JP2010091760A - Display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress degradation in the image quality due to changes in the hue of the video signal after setting the gain and achieve low power consumption, in a display that controls gain setting of a video signal and brightness of a backlight light source linked with each other based on a brightness histogram of the input video signal. <P>SOLUTION: The display includes a configuration design unit 13 for setting a gain of an input video signal and a distortion module 5 for selecting a light source brightness level of the backlight of a liquid crystal panel. The gain setting in the configuration design unit 13 is linked with the selected light source brightness level. The distortion module 5 regulates a lower limit of the emission brightness level of the backlight in accordance with the maximum brightness value in the histogram of the input video signal. Thus, saturation in an output value due to an excessive increase in the gain can be suppressed to maintain video quality. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a display device, and more particularly to a display device capable of realizing a desired gradation characteristic while maintaining video quality and performing video display with high contrast.

Conventionally, various techniques for increasing the gain of an input video signal in accordance with the luminance level of the input video signal have been proposed for the purpose of increasing the contrast of the display device.
For example, Patent Document 1 discloses a liquid crystal display device aimed at solving the problem of dynamic range caused by gamma correction characteristics when digital image data is corrected by a digital gamma correction circuit. Here, the digital image data is corrected by a digital gamma correction circuit to obtain digital image data suitable for the applied voltage-transmittance characteristics of the liquid crystal display unit. The digital image data is D / A converted and displayed with gain. At this time, when saturation (saturation) occurs in which the gradation value of the output digital image data from the gamma correction circuit reaches the maximum value before the gradation value of the input digital image data reaches the maximum value, the gamma correction circuit The dynamic range of the display image is adjusted by setting the gain to be low in the pre-stage front circuit.

Recently, especially in liquid crystal display devices, not only gaining the input video signal but also modulating the brightness of the backlight light source in conjunction with the gain of the input video signal reduces the power consumption and increases the sense of contrast. A technique for improving the above has been proposed.
For example, in Patent Documents 2 and 3, when the histogram of the video signal is equal to or less than a predetermined ratio of the displayable band (when the luminance distribution is dark), the backlight luminance is decreased and the gain of the input video signal is increased. A technique for improving the feeling of contrast while maintaining image display brightness is shown.
Japanese Patent Laid-Open No. 10-319922 JP 2006-276677 A US Patent Application Publication No. 2006/0274026

  Regarding the technology for modulating the luminance of the backlight light source in conjunction with the gain of the input video signal as described above, adjusting the luminance of the backlight light source and the video gain according to a histogram which is one of the feature quantities of the video. Thus, there is known a technique for reducing power consumption and increasing the dynamic range of video. In a liquid crystal display device using such a technique, a luminance value histogram of a video signal is calculated as a histogram, and the luminance value histogram is used to calculate the backlight luminance and the gain without calculating the histogram of each color or the color difference histogram. Some adjustments have been made.

In such a liquid crystal display device, the luminance of the backlight light source is determined according to the histogram of the video signal, and the gain for expanding the video signal is determined based on the luminance.
At this time, in the gain setting for expanding the video signal, if the output value cannot be expressed after expansion, saturation (saturation) occurs in the output value. For example, in a display panel that performs output display with 256 gradations (0 to 255), when the RGB output signals are expanded by giving gains to the RGB signals, if the output value after expansion exceeds 255 gradations , All are clipped at 255 and output in 255 gradations. This portion is the portion where the above saturation occurs.
Some liquid crystal display devices saturate the peak portion of the video in an inconspicuous range in the gain setting for expanding the video signal as described above.

As an example, when the gain set based on the luminance histogram is 2.0 and RGB of the video signal is (R, G, B) = (200, 100, 50), the video signal after giving the gain is (R, G, B) = (400, 200, 100). Here, in a display device that displays an image with 256 gradations, saturation occurs in R and is clipped to 255. That is, (R, G, B) = (255, 200, 100). That is, the hue of the video signal portion changes due to the gain.
Particularly for flesh-colored images, the change in hue due to saturation at the time of gain setting as described above is easily perceived by the human eye, and the degradation of video quality is noticeable.

  At this time, if there is an RGB color histogram or a color difference histogram, the saturation amount of the peak portion of the video signal can be reduced based on the value, but such correspondence is performed only with the luminance value histogram. I can't. For this reason, in a display device that adjusts the gain of the video signal and the brightness of the backlight light source by the histogram of the brightness value, it is required to improve the deterioration of the video quality when the gain is set.

  The present invention has been made in view of the above situation, and in a display device that adjusts the gain setting of a video signal and the luminance of a backlight light source in conjunction with each other based on a luminance histogram of an input video signal, It is an object of the present invention to provide a display device that suppresses a reduction in video quality due to a change in hue of a video signal after setting and realizes low power consumption.

  In order to solve the above problems, a first technical means of the present invention includes a liquid crystal panel that displays an image based on an input video signal, and a light source that illuminates the liquid crystal panel. A brightness level setting unit that sets a reference light emission luminance level based on a luminance control characteristic that defines a light emission luminance of a light source with respect to a feature amount of an input video signal; A histogram detection unit that detects a histogram of the luminance value of the signal, a luminance level selection unit that selects a light emission luminance level used for light source control from the histogram of the input video signal, and a light emission luminance level selected by the luminance level selection unit And a light emission luminance level for reference set in the luminance level setting unit, and a gain setting unit that sets the gain of the video signal, and the luminance level setting unit It is obtained by and limits the lower limit of the light emission luminance level of the light source according to the maximum luminance value of Sutoguramu.

  The second technical means has means for preliminarily storing a control characteristic for determining a lower limit value of the light emission luminance level according to the maximum luminance value of the histogram in the first technical means, and the luminance level selection unit is set in advance. In the histogram of the input video signal using the target contrast of the liquid crystal panel, the video brightness that cannot be expressed at the specific light emission brightness level of the light source within the video brightness range of the input video signal that can be displayed on the liquid crystal panel at the target contrast A process of multiplying the frequency of the image in the range by a predetermined weighting factor to obtain an evaluation value is performed for all the light emission luminance levels of the selectable light sources, and the light emission luminance level with the lowest evaluation value is obtained by the luminance level setting unit. Extract within the range that does not exceed the set emission luminance level for reference, refer to the control characteristics, and according to the maximum luminance value of the histogram of the input video signal The lower limit is higher than the extracted light emission luminance level is obtained by and outputs as a light emission luminance level selected lower limit.

