JP5236622B2 - Display device - Google Patents

Description

  The present invention relates to a display device, and more particularly to a display device having a function of adjusting a light emission luminance of a light source that illuminates a display panel in conjunction with a feature amount of a video signal and a tone curve of the video signal.

In a display device using a liquid crystal panel such as a liquid crystal television, as a measure for reducing power consumption while maintaining video quality, the luminance of the backlight that illuminates the liquid crystal panel and the gain of the video signal are linked. Those that have a function to set are known.
For example, by detecting the feature amount of the video signal and reducing the backlight emission brightness in the case of low dynamic range video (dark video), the video gain is compensated for the decrease in backlight emission brightness. As a result, the brightness of the display screen is kept constant while reducing the power consumption of the backlight.

  For example, Patent Document 1 discloses a technique for detecting a peak level of a video signal and improving a white peak luminance in a bright scene. There has been disclosed a display device that controls the light while adjusting the video amplitude gain to improve the peak luminance in a dark scene.

WO 03/075257

  In the display device having the function of controlling the backlight luminance and the video signal in conjunction with each other as described above, the surroundings are in a very bright environment, and a bright image is displayed and the backlight luminance is displayed. When the level was lowered, it sometimes seemed dark to the human eye. For example, in stores that sell display devices, etc., the environment is brighter in each stage than the viewing environment such as ordinary homes. In such an environment, when the backlight for power saving is reduced, the screen brightness is reduced. Even if the video signal was gained and amplified in order to maintain it, the bright screen looked dark and it seemed that the quality was degraded.

  Further, in a display device that controls the light emission luminance of a backlight and a video signal as described above, it is common to use APL (Average Picture Level) as a video feature quantity for controlling the light emission luminance and the video signal. Is. However, since APL is the average level of signal luminance, it is different from the average level of display luminance of the liquid crystal panel. It is known that when the luminance gain of a video signal is determined using APL, it may be dark at an intermediate luminance.

  In such a case, with the conventional control function, the dynamic range of the video is extended to the limit in accordance with the decrease in the light emission luminance of the backlight, and therefore other video processing functions cannot be used. As a result, maintaining the brightness of the input video signal is a limit, and sufficient video processing cannot be performed, and the viewer feels that the display device with the above interlocking function has a dark screen. Was given. Here, if the gain of the video signal is simply increased, for example, the high gradation white is crushed and cannot be expressed, and there is a problem that the video is broken.

  For this reason, when controlling the luminance of the backlight and the video signal in conjunction with each other, even in a bright environment, the screen can be further displayed to the viewer without increasing the power consumption and without destroying the video. There was a problem of wanting to look bright. In particular, it is preferable to perform luminance and signal control so that humans do not feel dark by performing control using video feature amounts that match human visual characteristics.

  The present invention has been made in view of the above circumstances, and has 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 the light emission luminance of the light source and the gain of the video signal. In a display device that is set in conjunction with the above, the display device is designed to make the screen appear bright even in a bright environment without increasing power consumption and without destroying the video. It is what.

In order to solve the above-described problem, a first technical means of the present invention provides a pixel of an input video signal in a display device including a liquid crystal panel that displays an image by an input video signal and a light source that irradiates the liquid crystal panel and geometric mean calculating section for calculating the geometric mean value of the mean logarithm of the value raised to the power, to dynamically control the emission luminance of the light source based on the geometric averages the geometric average calculation unit has calculated A light source luminance control unit; and a luminance level selection unit that selects a light emission luminance level used for controlling the light emission luminance of the light source based on a feature amount of the input video signal, the light source luminance control unit including the luminance The light emission luminance level selected by the level selection unit is corrected based on the geometric average value calculated by the geometric average calculation unit, thereby dynamically controlling the light emission luminance of the light source. It is obtained by, characterized in that.

According to a second technical means, in the first technical means, the light source luminance control unit is based on a relationship between a predetermined geometric average value and a correction value for correcting the light emission luminance of the light source. The light emission luminance level selected by the luminance level selection unit is corrected, and the relationship between the geometric average value and the correction value increases the light emission luminance when the geometric average value is larger than a predetermined value. When the geometric average value is less than or equal to the predetermined value, the light emission luminance level selected by the luminance level selection unit is maintained.

According to a third technical means, in the first or second technical means, a tone curve generation unit for generating a tone curve to be applied to a video signal in conjunction with the light emission luminance of the light source, and the geometric average value A black level correction unit for converting a video signal of a low gradation region in a range determined based on a correction value of light emission luminance of the light source based on a black gradation value of zero gradation, The tone curve generated by the tone curve generation unit is corrected by the black correction data generated by the black level correction unit.

According to a fourth technical means, in the third technical means, a gain corresponding to the geometric average value calculated by the geometric average calculation unit is calculated based on a predetermined relationship between the geometric average value and the gain. A gain calculation unit that calculates, and a correction tone curve generation unit that generates a correction tone curve for correcting the tone curve generated by the tone curve generation unit, using the gain calculated by the gain calculation unit. a tone curve which is pre-Symbol correction tone curve generator for generating the horizontal axis the input tone, has a convex shape on the when taking output gradation on the vertical axis, the intermediate gray level region A convex toe curve having a convex apex, wherein the convex tone curve applies the gain calculated by the gain calculation unit in a low gradation specific region, and the applied gain is increased as the gradation becomes higher. To the tone curve of a convex shape that is generated by reducing the emissions is obtained by being a tone curve modified so that the 1 the gain of a specific region of the low gradation portion.

