KR101633110B1 - Liquid crystal display and dimming control method of thereof - Google Patents

Abstract

The present invention relates to a liquid crystal display device capable of adjusting the brightness of a backlight according to a display image and a dimming control method thereof.

This liquid crystal display device includes a liquid crystal display panel; A backlight unit including a plurality of light sources for irradiating light to a rear surface of the liquid crystal display panel; And the image data are analyzed to calculate an image JND scale having a plurality of recognition levels and the dimming JND scale is calculated by down-adjusting the image JND scale while keeping the number of recognition levels the same, And a dimming control circuit for determining a dimming value for controlling the light sources.

Description

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

The present invention relates to a liquid crystal display device capable of adjusting the brightness of a backlight according to 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, a backlight dimming method of adjusting the brightness of the backlight according to an image has been proposed. The backlight dimming method calculates the APL (Average Picture Level) of the input image and adjusts the brightness of the backlight based on the APL. The backlight dimming method increases the brightness of the backlight when the APL is high and reduces the brightness of the backlight when the APL is low, thereby reducing power consumption.

However, since the conventional backlight dimming method adjusts the brightness of the backlight based on APL, it is impossible to consider the fine gradation level of the entire image. For example, according to the conventional backlight dimming method, all of the cases of FIG. 1 in which the APL is the same but the gradation level of the entire image is different are all adjusted to the same brightness. In this case, the gradation level after dimming is inevitably lowered compared to the gradation level before dimming.

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 which are capable of preventing degradation of gradation level upon backlight dimming.

According to an aspect of the present invention, there is provided a liquid crystal display device including: a liquid crystal display panel; A backlight unit including a plurality of light sources for irradiating light to a rear surface of the liquid crystal display panel; And the image data are analyzed to calculate an image JND scale having a plurality of recognition levels and the dimming JND scale is calculated by down-adjusting the image JND scale while keeping the number of recognition levels the same, And a dimming control circuit for determining a dimming value for controlling the light sources.

Wherein the dimming control circuit comprises: an image information analyzing unit for deriving the image JND scale; And a dimming value determining unit for determining the dimming value.

Wherein the image information analyzing unit comprises: a gradation analyzer for analyzing the image data of one frame and deriving a minimum gradation value and a maximum gradation value of the image; A first luminance calculator for mapping the minimum gradation value and the maximum gradation value to a preset gamma curve to calculate a minimum luminance value and a maximum luminance value of the image; And an image JND scale calculator for calculating the image JND scale by applying the minimum luminance value and the maximum luminance value to a predetermined luminance-JND relation.

Wherein the dimming value determiner calculates a lower limit JND level by applying a lower limit luminance value of the display to the luminance-JND relation, and calculates a dimming value using the dimming value including the lower limit JND level as the minimum recognition level A dimming JND scale calculating unit for calculating a JND scale; A second luminance calculation unit for calculating a maximum dimming luminance value by applying the maximum recognition level of the dimming JND scale to the luminance-JND relation; And a dimming value calculator for calculating the dimming value by comparing the maximum luminance value of the image with the maximum dimming luminance value.

According to an embodiment of the present invention, a dimming control method of a liquid crystal display device having a liquid crystal display panel and a plurality of light sources for irradiating light on the back surface of the liquid crystal display panel includes analyzing image data to generate an image JND Calculating a scale; Calculating a dimming JND scale by adjusting the image JND scale down while keeping the number of recognition levels the same; And determining a dimming value for controlling the light sources based on the dimming JND scale.

A liquid crystal display apparatus and a dimming control method thereof according to the present invention calculate a JND scale of a frame image having a plurality of recognition levels based on the analysis result of an input image and adjust the image JND scale down while maintaining the same number of recognition levels To calculate the dimming JND scale, and then determines the dimming value for controlling the light sources based on the dimming JND scale. As a result, even if the backlight luminance is lowered through dimming, the liquid crystal display device and its dimming control method according to the present invention can maintain the gradation level of the image in comparison with that before the dimming, It is possible to prevent degradation and deterioration of gradation quality.