  According to a third technical means, in the second technical means, the control characteristic for determining the lower limit value of the light emission luminance level is such that the lower limit value is within a predetermined high luminance side range of the maximum luminance value of the histogram. It is characterized by being constant.

  According to a fourth technical means, in any one of the first to third technical means, the gain setting unit is set by dividing the light emission luminance level for reference by the selected light emission luminance level and then raising to the 1 / γ power. It is characterized in that the gain to be calculated is calculated.

  According to the present invention, in a display device that adjusts the gain setting of a video signal and the luminance of a backlight light source in conjunction with each other based on the luminance histogram of the input video signal, the video quality due to the change in the hue of the video signal after the gain setting is achieved. It is possible to provide a display device that suppresses the decrease in power consumption and realizes low power consumption.

  According to the present invention, in the display device that adjusts the gain setting of the video signal and the luminance of the backlight light source based on the luminance histogram, the reduction of the luminance of the backlight light source is limited according to the characteristic of the luminance histogram. As a result, when the image is dark, it is possible to sufficiently reduce power consumption and improve the black level without impairing the image quality, and in particular, display an appropriate image without causing a change in the hue of the skin color. The video quality can be improved by reducing the power consumption and increasing the dynamic range of the video signal.

  Hereinafter, preferred embodiments according to a display device of the present invention will be described with reference to the accompanying drawings. The embodiment according to the present invention described below adjusts the degree of amplification (gain) of an input video signal in accordance with the video feature amount of the input video signal in a video display device having a backlight as a light source. At this time, a target contrast (target CR) is set, and video expression is performed so as to approach the target CR by controlling the light emission luminance of the backlight light source and controlling the gain. In this specification, such a video signal and backlight luminance modulation processing is referred to as advanced luminance modulation processing.

<Outline of Luminance Modulation Processing According to the Present Invention>
Ideally, the amount of light emitted from the display device faithfully reproduces the level of the video signal to be displayed. That is, when displaying a black screen, the amount of light emitted from the screen should ideally be zero. However, in an actual liquid crystal display device, there is some light leakage, and even when a black screen is displayed, gray display is performed instead of black.

  One of the important performances of the display device is a contrast ratio (hereinafter also referred to as CR). In the display device, CR is a ratio between the maximum luminance and the minimum luminance on the display panel. In the case of a liquid crystal display device, the maximum luminance is determined by the maximum light emission luminance of the light source, and the minimum luminance is determined by the amount of light leakage during black display. Therefore, when the light emission luminance of the light source is constant, the contrast ratio is constant in the same liquid crystal panel.

  FIG. 1 is a graph showing the relationship between the input gradation (video signal level) and the luminance value on the liquid crystal panel for liquid crystal panels with CR of 3000 and 6000. The maximum luminance is the same 450 cd, but the display luminance (minimum luminance) on the liquid crystal panel at the input gradation (pixel value) 0 is 0.15 cd for CR3000 and 0.075 cd for CR6000. There is a difference.

  Here, when the light emission luminance of the light source is reduced to 50% and the input video signal is amplified by a predetermined amount when the CR3000 liquid crystal panel is used, the relationship between the pixel value of the input video signal and the display luminance value of the liquid crystal panel is The relationship is as shown by the dotted line in FIG. 1, and with pixel values 0 to 128, it is possible to express the luminance close to that of a CR6000 liquid crystal panel. However, an image with a pixel value greater than 128 cannot be expressed in gradation, and so-called whitening occurs. Therefore, it is necessary to adjust the light emission luminance of the backlight and set the gain according to the feature amount of the input video signal.

  For example, when the luminance histogram of the input video signal shows a luminance distribution with a pixel value of 128 or less, the backlight luminance is reduced to 50% as shown in FIG. Control to be amplified may be performed. By controlling the backlight emission luminance and setting the gain in accordance with the feature amount of the input video signal in this way, it is possible to increase the contrast feeling and at the same time to save power by lowering the backlight emission luminance. It becomes possible.

  In the above description, the case where the luminance histogram of the input video signal shows a luminance distribution with a pixel value of 128 or less has been described as an example. However, other than the above example, for example, when the white portion in the video is extremely small, The degree of importance can be lowered and black expression can be improved in the same manner. At this time, the unsharp portions of the white area may be ignored, or the gain in the white area may be determined so that the white area can be reduced by the gain setting for realizing the target CR.

  Further, in the control of the luminance of the backlight and the gain setting according to the present invention, the light emission of the backlight is controlled according to the maximum value of the Y histogram of the video signal in order to suppress the hue change due to the saturation of the RGB output value at the time of the gain setting. The lower limit value of the luminance level is limited to suppress the color change by suppressing the gain increase.

  In addition, in the luminance modulation processing according to the present invention, in order to save power, processing for dynamically suppressing the light emission luminance of the light source according to the feature amount such as APL of the video obtained from the video signal as described later is also performed. And execute. In other words, the reference emission luminance level for setting the gain setting and the emission luminance level of the backlight light source is first set in accordance with the video feature amount (APL, peak (maximum luminance value), histogram information, etc.), thereby saving power. In addition, a process for producing a contrast feeling as described above (that is, setting the light emission luminance level to an appropriate value equal to or lower than the reference light emission luminance level) is performed on the reference light emission luminance level. Thus, CR improvement and further power saving are achieved, and the gain of the video signal is set in conjunction with the processing to maintain the visual brightness.

<System configuration example of a liquid crystal display device that performs luminance modulation processing according to the present invention>
FIG. 2 is a block diagram showing a system configuration example of the liquid crystal display device according to the present invention. The liquid crystal display device illustrated in FIG. 2 includes a scaling unit 1, a Y histogram detection unit 2, an APL detection unit 3, a BL (backlight) luminance level setting unit 8, a CPU (Central Processing Unit) / CPLD (Complex Programmable Logic Device). 11, a BL light control unit 12, an image quality correction unit 14, an RGBγ / WB (White Balance) adjustment unit 15, an FRC (Frame Rate Control) unit 16, and a video output unit 17.