According to a fifth technical means, in the fourth technical means, the relationship between the predetermined geometric average value and the gain is set to a gain greater than 1 when the geometric average value is a specific value or less. When the geometric average value is larger than the specific value, the gain is set to 1.

In a sixth technical means, in any one of the third to fifth technical means, a reference light emission luminance level is set based on a luminance control characteristic defining the light emission luminance of the light source with respect to a feature amount of an input video signal. A luminance level setting unit to be set, and the luminance level selection unit uses a histogram of the input video signal, and emits light from the light source in a video luminance range that can be displayed at a target contrast set in the liquid crystal panel. The frequency of an image that cannot be expressed on a histogram that changes according to the luminance level is evaluated, and based on the evaluated value, the light emission luminance level used for controlling the light emission luminance of the light source is selected, and the tone curve generation is performed. A luminance level selected by the luminance level selection unit and a reference light emission luminance level output from the luminance level setting unit. A gain to be used is determined, the determined gain is applied to a low gradation portion, and a tone curve is generated in which a gain smaller than the determined gain is applied to a high gradation portion.

A seventh technical means is the sixth technical means, wherein the brightness level selection unit is expressed in the histogram at a specific light emission brightness level of the light source in a video brightness range that can be displayed at the target contrast. Evaluation is performed by multiplying a frequency of an image in a video luminance range that cannot be performed by a predetermined weighting factor, and the evaluation is performed for all the light emission luminance levels of the light sources that can be selected, and the evaluation value obtained by the evaluation is the lowest. The luminance level is selected within a range that does not exceed the reference emission luminance level set by the luminance level setting unit.

  According to the present invention, the display device 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 light emission luminance of the light source and the gain of the video signal in conjunction with each other. In this case, it is possible to make the screen appear bright even in a bright environment without increasing the power consumption and without destroying the video.

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 display apparatus 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 display apparatus of FIG. FIG. 3 is a diagram for describing a specific example of advanced luminance modulation processing in the 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 which shows the example of the tone curve produced | generated based on a gain setting. It is a figure which shows the response curve with respect to the brightness | luminance of a human photoreceptor cell. It is a figure for demonstrating the gain calculated by a gain calculation part, and shows the relationship between GAve and a gain. It is a figure which shows an example of the tone curve produced | generated in the tone curve design part of a geometric correction | amendment part. It is a figure which shows the tone curve corrected by the low gradation linearization process part. It is a figure which shows the more specific structural example of a backlight enhancement part. It is a figure which shows an example of the table used for calculation of a backlight gain in BL gain calculation part. It is a figure which shows the example of LUT produced | generated in the black level correction | amendment part of the backlight enhancement part.

  Hereinafter, preferred embodiments according to a display device of the present invention will be described with reference to the accompanying drawings. An 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 display device including a backlight as a light source, and the gain. Therefore, a tone curve is generated. 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.

  In the advanced luminance modulation processing, a light emission luminance level for reducing the backlight luminance is obtained from the video feature amount. Further, a reference light emission luminance level for setting the light emission luminance level of the backlight light source is set from the image feature quantity such as APL. Using the light emission luminance level for reference and the light emission luminance level for reducing the backlight luminance, a gain for maintaining visual luminance is set, and a tone curve is generated based on the gain.

On the other hand, for a video generated based on video features (here, a histogram), a geometric average value is obtained, a gain is calculated based on the geometric average value, and a correction tone curve is calculated based on the calculated gain. Generate.
Furthermore, the gain of the backlight is calculated using the geometric average value obtained from the video feature amount, the light emission luminance level obtained by the advanced luminance modulation processing is further corrected, and finally the backlight light source is controlled. Outputs the luminance level. At the same time, a tone curve (black correction tone curve) for correcting the black level of the video signal is generated using the backlight gain value calculated based on the geometric average.
Then, the correction tone curve generated by the gain based on the geometric average value and the black correction tone curve generated based on the backlight gain are applied to the tone curve obtained by the advanced luminance modulation processing. To obtain a tone curve to be applied to the input video signal.

  The geometric average value is an image feature amount that is close to the human sensory amount, and by using this, the tone curve applied to the image signal is corrected to increase power consumption even when the surrounding environment is bright. This makes it possible to make the screen look brighter without breaking the image.

<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 / m ² , but the display luminance (minimum luminance) on the liquid crystal panel at the input gradation (pixel value) 0 is 0.15 cd / m ² in the case of CR3000 and 0 in the case of CR6000. 0.075 cd / m ² , which is twice as large.

  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 it is possible to express the luminance close to that of a liquid crystal panel of CR6000 when the pixel value is 0 to 187 (γ = 2.2). However, an image with a pixel value larger than 187 cannot be expressed with 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 187 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 having a pixel value of 187 or less has been described as an example. However, in addition to the above example, for example, when the white portion in the video is extremely small, It is possible to reduce the importance of the portion and improve the black expression 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.