Hereinafter, preferred embodiments of the present invention will be described with reference to FIGS. 2 to 8. FIG.

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

2, 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, a dimming control circuit 14, A backlight driving circuit 15, and a backlight unit 16. [

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. Each of the liquid crystal cells Clc includes a TFT, a pixel electrode 1 connected to the TFT, and a storage capacitor Cst. On the upper glass substrate of the liquid crystal display panel 10, a black matrix, a color filter, a common electrode 2, and the like 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. The liquid crystal cells Clc include R liquid crystal cells for red display, G liquid crystal cells for green display, and B liquid crystal cells for blue display. The R liquid crystal cell, the G liquid crystal cell, and the B liquid crystal cell constitute one unit pixel. 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 timing controller 11 arranges image data RGB input from a system board on which an external video source is mounted, and supplies the image data to the data driving circuit 12 and the dimming control circuit 14. 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 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 image data RGB under the control of the timing controller 11 and outputs the latched data RGB to the positive polarity / negative polarity analog data And then supplies the data 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 dimming control circuit 14 analyzes the image data RGB of one frame to calculate a JND (Just Noticeable Difference) scale of the frame image having a plurality of recognition levels, To determine a dimming value (DIM) for controlling the light sources of the backlight unit 16 based on the dimming JND scale. The dimming control circuit 14 will be described in detail with reference to FIG. 3 to FIG.

The backlight driving circuit 15 drives the light sources of the backlight unit 16 by pulse width modulation (PWM) in which the duty ratio is varied according to the dimming value DIM input from the dimming control circuit 14. [ The lighting time of the light sources is controlled according to the PWM duty ratio.

The backlight unit 16 includes a plurality of light sources to irradiate the liquid crystal display panel 10 with light. The backlight unit 16 may be implemented as either a direct type or an edge type. 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 edge type backlight unit 16 has a structure in which a plurality of optical sheets and a light guide plate are stacked below a 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.

3 shows the dimming control circuit 14 in detail.

Referring to FIG. 3, the dimming control circuit 14 includes an image information analyzing unit and a dimming value determining unit.

The image information analyzing unit includes a tone analyzer 141, a first luminance calculator 142, and an image JND scale calculator 143.

The grayscale analysis unit 141 analyzes the image data RGB of one frame and derives the minimum grayscale value Gmin and the maximum grayscale value Gmax of the frame image. 6, when the image data RGB of 16 gradation to 255 gradation is input, the gradation analysis unit 141 sets the 16 gradation to the minimum gradation value Gmin and the 255 gradation to the maximum gradation value Gmax .

The first luminance calculator 142 maps the minimum gradation value Gmin and the maximum gradation value Gmax inputted from the gradation analysis unit 141 to preset gamma curves and outputs the minimum luminance value Lmin and the minimum luminance value Lmin of the frame image, And calculates the maximum luminance value Lmax. For example, the first luminance calculator 142 may calculate the minimum luminance value (Lmin) of the frame image by 1.43 nits corresponding to the 16th gray level as shown in FIG. 6 and the maximum luminance value Lmax ). The gamma curve is a curve indicating grayscale to luminance, and can be determined by a predetermined gamma value and a lower limit luminance value and a lower limit luminance value of a display that can be implemented in the display device. In FIG. 6, the gamma value is 2.2 gamma, the lower limit luminance value is 0.3 nit, and the upper limit luminance value is 500 nit.

The image JND scale calculator 143 applies the minimum luminance value Lmin and the maximum luminance value Lmax input from the first luminance calculator 142 to either one of the following equations 1 and 2 Calculate the JND scale of the frame image.

In Equation 1, "L" represents a luminance value, and "A, B, and C" represents arbitrary constant values that are determined according to the surrounding environment. Equation (1) can be implemented as a look-up table.

In Equation (2), "L" represents a luminance value, and "A, B, C, D, E, F, G, H and I" Equation (2) can be implemented as a look-up table.