The liquid crystal display device illustrated in FIG. 2 further includes an advanced luminance modulation unit 20 that executes the main part of the luminance modulation processing according to the present invention. The advanced luminance modulation unit 20 includes a histogram stretching unit 4, a distortion module 5, a scene change detection unit 6, a first temporary filter 7, a second temporary filter 9, a variable delay 10, and a configuration design unit 13. Note that, as described above, the luminance modulation processing according to the present invention is determined not only by dynamic light emission luminance control according to a feature quantity such as APL but also by a predetermined condition of the video feature quantity. This is a luminance modulation process in which a light emission luminance level BL _reduced that gives a sense of contrast to the light emission luminance level BL _ref for reference of the light source is selected and the gain of the video signal is also set. A part that executes this process is called an “advanced” luminance modulation unit 20.

First, the outline of each block in the liquid crystal display device of FIG. 2 will be described.
The video output unit 17 includes a liquid crystal panel that displays video based on the video signal, and a liquid crystal control circuit that converts the video signal into a signal for driving the liquid crystal panel and outputs the signal to the liquid crystal panel. Although the details will be described later, the video signal is converted using the gain set by the advanced luminance modulation unit 20 and then input to the video output unit 17. That is, in the luminance modulation processing according to the present invention, a video signal indicating a video to be displayed by the video output unit 17 is a processing target. The gain and its setting will be described later.

  The BL adjustment unit 12 includes a lamp composed of a fluorescent tube and a lamp driving circuit that drives the lamp, and a light source that irradiates the liquid crystal panel from the back and side surfaces (also referred to as a backlight light source or simply a backlight). Configure. In the luminance modulation processing according to the present invention, this lamp is a target of emission luminance control.

The BL adjustment unit 12 is controlled by the CPU / CPLD 11. The CPU / CPLD 11 uses the lamp driving circuit (for example, an inverter circuit) of the BL adjusting unit 12 to actually perform light control according to a signal (for example, a duty signal) indicating the light emission luminance level BL _reduced output from the advanced luminance modulation unit 20. The signal is converted into a signal (for example, a signal suitable for driving such as pulse width modulation) and output to the BL adjustment unit 12. The backlight dimming value is converted into a signal for actual backlight dimming. Further, as the lamp, for example, a lamp composed of an LED (Light Emitting Diode) or a combination of an LED and a fluorescent tube may be adopted, and at the same time, a lamp driving circuit corresponding to the lamp may be provided. Good.

  The parts for processing the video signal output to the video output unit 17 and controlling the BL adjustment unit 12 via the CPU / CPLD 11 are the scaling unit 1, the Y histogram detection unit 2, the APL detection unit 3, and the BL luminance level setting unit. 8, an image quality correction unit 14, an RGBγ / WB adjustment unit 15, an FRC unit 16, and an advanced luminance modulation unit 20.

  First, the scaling unit 1 changes the number of pixels of the video frame indicated by the input video signal (input video signal) or the aspect ratio of the video frame by calculation according to the resolution of the liquid crystal panel and the like.

  Here, as the input video signal, for example, a signal obtained by demodulating a video signal received as a broadcast wave, a video signal received via a communication network, a signal obtained by reading a video signal stored in an internal storage device, various recorders and various players This corresponds to a video signal received from an external device such as a tuner device or a tuner device, or a video signal obtained by performing various video processes on the video signal. Although not shown, the liquid crystal display device of FIG. 2 may be configured to be able to acquire any of such video signals.

  The image quality correction unit 14 changes the contrast, color, and the like of the video with respect to the video signal output from the scaling unit 1 according to user settings and the like.

  The RGB γ / WB adjustment unit 15 adjusts γ, WB (white balance) / CT (color temperature), etc., of the video signal output from the image quality correction unit 14. Further, the RGBγ / WB adjustment unit 15 changes the gain of the signal by a gain setting signal from the advanced luminance modulation unit 20 (actually the configuration design unit 13).

  The RGBγ / WB adjustment unit 15 converts the video signal based on the gain, and compensates for the luminance reduction by the gain with respect to the control for reducing the emission luminance level by the advanced luminance modulation unit 20 as described later. Here, in order to suppress noise in the low gradation part, this conversion is performed after γ adjustment and before WB adjustment. Alternatively, γ adjustment is performed after the gain is changed, and further, gradation correction is performed before the gain change in order to correct a shift in gradation characteristics due to γ adjustment after the gain change. May be provided.

  The gain setting signal from the advanced luminance modulation unit 20 is a signal indicating a conversion coefficient for converting the pixel value (video signal level) of the video signal to be output to the liquid crystal panel. This gain setting signal is a common conversion coefficient for multiplying each video signal level (in this example, a value of 0 to 255) as shown in the following example, and the video signal level that reaches the peak by gaining is obtained. The gain may be corrected by the RGBγ / WB adjustment unit 15 for a certain video signal level range obtained based on the above range.

  The FRC unit 16 is a frame rate converter, and detects the motion vector of the video from the adjusted video signal output from the RGB γ / WB adjustment unit 15 to generate a complementary video, thereby generating a normal video from a display frequency of 60 Hz. The display frequency is converted to 120 Hz. Of course, the display frequency to be processed in the FRC unit 16 and the display frequency after processing are not limited to this. In the example of FIG. 2, the liquid crystal driving circuit of the video output unit 17 converts the video signal output from the FRC unit 16 into a signal for driving the liquid crystal panel and outputs the signal to the liquid crystal panel.

The Y histogram detection unit 2 divides the video frame into pixel units and generates a histogram representing the frequency of occurrence of the luminance value of each pixel. The histogram generated by the Y histogram detection unit 2 has a frequency value for each of luminance values (Y) 0 to 255, for example.
The APL detection unit 3 calculates the average luminance level of the video signal for each video frame. The value calculated by the APL detection unit 3 is a value indicating 0% when the entire screen is black, and a value indicating 100% when the entire screen is white.

  The histogram stretching unit 4 sets a range to be used by the advanced luminance modulation unit 20 from the histogram generated by the Y histogram detection unit 2. For example, it is assumed that the distortion module 5 is a module that performs an operation with a minimum value 0 to a maximum value 255, and the input video signal is originally a signal having a minimum value 10 to a maximum value 235. In such a case, the histogram stretching unit 4 sets the frequency value for each of the minimum value 10 to the maximum value 235 to the frequency for each of the minimum value 0 to the maximum value 255 in order to match the calculation in the distortion module 5. It stretches to fit the value.