  In the embodiment according to the present invention, the degree of amplification (gain) of the input video signal is adjusted according to the video feature amount of the input video signal. 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 gain. Here, the light emission luminance level for reference for setting the light emission luminance level of the backlight light source is set from the image feature quantity such as APL. The backlight brightness level is determined using the reference brightness level and the image histogram, and a gain is set to maintain visual brightness in conjunction with the brightness level. Based on the gain, To generate a tone curve.

Further, for the video generated based on the video feature amount (here, a histogram), a geometric average value is obtained, a gain for amplifying the video signal is calculated based on the geometric average value, and an intermediate value is calculated based on the gain. A correction tone curve that raises the gradation is generated. Separately from this, the backlight gain is calculated based on the above geometric average value, and the light emission luminance level of the backlight is corrected based on the backlight gain. In particular, here, the light emission luminance level is increased on a bright screen so that a bright screen does not look dark in a bright environment. At this time, in order to maintain the contrast of the low gradation image, a tone curve for black correction is generated based on the backlight gain.
Then, the tone curve linked to the light emission luminance level of the backlight is corrected by the correction tone curve and the black correction toe curve and applied to the video signal.

The geometric average value is an image feature amount that is close to the human sensory amount, and by using this to correct the tone curve applied to the image signal, the image is broken without increasing power consumption. You can make the screen look brighter.
Hereinafter, the configuration and operation of the embodiment according to the present invention will be described more specifically.

<System Configuration Example of Display Device for Performing Luminance Modulation Processing According to the Present Invention>
FIG. 2 is a block diagram showing a system configuration example of the display device according to the present invention. 2 includes a histogram / APL detection unit 1, a BL (backlight) luminance level setting unit 2, an advanced luminance modulation unit 3, a geometric correction unit 4, an image quality correction unit 5, an RGB adjustment unit 6, and an FRC. (Frame Rate Control) unit 7, multiplier 8, and backlight enhancement unit 9.

  The advanced luminance modulation unit 3 includes a distortion module 31, a temporary filter 32, and a tone curve design unit 33. The geometric correction unit 4 includes a geometric Ave calculation unit 41, a gain calculation unit 42, a temporary filter 43, a tone curve design unit 44, and a low gradation linearization processing unit 45.

The processing by the advanced luminance modulation unit 3 not only performs dynamic light source luminance control according to the feature quantity such as APL but also the light emission reference light source brightness determined by a predetermined condition of the video feature quantity. This is a process of selecting a light emission luminance level BL _reduced that gives more contrast to the level BL _ref . Here, a gain to be applied to the video signal is selected based on the light emission luminance level BL _reduced and the reference light emission luminance level BL _ref, and a tone curve based on the gain is generated.

  Further, the geometric correction unit 4 calculates a geometric average value (hereinafter referred to as GAve or geometric Ave) of the video signal using a histogram of the video signal, calculates a gain based on the GAve, and A correction tone curve for correcting the video signal is generated based on the gain. By correcting the tone curve generated by the advanced luminance modulation unit 3 with the correction tone curve, it is possible to perform video setting more suitable for human vision.

  Further, the backlight enhancement unit 9 calculates the gain of the backlight using the geometric average value obtained by the geometric calculation unit 41, and applies it to the light emission luminance level obtained by the advanced luminance modulation processing. A light emission luminance level for controlling the backlight light source is output. At the same time, the backlight enhancement unit 9 uses the backlight gain value calculated based on the geometric average to generate a tone curve (black correction tone curve) for correcting the black level of the video signal. The tone curve generated by the advanced luminance modulation unit 3 is corrected.

First, the outline of each block in the display device of FIG. 2 will be described.
The input video signal includes, for example, a demodulated video signal received as a broadcast wave, a video signal received via a communication network, a signal read from a video signal stored in an internal storage device, various recorders, various players, and tuner devices For example, a video signal received from an external device or the like, or a video signal obtained by performing various types of video processing on the video signal. The display device of FIG. 2 may be configured to be able to acquire any of such video signals. The input video signal can be obtained by scaling 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. .

  The histogram / APL detector 1 is a block that detects a Y histogram and APL from an input video signal. In the histogram detection, a video frame is divided into pixel units or the like, and a histogram representing the frequency of occurrence of the luminance value of each pixel is generated. The generated histogram has a frequency value for each of the luminance values (Y) 0 to 255, for example. In APL detection, the average luminance level of the video signal is calculated for each video frame. The calculated value is 0% when the entire screen is black, and 100% when the entire screen is white.

  Further, the histogram / APL detection unit 1 can perform histogram stretching for setting a range to be used by the advanced luminance modulation unit 3 from the generated Y histogram. For example, it is assumed that the distortion module 31 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 frequency value for each of the minimum value 10 to the maximum value 235 is stretched so as to be applied to the frequency value for each of the minimum value 0 to the maximum value 255 in order to match the calculation in the distortion module 31. Stretch.