The JND scale refers to the change in brightness that can be distinguished by the human eye from the HVS (Human Visual System) viewpoint at the JND level. FIG. 4 shows an example of the JND scale, in which between 0.005 nit to 0.02 nit is represented by seven JND levels. Equation (1) is a result of deriving the relationship between luminance and JND by performing a cognitive experiment on an actual display as LEE's HVS. Equation 2 is a result of deriving the relationship between brightness and JND based on the contrast threshold value as NEMA-DICOM in VESA 2.0. Equations (1) and (2) show similar results as shown in FIG. However, Equation (1) has an advantage that it can be implemented more accurately and simply than Equation (2).

For example, the image JND scale calculator 143 calculates 619 JND levels between 87 JND levels corresponding to 1.43 nits and 706 JND levels corresponding to 500 nits, as shown in FIG. 6, on the JND scale of the frame image.

The dimming value determining unit includes a dimming JND scale calculating unit 144, a second brightness calculating unit 145, and a dimming value calculating unit 146. [

The dimming JND scale calculation unit 144 calculates the dimming JND scale for maintaining the gradation level. The dimming JND scale calculation unit 144 calculates a lower limit JND level of a display that can be implemented in a display device by applying a lower limit luminance value of a display to Equation 1 or Equation 2, 143 to calculate the dimming JND scale by adding the number of recognition levels of the image JND scale. In other words, the dimming JND scale calculator 144 calculates the dimming JND scale by adjusting the image JND scale down while maintaining the same number of recognition levels. For example, the dimming JND scale calculator 144 calculates the dimming JND scale by adjusting the image JND scale including 87 JND level to 706 JND level from 32 JND level to 651 JND level as shown in FIG.

The second luminance calculator 145 calculates the maximum dimming luminance value DLmax by applying the maximum recognition level of the dimming JND scale inputted from the dimming JND scale calculator 144 to Equation 1 or Equation 2 inversely . For example, as shown in FIG. 6, the second luminance calculator 145 calculates 345 nit corresponding to the 651 JND level as the maximum dimming luminance value DLmax.

The dimming value calculation unit 146 compares the maximum luminance value Lmax input from the first luminance calculation unit 142 with the maximum dimming luminance value DLmax input from the second luminance calculation unit 145, (DIM). For example, the dimming value calculator 146 calculates the dimming value DIM so that the display luminance of 500 nit is reduced by 31% to 345 nit in the case of FIG. Here, since the maximum dimming luminance value DLmax of 345 nit is based on the dimming JND scale having the same number of recognition levels as the image JND scale, even if the display luminance is reduced by 31% by the dimming value DIM as shown in FIG. 7 There is no problem of low gradation clipping, degradation of image quality, and degradation of gradation quality.

FIG. 8 shows a dimming control method capable of preventing degradation of gradation level upon backlight dimming.

8, the dimming control method analyzes image data (RGB) of one frame to derive a minimum gradation value and a maximum gradation value of a frame image. (S10, S20) Value and the maximum gradation value are respectively mapped to a preset gamma curve to calculate a minimum luminance value and a maximum luminance value of the frame image.

The dimming control method calculates the JND scale of the frame image by applying the minimum luminance value and the maximum luminance value to Equation (1) or Equation (2), respectively. (S40) The lower limit luminance value of the display is applied to Equation 2 to calculate the lower limit JND level of the display that can be implemented in the display device.

The dimming control method calculates the dimming JND scale by adding the number of recognition levels of the image JND scale to the lower limit JND level. In other words, the dimming control method calculates the dimming JND scale by adjusting the image JND scale down while maintaining the same number of recognition levels (S60)

The dimming control method calculates the maximum dimming luminance value by applying the maximum recognition level of the dimming JND scale back to Equation 1 or Equation 2 above (S70)

The dimming control method calculates the dimming value (DIM) by comparing the maximum dimming luminance value with the maximum luminance value of the image, and outputs the calculated dimming value (S80, S90). The maximum dimming luminance value corresponds to the dimming value Since it is based on the JND scale, even if the display luminance is lowered by the dimming value (DIM), there is no problem of low gradation clipping, deterioration of image quality, and degradation of gradation quality.