The distortion module 5 corresponds to a luminance level selection unit of the present invention, and includes a histogram input from the histogram stretching unit 4 and a reference emission luminance level (backlight target) set by a BL luminance level setting unit 8 described later. The light emission luminance level (also referred to as a backlight value) BL _reduced that is actually set, that is, the light emission luminance level used for backlight control, is selected from BL _ref . The selection is performed in a range that does not exceed the reference light emission luminance level BL _ref set by the BL luminance level setting unit 8 from a plurality of predetermined light emission luminance levels. Here, the backlight value BL _reduced that can realize a display image closer to the liquid crystal panel having the target CR is selected.
At this time, in order to suppress a change in hue due to saturation of the RGB output value at the time of gain setting, the reduction of the backlight value BL _reduced is limited to a predetermined level according to the lower limit value according to the maximum value of the histogram of the video signal, and the gain The color change is suppressed by suppressing the rise of the color. The distortion parameters such as the target CR may be set from a main CPU (not shown).

  The scene change detection unit 6 detects the presence or absence of a scene change from the degree of change between the histogram one frame before and the current histogram. For example, the cumulative value of the frequency change of each luminance value is calculated, and if it is larger than a specific value, it is determined that the scene has changed.

The first temporary filter 7 is provided to prevent visual discomfort caused when the above-described actually set light emission luminance level BL _reduced selected by the distortion module 5 changes abruptly. after the amount of change in light emission luminance level BL _reduced to those temporally slow, output to the subsequent stage as a light emission luminance level BL _reduced to be actually set. Further, at the time of a scene change, if a slow light emission brightness level BL _reduced is applied, the result is uncomfortable. _Therefore , the value of the first temporary filter 7 is changed according to the scene change detection signal from the scene change detection unit 6. Make fast changes possible.

The BL luminance level setting unit 8 corresponds to the luminance level setting unit of the present invention, and includes video feature amounts such as an APL value output from the APL detection unit 3 or histogram information output from the Y histogram detection unit 2, and not shown. The maximum value of the light emission luminance level of the backlight is determined with reference to the value of OPC (Optical Picture Control; also referred to as a brightness sensor) output from the main CPU, the user setting value, and the like. For example, when the APL is high, the maximum value of the light emission luminance level of the backlight is set to a low value so that an image without feeling dazzling can be obtained. The maximum value of the backlight emission luminance level is the reference emission luminance level (backlight target value) BL _ref for advanced luminance modulation executed by the advanced luminance modulation unit 20. As the video feature amount for determining the light emission luminance level BL _ref for reference, APL or histogram information can be used as described above, and the feature amount to be used is selected according to the embodiment. For the histogram information, the peak value (maximum luminance value) of the video, the ratio of the video signal included between the predetermined luminances smaller than the maximum luminance, and the like are used.

Since the selection by the distortion module 5 is performed within a range that does not exceed the reference emission luminance level BL _ref set by the BL luminance level setting unit 8, the BL luminance level setting unit 8 uses the reference emission luminance level. It is described that the maximum value of the light emission luminance level of the backlight is set as BL _ref . In the example of FIG. 2, the reference light emission luminance level that has passed through the second temporary filter 9 is BL _ref .

The second temporary filter 9 is a filter having a function equivalent to that of the first temporary filter 7. In brief, when the APL changes rapidly and the change does not affect the selection in the distortion module 5, the emission luminance level BL _reduced output from the first temporary filter 7 is the time. Change has been mitigated. However, since the gain setting is calculated based on the reference emission luminance level output from the BL luminance level setting unit 8, the gain changes and the display luminance on the liquid crystal panel changes abruptly. In order to eliminate or alleviate such a sudden change in display luminance, a second temporary filter 9 is provided.

  The variable delay 10 is a delay unit for timing the video output from the video output unit 17 and the backlight dimming in the BL dimming unit 12. In backlight dimming, backlight luminance control is performed after a relatively small amount of processing if the dimming value is determined. On the other hand, the video signal gain is determined by advanced luminance modulation, the frame rate is controlled by the FRC unit 16 and converted to the panel control signal by the liquid crystal control circuit after the luminance level of the video signal is changed. Since many processes are performed, a time delay occurs. Then, the timing of the backlight dimming control and the video gain control that should be performed at the same time is shifted, and the balance between the backlight and the video is lost. Therefore, the backlight dimming is intentionally delayed by the variable delay 10 to match the timing of the backlight dimming control and the video gain control.

The configuration design unit 13 corresponds to the gain setting unit of the present invention, and the reference emission luminance level BL _ref determined by the BL luminance level setting unit 8 and the emission luminance level BL _reduced selected by the distortion module 5. Based on the above, the gain of the video signal is determined. In the example of FIG. 2, the levels BL _reduced and BL _ref are levels that have passed through the temporary filters 7 and 9, respectively. If the reference light emission luminance level (backlight target value) BL _ref and the selected light emission luminance level (backlight value) BL _reduced are the same, it is not necessary to change the luminance level of the video signal, and the gain is 1. . Further, when the selected light emission luminance level is lower than the reference light emission luminance level, the gain is set in a direction to increase the luminance level of the video signal according to the value.

<Detailed Description of Main Blocks for Performing Luminance Modulation Processing According to the Present Invention>
The BL luminance level setting unit 8, the configuration design unit 13, and the RGBγ / WB adjustment unit 15 will be described in this order as main blocks in the liquid crystal display device of FIG.

<< BL brightness level setting section 8 >>
The BL luminance level setting unit 8 receives the APL of the video signal detected by the APL detection unit 3 and controls based on detection information from a brightness sensor (not shown) that measures the ambient brightness (ambient illuminance). A signal and a control signal based on a user setting for setting the brightness of the liquid crystal panel are input. Also, when information such as the frequency that cannot be expressed when the video signal is expanded or information such as the minimum luminance and the maximum luminance of the video signal is used as the video feature amount, the histogram detection unit 2 outputs the video signal screen unit ( These pieces of information necessary for each frame (to be referred to as histogram information) are input. When both APL and histogram information are used, each information is input to the BL luminance level setting unit 8.

Then, the BL luminance level setting unit 8 outputs the reference light emission luminance level BL _ref based on these control signals and APL. More specifically, a method of dynamically adjusting the backlight luminance according to the APL of the input video signal that changes in screen units (frame units) is applied, and the light emission luminance level obtained thereby is used for reference light emission. Output as luminance level BL _ref .

A luminance control table (lookup table) held in the BL luminance level setting unit 8 is used to generate the reference emission luminance level BL _ref . The luminance control table defines the relationship of the light emission luminance level of the backlight according to the video feature amount (APL in this case) of the input video signal, that is, the luminance control characteristic. A plurality of selectable brightness control tables are prepared and held in a table storage memory such as a ROM (Read Only Memory) provided in the BL brightness level setting unit 8.