The image quality correction unit 5 performs IP conversion or noise reduction on the video signal output from the histogram / APL detection unit 1 as necessary. Also, the contrast and color of the video are changed according to user settings and the like.
The RGB adjustment unit 6 applies a correction tone curve generated based on the processing of the advanced luminance modulation unit 3, the geometric correction unit 4, and the backlight enhancement unit 9 to the video signal output from the image quality correction unit 5. Apply to convert video signal. The tone curve is a data that is generated as an LUT (Look Up Table), in which an output value for an input value is determined in order to convert an image value of a video signal. The tone curve input to the RGB adjustment unit 6 includes a tone curve generated by the advanced luminance modulation unit 3, a tone curve generated by the geometric correction unit 4, and a black correction tone curve generated by the backlight enhancement unit 9. And obtained by correction (multiplication by the multiplier 8).

The RGB adjustment unit 6 converts the video signal output from the image quality correction unit 5 using the above-described tone curve, γ adjustment processing for image creation, WB adjustment processing, and CT ( The color temperature can be adjusted. Each process executed by the RGB adjustment unit 6 is executed independently for each of R, G, and B of the video signal. At that time, the tone curve conversion process and the γ adjustment process are performed by the same curve for R, G, and B, and the WB adjustment process / CT adjustment process is performed by a curve having different characteristics for R, G, and B respectively. Is made.
By applying the tone curve in the RGB adjustment unit 6, the correction by the geometric correction unit 4 and the backlight enhancement unit 9 are generated for the control to reduce the light emission luminance level of the backlight in the advanced luminance modulation unit 3. The image is expressed with the black correction tone curve.

  The FRC unit 7 is a frame rate converter, and detects a motion vector of a video from the adjusted video signal output from the RGB adjustment unit 6 to generate a complementary video, thereby generating a normal video from a display frequency of 60 Hz to 120 Hz. Or a display frequency higher than that. The video signal output from the FRC unit 17 is converted into a signal for driving the liquid crystal panel, and is output and displayed on the liquid crystal panel.

The distortion module 31 of the advanced luminance modulation unit 3 includes a histogram output from the histogram / APL detection unit 1 and a reference emission luminance level (also referred to as a backlight target value) set by a BL luminance level setting unit 2 described later. From BL _ref , the gain is determined, and a light emission luminance level (also referred to as a backlight value) BL _reduced for output to the backlight enhancement unit 9 is selected. The selection is performed within a range that does not exceed the reference light emission luminance level BL _ref set by the BL luminance level setting unit 2 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. The distortion parameters such as the target CR may be set from a main CPU (not shown). The distortion module 31 is a block corresponding to the luminance level selection unit of the present invention.

The temporary filter 32 is provided to prevent visual discomfort caused when the light emission luminance level BL _reduced selected by the distortion module 31 changes suddenly, and the amount of change in the light emission luminance level BL _reduced is determined. After being slowed down in time, it is output to the subsequent stage as the actually set emission luminance level BL _reduced . Here, a low-pass filter is used. In addition, since a slow change in the light emission luminance level BL _reduced may cause a sense of incongruity, a scene change detection unit (not shown) is provided, and the value of the temporary filter 32 is changed by the scene change detection signal. You may be able to make quick changes.

The BL luminance level setting unit 2 is a block corresponding to the luminance level setting unit of the present invention, and refers to video feature quantities such as APL and histogram output from the histogram / APL detection unit 1 to determine the emission luminance level of the backlight. Determine the maximum value of. At this time, the maximum value of the light emission luminance level may be determined by further referring to an OPC (Optical Picture Control; also referred to as a brightness sensor) value or a user setting value output from a main CPU (not shown). . For example, when the APL is high, an image that does not feel dazzling can be obtained by setting the maximum value of the light emission luminance level of the backlight to a low value. The maximum value of the light emission luminance level of the backlight is the reference light emission luminance level (backlight target value) BL _ref for advanced luminance modulation executed by the advanced luminance modulation unit 3.

As the video feature amount for determining the light emission luminance level BL _ref for reference, an APL or a histogram can be used as described above, and the feature amount to be used is selected according to the embodiment. For the histogram, the peak value (maximum luminance value) of the video, the ratio of the video signal included between predetermined luminances smaller than the maximum luminance, and the like are used. In this embodiment, it is assumed that APL is used as a video feature amount for determining BL _ref .

Since the selection by the distortion module 31 is performed within a range that does not exceed the reference emission luminance level BL _ref set by the BL luminance level setting unit 2, the BL luminance level setting unit 2 uses the reference emission luminance level. The maximum value of the light emission luminance level of the backlight is set as BL _ref . Further, another temporary filter (not shown) may be provided between the BL luminance level setting unit 2 and the distortion module 31.

The light emission luminance level BL _reduced output from the temporary filter 32 at the subsequent stage of the distortion module 31 is input to the backlight enhancement unit 9 and is generated based on the geometric average value output from the geometric correction unit 4. And is output as a light emission luminance level for actually controlling the backlight.
The light emission luminance level output from the backlight enhancement unit 9 is sent to a light source unit (not shown), and the backlight light emission luminance is controlled. The light source unit includes a lamp composed of an LED, a fluorescent tube, and the like, and a lamp driving circuit that drives the lamp, and constitutes a light source (backlight light source) that irradiates the liquid crystal panel from the back and side surfaces. In the luminance modulation processing according to the present embodiment, this lamp is a target for emission luminance control.