As described above, the liquid crystal display and dimming control method thereof according to the present invention calculate the JND scale of a frame image having a plurality of recognition levels based on the analysis result of an input image, After lowering the JND scale to calculate the dimming JND scale, determine the dimming value to control the light sources based on the dimming JND scale. As a result, even if the backlight luminance is lowered through dimming, the liquid crystal display device and its dimming control method according to the present invention can maintain the same gradation level of the image compared to before the dimming, , It is possible to prevent degradation of gradation quality.

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 diagram showing an example in which the APL is the same but the gradation level of the entire image is different.

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

3 is a detailed circuit diagram of a dimming control circuit;

4 is a view showing an example of a JND scale;

5 is a diagram showing the relationship between luminance and JND;

6 is a view showing an application example of the present invention.

Figure 7 shows the effect of the present invention.

8 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: dimming control circuit 15: light source driving circuit

16: backlight unit 141: gradation analysis unit

142: First luminance calculation unit 143: Image JND scale calculation unit

144: dimming JND scale calculation unit 145: second luminance calculation unit

146: Dimming value calculation unit

Claims (8)

A liquid crystal display panel; A backlight unit including a plurality of light sources for irradiating light to a rear surface of the liquid crystal display panel; And The image data is analyzed to calculate an image JND scale having a plurality of recognition levels and the dimming JND scale is calculated by adjusting the image JND scale down while maintaining the same number of recognition levels, And a dimming control circuit for determining a dimming value for controlling the light sources, The dimming control circuit calculates a lower limit JND level by applying a lower limit luminance value of a display to a predetermined luminance-JND relation, adds the number of recognition levels of the image JND scale to the lower limit JND level, calculates the dimming JND scale And applying the maximum recognition level of the dimming JND scale back to the brightness-JND relation to lower the maximum dimming luminance value. The method according to claim 1, Wherein the dimming control circuit comprises: An image information analyzer for deriving the image JND scale; And And a dimming value determination unit for determining the dimming value. 3. The method of claim 2, Wherein the image information analyzing unit comprises: A gray level analyzer for analyzing the image data of one frame and deriving a minimum gray level value and a maximum gray level value of the image; A first luminance calculator for mapping the minimum gradation value and the maximum gradation value to a preset gamma curve to calculate a minimum luminance value and a maximum luminance value of the image; And And an image JND scale calculator for calculating the image JND scale by applying the minimum luminance value and the maximum luminance value to the preset luminance-JND relation. The method of claim 3, Wherein the dimming value determining unit A dimming JND scale calculator for calculating the dimming JND scale; A second luminance calculator calculating the maximum dimming luminance value; And And a dimming value calculator for calculating the dimming value by comparing the maximum luminance value of the image with the maximum dimming luminance value. A dimming control method for a liquid crystal display having a liquid crystal display panel and a plurality of light sources for irradiating light on a back surface of the liquid crystal display panel, Analyzing the image data to calculate an image JND scale having a plurality of recognition levels; Calculating a dimming JND scale by adjusting the image JND scale down while keeping the number of recognition levels the same; And Determining a dimming value for controlling the light sources based on the dimming JND scale, The step of calculating the dimming JND scale may include calculating a lower limit JND level by applying a lower limit luminance value of a display to a predetermined luminance-JND relation, adding the number of recognition levels of the image JND scale to the lower limit JND level Lt; / RTI > Wherein the step of determining the dimming value comprises applying the maximum recognition level of the dimming JND scale back to the brightness-JND relationship to lower the maximum dimming luminance value. 6. The method of claim 5, Wherein the step of calculating the image JND scale comprises: Analyzing the image data of one frame to derive a minimum gradation value and a maximum gradation value of the image; Calculating a minimum luminance value and a maximum luminance value of the image by mapping the minimum and maximum gradation values to a preset gamma curve; And And calculating the image JND scale by applying the minimum luminance value and the maximum luminance value to the preset luminance-JND relation. delete 6. The method of claim 5, The step of determining the dimming value comprises: And compares the maximum luminance value of the image with the maximum dimming luminance value to calculate the dimming value.

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