  For example, a photodiode is applied to a brightness sensor that measures the brightness around the liquid crystal display device. The brightness sensor generates a DC voltage signal corresponding to the detected ambient light and outputs it to a main CPU (not shown). The main CPU outputs a control signal for selecting the brightness control table to the BL brightness level setting unit 8 according to the DC voltage signal corresponding to the ambient light.

  Further, the main CPU outputs a luminance adjustment coefficient for adjusting the luminance control value of the luminance control table as a control signal based on a user setting for setting the brightness of the liquid crystal panel. The brightness adjustment coefficient is used for setting the brightness of the entire screen in accordance with a user operation. For example, screen brightness adjustment items are set on a menu screen held by the liquid crystal display device. The user can set an arbitrary screen brightness by operating the setting item. The main CPU recognizes the brightness setting and outputs a brightness adjustment coefficient to the BL brightness level setting unit 8 according to the set brightness.

The BL luminance level setting unit 8 selects a luminance control table by designating a table No. according to a control signal output from the main CPU according to the detection information of the brightness sensor. Alternatively, the luminance control table to be selected may be generated by calculation. Then, the brightness conversion value of the selected brightness control table is multiplied by the brightness adjustment coefficient obtained as a control signal based on the user setting, and the slope of the brightness control characteristic of the brightness control table is changed, and finally, for reference A brightness control table used to generate the light emission brightness level BL _ref is determined. Then, the BL luminance level setting unit 8 uses the luminance control characteristic of the determined luminance control table to generate and output a reference emission luminance level BL _ref according to the APL output from the APL detection unit 3.

As described above, the luminance control table defines the relationship between the APL that is the feature amount of the input video signal and the light emission luminance level of the backlight. For example, when the APL is large, the light emission luminance level of the backlight is small. By setting so as to be, the luminance of the backlight is suppressed so as not to feel dazzling in the case of a high-luminance image. In accordance with the luminance control characteristics of the luminance control table, the light emission luminance level BL _ref dynamically changes according to the change in the APL of the video signal. In the present invention, the brightness control characteristics in the brightness control table are not particularly limited, and those that prescribe characteristics that dynamically change the light emission brightness level of the backlight according to the feature amount of the input video signal are appropriately selected. Can be applied.

In this way, the BL luminance level for reference light emission luminance level BL _ref output from the setting section 8, after being delayed by the action of the first temporary filter 7, and input to the configuration design portion 13, operation of the image gain And is input to the distortion module 5 and used to determine the emission luminance level BL _reduced according to the histogram.

Hereinafter, an example of the process of selecting the light emission luminance level of the backlight executed by the distortion module will be described.
In the present invention, as described above, in order to suppress a change in hue due to saturation of the RGB output value at the time of gain setting, the lower limit value of the light emission luminance level of the backlight is limited according to the maximum value of the histogram of the video, The rise is suppressed and the color change is suppressed. That is, the lower limit value is limited with respect to the light emission luminance level of the backlight calculated based on the evaluation value by the distortion module.
In the following, first, a process for selecting the light emission luminance level based on the evaluation value of the histogram of the video before limiting the lower limit value of the light emission luminance level will be described.

  FIG. 3 is a diagram for explaining an example of the light emission luminance level selection process executed by the distortion module in the liquid crystal display device of FIG. h1 represents a Y histogram of the video signal. Here, the horizontal axis represents the input gradation of the video signal (also referred to as a pixel value or video signal level that can be taken as the video signal), and the vertical axis represents the frequency of each video signal level.

  For such a video histogram h1, A is a video luminance range that can be displayed when the light emission luminance level of the backlight is 100% in the liquid crystal panel to be used. Also, let B be the video luminance range that can be displayed on the liquid crystal panel of the target CR. Also, let C be an image luminance range that can be displayed at a specific light emission luminance level among the light emission luminance levels that can be selected by the distortion module 5. In the histogram h1, the portions that overlap the video luminance range B on both sides of the video luminance range C are the portions to be subjected to the above-mentioned numericalization, and are evaluation value calculation portions. Of the evaluation value calculation portion, the low luminance portion is D1 and the high luminance portion is D2.

The evaluation value (Distortion) is calculated by the following equation (1) based on the frequency and weighting with respect to the selectable light emission luminance level.
Distortion = Σ {(frequency of image luminance range D1 + D2) × (distance weight)} (1)

  As the weight, a distance weight that increases as the distance from the image luminance range C that can be displayed at the light emission luminance level that is an evaluation value calculation target increases. Here, the distance weight of the low luminance portion D1 is E1, and the distance weight of the high luminance portion D2 is E2. Therefore, even if the frequency value is the same, the evaluation value becomes larger as it is far from the range that can be expressed. This is because the farther from the range that can be expressed, the greater the influence that cannot be expressed as video. The values calculated by the frequency and the weight are F1 (low luminance part) and F2 (high luminance part). The evaluation value is the sum of the areas (cumulative total) of F1 and F2.

In the distortion module 5, the light emission luminance level corresponding to the video luminance range C having the lowest evaluation value among the evaluation values calculated for each light emission luminance level is selected as the light emission luminance level BL _reduced to be output. At this time, the distortion module 5 corresponds to the video luminance range C having the lowest evaluation value within a range not _exceeding the light emission luminance level BL _ref set by the BL luminance level setting unit 8 and relaxed by the second temporary filter 9. The emission luminance level BL _reduced to be selected is selected.

  Ideally, the evaluation value is calculated for all selectable light emission luminance levels in the distortion module 5. However, since there is a limitation on the processing time or the like, the luminance control range of selectable light emission luminance levels may be equally divided, and for example, the light emission luminance level may be calculated for about 10%.

That is, the video luminance range that can be displayed at the specific light emission luminance level of the above equation (1) is set as C, and the selectable light emission luminance levels are sequentially applied, and the evaluation value is calculated for each light emission luminance level. Then, the light emission luminance level having the lowest evaluation value among the calculated evaluation values is set as the selected light emission luminance level BL _reduced . Here, if the selected light emission luminance level is higher than the lower limit value of the light emission luminance level according to the maximum value of the histogram, this value is output to the first temporary filter 7 to be used for dimming control of the backlight. This is output to the unit 13 and used for setting (calculation) of the video gain.
If the emission brightness level with the lowest evaluation value is lower than the lower limit value of the emission brightness level according to the maximum value of the histogram, the lower limit value determined according to the maximum value is used for gain setting. _Is output as the emission luminance level BL _reduced .