The light emission luminance level BL _ref set by the BL luminance level setting unit, the light emission luminance level BL _reduced generated by the distortion module 31, and the light emission luminance level output from the backlight enhancement unit 9 are, for example, duty signals. In the light source unit, it is converted into a signal (for example, a signal suitable for driving such as pulse width modulation) that is actually dimmed by a lamp driving circuit (for example, an inverter circuit) in accordance with the light emission luminance level output from the backlight enhancement unit 9. And the light emission luminance of the backlight is controlled. As the lamp, for example, an LED (Light Emitting Diode) can be applied, or a conventional fluorescent tube or a combination of an LED and a fluorescent tube may be used. It should be provided.

The tone curve design unit 33 is a block corresponding to the tone curve generation unit of the present invention, and the reference emission luminance level BL _ref determined by the BL luminance level setting unit 2 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, and a tone curve to be applied to the video signal is generated based on the gain. In the example of FIG. 2, BL _reduced uses a level that has passed through the temporary filter 32.
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. Specific tone curve generation processing based on the gain setting will be described later.

The geometric ave calculation unit (geometric average value calculation unit) 41 is based on a histogram that is a feature amount of the video signal detected by the histogram / APL detection unit 1 and based on a predetermined arithmetic expression (geometric average value ( Gave) is calculated. The GAve value output from the geometric ave calculation unit 41 is output to the gain calculation unit 42 of the geometric correction unit 4 and the backlight enhancement unit 9.
The gain calculation unit 42 of the geometric correction unit 4 calculates the gain based on the value of GAve. The calculated gain is alleviated by the temporary filter 43, and the tone curve design unit 44 generates a basis for the tone curve for correction. The tone curve design unit 44 corresponds to the correction tone curve generation unit of the present invention.

  The low gradation linearization processing unit 45 linearizes the low gradation of the tone curve generated by the tone curve design unit 44, thereby increasing noise and increasing black floating amount when gain is applied to the low luminance region. Try to suppress. The size of this linear region is changed according to the value of Gave. By the processing of the low gradation linearization processing unit 45, a correction tone curve for correcting the tone curve output from the advanced luminance modulation unit 3 is obtained.

  The backlight enhancement unit 9 is a block corresponding to the light source luminance control unit of the present invention, calculates the gain of the backlight using the geometric average value obtained by the geometric calculation unit 41, and is obtained by advanced luminance modulation processing. The light emission luminance level for controlling the backlight light source is finally output by applying to the light emission luminance level. The backlight gain increases the light emission luminance of the backlight when the geometric average value is larger than the predetermined value, and maintains the light emission luminance level selected by the advanced luminance modulation process when the geometric average value is lower than the predetermined value. Is set. As a result, on a bright screen, a bright image can be seen even in a bright environment due to an increase in the light emission luminance level of the backlight.

  At the same time, the backlight enhancement unit 9 uses the backlight gain value calculated based on the geometric average value to generate a tone curve (black correction tone curve) for correcting the black level of the video signal. Then, the tone curve generated by the advanced luminance modulation unit 3 is corrected. The tone curve for black correction is used to prevent the black float and maintain the contrast when the backlight brightness is brightened by applying the gain according to the geometric average value again to the light emission brightness level reduced by the advanced brightness modulation processing. This is mainly generated as a tone curve in which the output of a specific range low gradation portion of the input signal is zero (black).

  The multiplier 8 performs a correction tone curve output from the geometric correction unit 4 and a black correction tone curve output from the backlight enhancement unit 9 on the tone curve output from the advanced luminance modulation unit 3. Is multiplied to obtain a corrected tone curve. That is, with respect to the output value for each image of the tone curve output from the advanced luminance modulation unit 3, the value of the displacement from the gain 1 in the correction tone curve output from the geometric correction unit 4 is multiplied by the multiplier 8. And the multiplier 8 multiplies the value of the displacement from the gain 1 in the black correction tone curve output from the backlight enhancement unit 9 and finally the tone applied to the video signal by the RGB adjustment unit 6 Generate a curve. The RGB adjustment unit 6 performs conversion by applying the tone curve output from the multiplier 8 to the video signal output from the image quality correction unit 5.

<Description of Main Blocks for Performing Luminance Modulation Processing According to the Present Invention>
The BL luminance level setting unit 2, tone curve design unit 33, geometric correction unit 4, and backlight enhancement unit 9 will be described in more detail as main blocks in the display device of FIG.

<< BL brightness level setting unit 2 >>
The BL luminance level setting unit 2 receives the APL of the video signal detected by the histogram / APL detection unit 1. At this time, a control signal based on detection information of a brightness sensor (not shown) that measures ambient brightness (ambient illuminance) and a control signal based on user settings for setting the brightness of the liquid crystal panel are input, and these are input. Can be used to further change the backlight emission luminance.
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 quantity, the histogram / APL detection unit 1 receives 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 2.

The BL luminance level setting unit 2 outputs a 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 the luminance level BL _ref .

In order to generate the reference light emission luminance level BL _ref , a luminance control table (lookup table) held in the BL luminance level setting unit 2 is used. 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 stored in a table storage memory such as a ROM (Read Only Memory) provided in the BL brightness level setting unit 2.

  When the brightness around the liquid crystal display device is used, for example, a photodiode is applied as a brightness sensor for detecting the brightness. 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 a brightness control table to the BL brightness level setting unit 2 according to a DC voltage signal corresponding to 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 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 2 according to the set brightness.

