuit comprises: An image analyzer for analyzing the input data on a frame-by-frame basis with respect to the analysis area; A representative value deriving unit for deriving the maximum gray level value in the analysis area as the frame representative value; A duty controller for adjusting the duty of the light sources to a maximum during the second sub frame period and adjusting the duty of the light sources according to the maximum gray level value during the first sub frame period; And And a data modulation unit for adjusting the input data to the modulated data in consideration of the maximum gray level value and the adjusted duty. 3. The method of claim 2, The duty controller may include: The duty of the light sources is controlled to 0% so that the light sources are extinguished during the first sub frame period if the maximum gradation value is less than a predetermined halftone value, To 50%. The method of claim 3, Wherein a duty (C%) of the light sources is controlled so as to increase in proportion to a luminance difference between the halftone value and the maximum gradation value. The method of claim 3, Wherein the data modulator comprises: Modulating the data input to the analysis area into black data and outputting the data during the first sub frame period if the maximum gradation value is less than the halftone value, Modulated with data adjusted to exhibit a luminance value equal to a value (a luminance value at a duty of 100%), and outputs the modulated data during the second sub frame period. 3. The method of claim 2, The analysis area is set to the entire screen or to each block; Wherein the duty of the light sources is adjusted in units of the analysis area. A timing controller for dividing one frame period into a first sub frame period and a second sub frame period, and a backlight for emitting light to the liquid crystal display panel including at least one light source A method of controlling an image quality of a liquid crystal display device, (A) analyzing the input data on a frame-by-frame basis with respect to an analysis area of a predetermined size; (B) deriving a maximum gradation value in the analysis area; (C) adjusting the duty of the light sources to a maximum during the second sub-frame period and adjusting the duty of the light sources according to the maximum gray-scale value during the first sub-frame period; And (D) adjusting the input data to the modulated data in consideration of the maximum gradation value and the adjusted duty, Wherein the step (C) controls the duty of the light sources to be 50% so that the light sources are fully lit during the second sub-frame period when the maximum gray-level value exceeds a predetermined gray-level value, Controlling the duty of the light sources to be C (C? 50%) so as to be turned on for a period corresponding to the luminance difference between the halftone value and the maximum gray level value during one subframe, Wherein the step (D) modulates data input to the analysis area into white data and outputs the data during the second sub frame period when the maximum gray value exceeds the halftone value, Modulating gray data upwardly adjusted to exhibit a luminance value equal to the original luminance value (luminance value at duty ratio 100%) at duty (50 + C)% and outputting the gray data during the first sub frame period A method for controlling image quality of a display device. 8. The method of claim 7, The step (C) The duty of the light sources is controlled to 0% so that the light sources are extinguished during the first sub frame period if the maximum gradation value is less than a predetermined halftone value, To 50%. ≪ / RTI > 9. The method of claim 8, Wherein the duty (C%) of the light sources is controlled so as to increase in proportion to a luminance difference between the halftone value and the maximum gradation value. 8. The method of claim 7, The step (D) Modulating the data input to the analysis area into black data and outputting the data during the first sub frame period if the maximum gradation value is less than the halftone value, Modulated with data adjusted to exhibit a luminance value equal to a value (a luminance value at a duty of 100%), and outputting the modulated data during the second sub frame period. 8. The method of claim 7, The analysis area is set to the entire screen or to each block; Wherein the duty of the light sources is adjusted in units of the analysis area.

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