  The selection process in the distortion module 5 will be described with specific numerical values with reference to FIGS. FIG. 4 is a diagram for explaining a specific example of the luminance modulation processing in the liquid crystal display device according to the present invention, and is a diagram showing an example of the relationship between the panel CR and the target CR in the video histogram. Here, the maximum luminance of the liquid crystal panel is 450 cd when the CR (panel CR) of the liquid crystal panel to be used is 2000, the target CR is 3500, the luminance control range of the backlight is 20 to 100%, and the backlight luminance is 100%. To do. Moreover, each alphabet symbol in FIG. 4 is based on FIG.

  In this example, the video luminance range A that can be displayed on the liquid crystal panel to be used is 450 cd to 0.225 cd. In addition, the target video luminance range B that can be displayed on the liquid crystal panel is 450 cd to 0.128 cd. Then, the frequency for each video signal level 0 to 255 is assigned so as to match the video luminance range B. In this case, the difference between the video luminance range A and the video luminance range B is about 5 digits.

If there is an image in the difference between the image luminance range B and the image luminance range A in the histogram h1, the luminance expression closer to the target CR can be expressed by lowering the light emission luminance level of the backlight. However, if an image is distributed on the high luminance side, a portion that cannot be expressed by reducing the light emission luminance level of the backlight occurs. Therefore, as described above, the evaluation value is calculated to obtain the optimum light emission luminance level BL _reduced .

  FIG. 5 is a diagram showing a video luminance range C when the light emission luminance level is 100%, which is one of the selection targets, and FIG. 6 is a video luminance range when the light emission luminance level is about 70%, which is one of the selection targets. FIG. 7 is a diagram showing a video luminance range C at a light emission luminance level of about 50%, which is one of the selection targets. Each alphabet symbol in FIGS. 5-7 is based on FIG.

  As shown in FIG. 5, when the light emission luminance level is 100%, the evaluation value F1 for the low luminance portion has a certain value, and the evaluation value F2 for the high luminance portion has no value. . Further, as shown in FIG. 6, when the emission luminance level is lowered to about 70%, both the low luminance portion evaluation value F1 and the high luminance portion evaluation value F2 have low values. Further, as shown in FIG. 7, when the emission luminance level is lowered to about 50%, the evaluation value F1 for the low luminance portion has no value, and the evaluation value F2 for the high luminance portion has a large value. Comparing the areas (cumulative values) of the evaluation value calculation results at the respective light emission luminance levels exemplified in FIGS. 5 to 7, the light emission luminance level is the lowest when it is 70%. Therefore, the distortion module 5 selects the light emission luminance level of 70%.

  Then, the distortion module 5 emits the value of the selected light emission luminance level of 70% if the value of the selected light emission luminance level of 70% is equal to or greater than the lower limit value of the light emission luminance level determined according to the maximum value of the evaluated histogram. Output as luminance level. Further, if the selected value of 70% of the light emission luminance level is lower than the lower limit value of the light emission luminance level determined according to the maximum value of the evaluated histogram, the value of the light emission luminance level determined as the lower limit value is output as the light emission luminance level. To do.

<< Configuration Design Section 13 >>
A basic model showing the relationship between the pixel value input to the liquid crystal panel and the display brightness on the liquid crystal panel is expressed by the following equation (2). Here, Y is the display luminance on the liquid crystal panel, BL is the light emission luminance level of the backlight (backlight DUTY), and CV (Code Value) is the pixel value input to the liquid crystal panel. In this example, it is assumed that the gradation of the video signal is quantized from 0 to 255.
Y = BL (CV / 255) ^γ (2)

The configuration design unit 13 adjusts the video gain so that the luminance on the screen is increased when the luminance of the backlight is _reduced by the luminance level BL _reduced selected by the distortion module 5. When the pixel value to which the gain is applied is CV _reduced , the screen brightness (display brightness on the liquid crystal panel) when the light emission brightness level is reduced is BL _reduced (CV _reduced / 255) ^γ . On the other hand, the brightness of the screen when the backlight is controlled at the reference emission luminance level BL _ref is BL _ref (CV _ref / 255) ^γ . The pixel values may be determined so that these values are equalized and the decrease in the light emission luminance of the backlight caused by the light emission luminance level BL _reduced is compensated. That is, the configuration design unit 13 may perform a gain setting that satisfies the following expression (3).
Y = BL _reduced (CV _reduced / 255) ^γ = BL _ref (CV _ref / 255) ^γ (3)

Therefore, the gain (G) is expressed by the following equation (4). For example, when the reference light emission luminance level BL _ref is 100%, the following equation (5) is obtained. Note that it is preferable to store the relationship between BL _ref and BL _reduced as a lookup table in the ROM of the configuration design unit 13 or the like so that the arithmetic processing of the following expression (4) is executed at high speed.

G = CV _reduced / CV _ref = (BL _ref / BL _reduced ) ^{1 / γ} (4)
G = (1 / BL _reduced ) ^{1 / γ} (5)

(Example of luminance modulation control using the lower limit of the backlight luminance level)
When the backlight emission luminance level and the video signal gain adjustment are executed in conjunction with each other, for example, as one tendency, the output value is saturated when the difference between the maximum RGB value and the Y value of the video image is large (saturation). Is likely to occur. This is because the maximum value of RGB is high and saturation is likely to occur even with a relatively small gain. In addition, when the backlight emission brightness level BL _ref is dynamically changed according to the change in the APL of the video signal, when the control is performed to increase the backlight emission brightness level in a darker screen. In other words, the lower the Y value of the video signal, the higher the emission luminance level BL _ref , and the gain setting value tends to increase from the above equation (4).

Here, an RGB value in which the difference between the maximum RGB value and the Y value of the video is large and the color change at the time of gain setting is large is considered. The above difference becomes maximum when (R, G, B) = (0, 0, 255), but saturation does not occur in the case of a monochrome image.
The color change is caused by setting the gain. The gain G set by advanced luminance modulation is G = (BL _ref / BL _reduced ) ^{1 / γ} , as shown in the above equation (4).