The BL brightness level setting unit 2 selects a brightness 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 2 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 histogram / APL detection unit 1. .

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 characteristic 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.

The reference light emission luminance level BL _ref output from the BL luminance level setting unit 2 in this manner is input to the tone curve design unit 33, used for calculating the video gain, and input to the distortion module 31. It is used to determine the emission luminance level BL _reduced according to the histogram.

  FIG. 3 is a diagram for explaining an evaluation value calculation processing example executed by the distortion module 31 in the display device of FIG. In FIG. 3, h1 represents the 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 31. In the histogram h1, portions overlapping the video luminance range B on both sides of the video luminance range C are portions that are digitized as distortion, 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 formula (1) based on the frequency and weighting with respect to the selectable luminance level.
Distortion = Σ {(frequency of video 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.

The distortion module 31 calculates an evaluation value for each light emission luminance level, calculates the relationship between the evaluation value and the gain by a method described later, and determines the light emission luminance to be set for the backlight from the relationship between the evaluation value and the gain. Select level BL _reduced . The light emission luminance level BL _reduced is then output to the tone curve design unit 33 via the temporary filter 32, a gain to be applied to the video signal is determined, and a tone curve is generated based on the gain. When choosing the light emission luminance level _{BL Reduced,} in the distortion module 31, in a range that does not exceed the light emission luminance level _{BL ref} set by the BL luminance level setting portion 2, selects a light emission luminance level _{BL Reduced.}

  Ideally, the evaluation value is calculated for all selectable light emission luminance levels in the distortion module 31. 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 to calculate the evaluation value for each light emission luminance level. Then, the relationship between the calculated evaluation value and the gain is calculated. 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, and this value is output to the temporary filter 32 and used for dimming control of the backlight. It outputs to the curve design part 33, and is used for the gain setting for producing | generating a tone curve.

The selection process in the distortion module 31 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 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, when the CR (panel CR) of the liquid crystal panel 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%, the maximum luminance of the liquid crystal panel is 450 cd / and m ^2. 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 / m ^{2 to} 0.225 cd / m ² . In addition, the target image luminance range B that can be displayed on the liquid crystal panel is 450 cd / m ^{2 to} 0.128 cd / m ² . 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 and outputs a light emission luminance level of 70%.

<< Tone curve design section 33 >>
The tone curve design unit 33 first calculates a gain used to generate a tone curve.
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 tone curve design unit 33 adjusts the video gain so as to increase the brightness on the screen when the light emission brightness of the backlight is _reduced by the light emission brightness level BL _reduced selected by the distortion module 31. When the pixel value to which the gain is applied is CV _reduced , the brightness of the screen (display luminance on the liquid crystal panel) when the emission luminance level is lowered 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 a decrease in the light emission luminance of the backlight caused by the light emission luminance level BL _reduced is compensated. That is, the tone curve design unit 33 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. It is preferable that the relationship between BL _ref and BL _reduced is stored as a look-up table in the ROM of the tone curve design unit 33 and 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)

The tone curve design unit 33 generates a tone curve to be applied to the video signal according to the gain calculated by the above processing.
FIG. 8 is a diagram illustrating an example of a tone curve generated based on the gain setting. As shown in FIG. 8A, when the gain setting of the video signal calculated by the tone curve design 33 is 1.0, there is no problem by setting a linear tone curve for all luminances.
However, when the gain is 1.0 or more, as shown in FIG. 8B, when the tone curve is uniformly applied, the high gradation (high luminance) portion has a value of 100% (output pixel value 255). Is clipped, and so-called white-out occurs.

In advanced luminance modulation processing, for example, the gain may be set so that the black portion is tightened at the expense of white-out of a small number of white luminance portions. The quality will be greatly reduced.
Therefore, it is preferable to reduce the gradation in the high-brightness part by performing signal expansion according to the gain setting for low and medium brightness and making the tone curve non-linear for high brightness. To prevent). This method has a trade-off relationship between brightness and whiteout. If the non-linear region is narrowed, the region in which normal brightness can be expressed increases, but the high luminance gradation is lowered. Conversely, if the non-linear region is widened, the region where normal brightness can be expressed decreases, but the high luminance gradation is maintained to some extent. In practice, the non-linear luminance may be made non-linear only in the portion affected by the white-out, for example, by setting the output by the gain setting to 90% or more or 95% or more.

  FIG. 8C shows a tone curve that is corrected so that a portion of 90% or more becomes non-linear when the gain setting is 1.2. FIG. 8D shows a tone curve that is corrected so that a portion of 90% or more becomes non-linear when the gain setting is 1.6.

In addition, as described above, in order to avoid whiteout of a high luminance portion that occurs when the gain setting exceeds 1.0, it is necessary to make the high gradation region of the tone curve partly non-linear.
At this time, the tone curve design unit 33 cannot calculate the nonlinear tone curve as described above if the proportional calculation is simply performed based on the gain setting. Therefore, in order to obtain a non-linear tone curve, the tone curve may be stored in the memory for each gain setting, or the linear portion of the tone curve is simply proportionally calculated from the gain setting value, as shown in FIG. As exemplified in C) and (D), for the portion of 90% or more, a non-linear portion may be calculated by interpolation or the like.

<< Geometric correction part >>
As a kind of the average luminance of the video signal, there is a GAve (Geometric Average) as described above. Gave is not the average of signal luminance, but is the average luminance value calculated as the average luminance of the liquid crystal panel as a value that matches the visual characteristics. Specifically, Gave. Is represented by the following formula (6).

In the above formula (6), δ is a minute value that cannot cause the calculation to diverge, for example, δ = 0.00001. _Ylum represents the panel luminance and has a value of 0 to 1.0. _Ylum can be expressed as (signal luminance / MAX luminance) ^ γ. N represents the number of pixels, and pixels represents the total number of pixels. As described above, the expression (6) is a power of the logarithmic average of the luminance values of the pixels of the image, in other words, indicates a geometric average value of the luminance, and the value greatly depends on black. .

FIG. 9 is a diagram showing a response curve with respect to luminance of human photoreceptor cells. As shown in Fig. 9, the response curve of the human photoreceptor depends on the logarithmic luminance value (luminance (log cd / m ² ). This is generally the Michaelis-Menten equation ( Micaelis-Menten Equation).
Gave is the power of the logarithmic average of the luminance values of the pixels as described above. Therefore, it can be said that Gave is a numerical value of the eye response to the image (that is, how bright it looks). That is, Gave is close to a human sensory amount, and this value is used as a video feature amount to generate a correction tone curve, which is linked to the backlight emission luminance generated by the advanced luminance modulation unit 3. Correct the tone curve.

In the configuration of FIG. 2, the histogram of the video signal detected by the histogram / APL detection unit 1 is input to the geometric correction unit 3, and first, GAve is calculated by the geometric Ave calculation unit 41.
Here, according to the above equation (6), GAve is calculated by performing the following processing.
(S1) Normalization is performed for each pixel of the histogram, and the panel luminance value is calculated by multiplying by γ. The minimum luminance value and the panel luminance value are added, and the value of log 10 is obtained.
(S2) The result of log10 is added for all pixels.
(S3) The average exp of the result of addition is taken.

In the embodiment according to the present invention, the GAve is calculated by table conversion so that the calculation amount of the GAve does not become enormous.
Here, the above-described S1 (processing for normalizing each pixel of the histogram and multiplying by γ, calculating the panel luminance value, adding the minimum luminance value and the panel luminance value, and taking the value of log10 of that value) ) And the above-described S3 (processing for taking the average exp of the result of addition for all pixels) by table conversion. That is, the calculation of equation (6) is not performed each time, but for S1 and S3, a conversion table is stored in the memory in advance, and a desired calculation is performed by using the conversion table.

In addition, with respect to the above S2, an operation of adding the results obtained in S1 for all the pixels is performed, but here the operation is performed using a histogram, so the number of additions per frame is suppressed to 256 times. (When quantized with 256 gradations), GAve can be calculated without enormous amount of computation.
Note that the embodiment according to the present invention does not exclude the adoption of a method of obtaining the Gave by performing the calculation of the formula (6) each time. Since the value of Gave changes depending on the window size, adjustment such as changing the calculation area according to the window size may be performed.

The GAve value output from the geometric ave calculation unit 41 is input to the gain calculation unit 42, and the gain is calculated based on the GAve value.
FIG. 10 is a diagram for explaining the gain calculated by the gain calculation unit 42, and shows the relationship between Gave and gain. The gain calculation unit 42 calculates the gain based on the GAve output from the geometric ave calculation unit 41 according to the relationship shown in FIG. In this case, it is assumed that a gain of a predetermined value (here, 1 <gain ≦ 2) is obtained in a range where Gave is low luminance to medium luminance, and in the high luminance region, the gain is 1, that is, the video signal is corrected by the gain. Shall not be performed.

A MAX point is defined as a boundary between a low and medium luminance region of Gave for setting a gain larger than 1 and a high luminance region for setting a gain of 1. The MAX point is the upper limit of the area where the luminance of the video signal is increased by the gain, and is set in the intermediate luminance area of Gave.
The position of this MAX point is determined by a sensitive test or the like on the design of the display device. Alternatively, a menu for setting a MAX point may be prepared, and the MAX point may be set by a user operation. The characteristic from the MAX point to the maximum gain on the low luminance side is determined based on a curve determined by the visual characteristic. In FIG. 10, r indicates a settable range of the gain characteristic.

  The gain value calculated by the gain calculation unit 42 is then input to the temporary filter 43. The temporary filter 43 is provided for the same purpose as the temporary filter 32 provided in the advanced luminance modulation unit 3, and is for suppressing an excessive gain change due to a sudden change in the input video signal. Here, a low-pass filter having the same characteristics as the temporary filter 32 of the advanced luminance modulation unit 3 can be used as the temporary filter 43.

  The tone curve design unit 44 generates a correction tone curve LUT according to the gain output from the temporary filter 43. The tone curve design unit 33 of the advanced luminance modulation unit 3 generates a polygonal tone curve as shown in FIG. 8C or 8D. The tone curve design unit 44 generates an upwardly convex curved tone curve as shown in FIG. This is mainly to ensure screen brightness in the intermediate luminance region.