Further, the adjustment range of the light emission luminance level of the backlight cannot be set without limitation, and a lower limit value is provided. For example, in a CCFL (Cold Cathode Fluorescent Lamp) used as a backlight, if the dimming level is lowered too much, it becomes unstable and uneven brightness occurs. An example of such a lower limit level is a dimming level of about 20%. For this reason, even when the advanced luminance modulation is performed, the light emission luminance level BL _reduced of the backlight is set in a range from a minimum of 20% to a maximum of 100%.

According to such a backlight adjustment range, the maximum gain that can be obtained with this advanced luminance modulation is when γ is 2.2.
G _max = (BL _ref / BL _reduced ) ^{1 / γ} = (100/20) ^{1 / 2.2} = 2.07
It becomes. That is, the gain may take a value exceeding 2. In some cases, the upper limit is limited to about 2 gains due to restrictions on hardware such as a CPU.

  In the example as described above, if the RGB values of the input video are all 128 or less, saturation does not occur even when the gain is set to the maximum of 2 (when the expression gradation is 0 to 255). Therefore, no color change occurs when the gain is set. Conversely, when any of RGB values is greater than 128, saturation occurs depending on the gain setting. Therefore, in a display panel in which the output value is likely to be saturated when the difference between the RGB maximum value and the Y value of the video is large, when any of the RGB values is 128 or more and the Y value is relatively small Saturation tends to occur, and color change occurs at this time.

In the embodiment according to the present invention, the lower limit value of the light emission luminance level BL _reduced of the backlight is determined according to the maximum value of the Y histogram for the purpose of suppressing the color change due to the saturation of the output value at the time of gain setting. That is, by preventing the emission luminance level BL _reduced from being lowered to a value lower than a predetermined lower limit value according to the characteristics of the video, the gain is prevented from becoming high and saturation is not caused.

Here, in the embodiment according to the present invention, attention is given to a skin color that is easily changed by human eyes, and control is performed so that the skin color does not change when gain is set.
For example, as an example of RGB values indicating skin color, bright skin color (R, G, B) = (239, 193, 143), intermediate skin color (R, G, B) = (230, 160, 80), Consider a dark skin color (R, G, B) = (150, 100, 90).
When Y values are calculated for these skin colors by a conversion formula according to ITUT-R BT.601, Y = 201 for (R, G, B) = (239, 193, 143), Y = for (230, 160, 80). 171 and (150, 100, 90), Y = 113.

Consider the dark skin color, ie, (R, G, B) = (150, 100, 90). Here, when Y = 113, the value of R, which is the maximum value of RGB, is 150. At this time, the gain setting value at which R does not saturate is 255/150 = 1.7.
In order to obtain a gain of 1.7, when BL _ref is 100%, the value of BL _reduced (referred to as limit) can be expressed as (100 / limit) ^{1 / 2.2} = 1.7. From this, limit = 79%.

That is, in the example of dark skin color, in order to suppress the gain to 1.7 or less that does not cause saturation, it is necessary to prevent the backlight luminance level from dropping below 79%.
In advanced luminance modulation, the light emission luminance level with the least distortion in the luminance control range of the backlight is selected according to the state of the Y histogram, but in order to suppress color changes such as dark skin color, Control is performed so as not to drop the light emission luminance level to a value lower than the lower limit.
Here, the control range is determined by focusing on dark skin color. When the skin tone is light, the image is bright and the maximum value of the luminance histogram is high, so that it does not matter so much to the eye. On the other hand, the dark skin color is likely to notice a color change when saturated.

  FIG. 8 is a diagram for explaining an example of lower limit control of the light emission luminance level of the backlight. As described above, in the embodiment according to the present invention, in the advanced luminance modulation control, the lower limit value of the backlight emission luminance level is set according to the maximum value of the histogram, and the backlight emission luminance level is set to a value lower than the lower limit value. Do not lower. This suppresses the saturation of the RGB output value due to the gain increase so that the color change (particularly the skin color change) at the time of gain setting is not noticeable.

Specifically, as shown in FIG. 8, a lower limit value of the backlight emission luminance level BL _reduced at the time of advanced luminance modulation is set according to the maximum value of the histogram (Y histogram), and the control range is limited. The maximum value of the histogram is the maximum luminance value counted in the histogram.
In the example of FIG. 8, when the maximum value of the histogram is in the range of 100 to 255, the lower limit value of the backlight emission luminance level BL _reduced is set to 80%. Thereby, in this range, when determining the backlight emission luminance level BL _reduced according to the distortion obtained from the histogram, it is not set to a value smaller than 80%, but is controlled in the range of 80 to 100%.

That is, the distortion module 5 selects, as BL _{reduced, the} light emission luminance level corresponding to the video luminance range with the lowest evaluation value among the evaluation values calculated for each light emission luminance level by the processing as described above. At times, when the maximum value of the histogram is in the range of 100 to 255, if the BL _reduced selected according to the evaluation value is lower than 80%, all are set to 80% and are not output.
Then, the configuration design unit 13 uses the set BL _reduced to determine the gain G according to G = (BL _ref / BL _reduced ) ^{1 / γ} . Thereby, saturation of RGB values due to gain increase can be suppressed, and color change at the time of gain setting can be suppressed.

By preventing BL _reduced from being _reduced to a value smaller than 80%, for example, even in the case of the dark skin color (Y = 113), it can be maintained at a value larger than 79% at which saturation does not occur. Color change can be suppressed.

Further, when the maximum value of the histogram is smaller than 100, the lower limit value of the light emission luminance level of the backlight is linearly reduced toward the maximum value 0 and controlled as shown by the dotted line in FIG. . In a dark image with a low maximum value, the RGB value is low, so it is difficult to saturate even if the gain is increased. Accordingly, the light emission luminance level BL _reduced of the backlight is _reduced according to the distortion evaluation value within a range not lower than the lower limit value, and the gain is increased accordingly.

For example, an image of (R, G, B) = (64, 64, 255) has Y = 85. For example, when a gain of 1.7 is applied to this image, (R, G, B) = (108, 108, 255) after the gain is set. That is, B is saturated and remains at 255. In this case, the color changes depending on the gain setting. The degree of change is a trade-off with the lower limit value of the light emission luminance level BL _reduced of the backlight. That is, if the lower limit level is high, the gain increase is suppressed and the degree of color change due to saturation is reduced.
However, the color change of the above image is not as noticeable as the skin color. Even when a gain of 1.7 is given as described above, the color changes, but it becomes a little lighter, and is a level that does not bother much to the eye.

  That is, by performing the above control, the color change due to the gain setting cannot be suppressed for all colors, but the skin color change that can be perceived sensitively to human eyes can be effectively suppressed. For an image with low luminance, it is possible to realize power saving while maintaining video quality by sufficiently reducing the light emission luminance level of the backlight.

FIG. 9 is a diagram for explaining another example of the lower limit control of the light emission luminance level of the backlight. In the control example of FIG. 9, when the maximum value of the histogram is in the range of 100 to 255, the lower limit value of the light emission brightness BL _reduced of the backlight is set to 80%. In this example, the BL _reduced in this range is always set to 100%. Yes. That is, in the case of this control example, when the maximum value of the histogram is 100 to 255, BL _{reduced is set} to 100% regardless of the evaluation value so as not to be lowered. As a result, a gain larger than 1 is not given, and the color change of a bright image can be completely suppressed. Especially, the skin color is completely changed regardless of whether the skin color is light or dark. Can be suppressed.

8 and 9, in the range where the maximum value of the histogram is smaller than 100, not only the linear control characteristic as shown by the dotted line in each figure but also stepwise from the maximum value 100 to 0. It may be controlled as will drop the lower limit while changing the inclination, or the maximum value 100 is smaller than the range, without imparting the lower limit of BL _reduced, there in the control characteristic to set the BL _reduced in accordance with the evaluation value May be. In addition, it is needless to say that the range in which the lower limit value of the histogram is constant is set to 100 to 255 is an example of control, and the range can be set as appropriate.

  Further, when detecting the maximum value of the histogram, in the above example, for example, the maximum value of the luminance value of 0 to 255 gradations is detected. However, the control is simplified and 0 to 255 are divided into a plurality of regions. (16 divisions, 32 divisions, etc.), a divided region including the maximum luminance value may be detected, and a lower limit value of the light emission luminance level of the backlight may be determined for each divided region.

Alternatively, when the maximum value is detected, when there are a predetermined number of the same luminance value, the luminance value may be detected as the maximum value. For example, when there are two or three or more counts in the same luminance value, it can be determined that the luminance value is detected as the maximum value. Alternatively, control such as detection as a maximum value when the total count exceeds a predetermined percentage may be performed. At this time, the gradation of 0 to 255 is divided into a plurality of areas as described above, and the histogram count in the area is a predetermined number (one, two, three or more, a predetermined% or more, etc.). In this case, the area may be detected as the maximum value area, and BL _reduced may be determined according to the control characteristic of the lower limit value determined according to the area.

It is a graph which shows the relationship between an input gradation level (video signal level) and the luminance value on a liquid crystal panel about liquid crystal panels with CR of 3000 and 6000. It is a block diagram which shows the system configuration example of the liquid crystal display device which concerns on one Embodiment of this invention. It is a figure for demonstrating an example of the light emission luminance level selection process performed with the distortion module in the liquid crystal display device of FIG. FIG. 3 is a diagram for explaining a specific example of advanced luminance modulation processing in the liquid crystal display device of FIG. 2. It is a figure which shows the video-luminance range C in the case of the light emission luminance level 100% which is one of the selection objects. It is a figure which shows the image luminance range C when the light emission luminance level which is one of the selection objects is about 70%. It is a figure which shows the image luminance range C when the light emission luminance level which is one of the selection objects is about 50%. It is a figure for demonstrating an example of the lower limit control of the light emission luminance level of a backlight. It is a figure for demonstrating the other example of lower limit control of the light emission luminance level of a backlight.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Scaling part, 2 ... Y histogram detection part, 3 ... APL detection part, 4 ... Histogram stretching part, 5 ... Distortion module, 6 ... Scene change detection part, 7 ... 1st temporary filter, 8 ... BL brightness level Setting unit, 9 ... second temporary filter, 9a ... third temporary filter, 10 ... variable delay, 11 ... CPU / CPLD, 12 ... BL dimming unit, 13 ... configuration design unit, 14 ... image quality correction unit, DESCRIPTION OF SYMBOLS 15 ... RGB (gamma) / WB adjustment part, 16 ... FRC part, 17 ... Video output part, 20 ... Advanced brightness | luminance modulation part.

Claims (4)

In a display device that includes a liquid crystal panel that displays an image based on an input video signal, and a light source that illuminates the liquid crystal panel, and sets the emission luminance of the light source and the gain of the video signal in conjunction with each other.
A luminance level setting unit that sets a reference emission luminance level based on a luminance control characteristic that defines the emission luminance of the light source with respect to a feature amount of an input video signal;
A histogram detector for detecting a histogram of luminance values of the input video signal;
A luminance level selection unit for selecting a light emission luminance level used for controlling the light source from the histogram;
A gain setting unit that sets a gain of a video signal using the light emission luminance level selected by the luminance level selection unit and the reference light emission luminance level set by the luminance level setting unit;
The display device according to claim 1, wherein the brightness level setting unit limits a lower limit value of a light emission brightness level of the light source according to a maximum brightness value of the histogram. The display device according to claim 1,
Means for preliminarily storing a control characteristic for determining a lower limit value of the light emission luminance level according to the maximum luminance value of the histogram;
The luminance level selection unit uses a preset target contrast of the liquid crystal panel, and, in the histogram of the input video signal, the video luminance range of the input video signal that can be displayed on the liquid crystal panel at the target contrast. Among them, a process of obtaining an evaluation value by multiplying a frequency of an image in a video luminance range that cannot be expressed at a specific light emission luminance level of the light source by a predetermined weighting factor is executed for all the light emission luminance levels of the selectable light source. , Extracting the lowest emission luminance level of the evaluation value in a range not exceeding the reference emission luminance level set by the luminance level setting unit,
With reference to the control characteristic, if the lower limit value according to the maximum luminance value of the histogram of the input video signal is higher than the extracted light emission luminance level, the lower limit value is output as the selected light emission luminance level. Characteristic display device.   3. The display device according to claim 2, wherein the control characteristic for determining the lower limit value of the light emission luminance level has a constant lower limit value in a range on a predetermined high luminance side of a range that the maximum luminance value of the histogram can take. A display device characterized by the above.   4. The display device according to claim 1, wherein the gain setting unit divides the light emission luminance level for reference by the selected light emission luminance level, and then raises the power by 1 / γ. A display device characterized in that a gain to be calculated is calculated.

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