For example, if the video signal is corrected with the tone curve being linear, the video signal may be saturated and the video may become unnatural. In the present embodiment, a correction tone curve is generated based on the gain calculated by the gain calculation unit 42 so that the display image looks bright while preventing saturation of the video signal.
Here, as shown in FIG. 11, the gain calculated by the gain calculation unit 42 is reflected as it is to obtain a linear tone curve from 0 to 10% of the input gradation. For an input gradation larger than 10%, the gain is gradually reduced as the gradation becomes higher, thereby generating an upwardly convex tone curve as shown in FIG.
In other words, the tone curve design unit 44 uses the gain calculated by the gain calculation unit 42, applies the gain calculated in the specific area of low gradation, and reduces the applied gain as the gradation becomes high, thereby reducing the convex tone. Generate a curve.

  Here, for example, it is conceivable to generate a tone curve as described above using APL as a feature quantity. However, since APL does not match the human sense compared to Gave, it is limited to an image that the user feels darker than the original. Correction cannot be performed. For this reason, correction is performed even for an image having sufficient luminance, and in that case, the luminance may be excessively increased and the image quality may be deteriorated. If a tone curve is generated based on Gave as in the present embodiment, it is possible to perform a correction for increasing the brightness of an image that is felt darker than the original, thereby enabling a more optimal correction.

<Low gradation linearization processing section>
The above geometric correction is characterized in that the gain is largely increased mainly in a dark scene, but a convex tone curve is used as it is generated by the tone curve design unit 44 of the geometric correction unit 4. Then, there arises a problem that the so-called black float and noise below halftone are conspicuous. In order to solve these problems, the low gradation linearization processing unit 45 performs processing for regenerating the tone curve generated by the tone curve design unit 44 so that the specific region on the low gradation side becomes linear. . In other words, the gain of the specific area on the low gradation side of the tone curve is set to 1, and correction is not performed. Here, as an example, the linear region where the gain is 1 can be set to a region up to 12% on the low gradation side of the input gradation.

FIG. 12 is a diagram illustrating a tone curve to be corrected by the low gradation linearization processing unit, FIG. 12A is a diagram illustrating an example of a tone curve generated by the tone curve design unit 44, and FIG. FIG. 13 is a diagram illustrating a state in which a low gradation portion is corrected by a low gradation linearization processing unit 45 to be linear with respect to the tone curve of FIG.
In the correction by the tone curve having a convex shape upward, it is natural in terms of visual characteristics that the gradation is smooth in the entire region of 0 to 255. However, in the low gradation area, even if the luminance is slightly increased, the appearance of noise and the amount of black floating often change greatly. Therefore, there is a case where it is better not to perform any correction for a region with extremely low gradation (here, luminance).

  Therefore, the low gradation linearization processing unit 45 sets a region (linear region) L where correction is not performed according to the value of Gave. The upper limit of the input gradation in the linear region L is defined as a knee point k. Then, the input gradation width of the linear region L is changed according to the state of the video. For example, when Gave is large, the linear region L is made large (that is, the knee point k is set to the high gradation side), and an increase in noise is suppressed while raising an intermediate gradation image. When GAve is small, the linear region L is made small (that is, by setting the knee point k on the low gradation side), and the intermediate gradation region having a relatively low gradation is also lifted.

<< Backlight enhancement part 9 >>
FIG. 13 is a diagram illustrating a more specific configuration example of the backlight enhancement unit 9. The backlight enhancement unit 9 receives the emission luminance level BL _reduced generated by the distortion module 31 and output through the temporary filter 32 and the GAve calculated by the geometric Ave calculation unit 41.
The backlight enhancement unit 9 includes a BL (backlight) gain calculation unit 91, a temporary filter 92, a multiplier 93, and a black level correction unit 94. The BL gain calculation unit 91 generates a backlight gain for correcting the light emission luminance level of the backlight using the input GAve value.

FIG. 14 is a diagram illustrating an example of a table used by the BL gain calculation unit 91 for calculating the backlight gain. In the display device, a table that defines the relationship between GAve and backlight gain (BL Gain) as shown in FIG. 14 is stored in a predetermined storage means, and the BL gain calculation unit 91 includes a geometric Ave calculation unit 41. Based on the GAve output from, the table is referred to, and the backlight gain value corresponding to the GAve is obtained.
As shown in FIG. 14, the value of the backlight gain is 1.0 in an area where the GAve is low (screen is dark), that is, no gain is applied, and in the area where the GAve is high (screen is bright), the backlight gain is set. For example, 2.0 is set to a value that increases the light source luminance of the backlight. As a result, even when a bright screen feels dark in a brighter environment, the brightness of the backlight can be increased to a bright screen when the screen is bright based on the geometric Ave.

  The temporary filter 92 is provided for the same purpose as the temporary filter 32 provided in the advanced luminance modulation unit 3 and the temporary filter 43 provided in the geometric correction unit 4, and the back-up caused by a sudden change in the input video signal is provided. This is for suppressing an excessive change in the light gain and smoothly changing the backlight gain.

The backlight gain output from the temporary filter 92 is multiplied by the duty indicated by the light emission luminance level BL _{reduced in} the multiplier 93.
Here, when the duty output to the light source unit is Duty ′ and the duty indicated by BL _reduced is Duty,
Duty ′ = Duty × BL Gain (7)
To amplify the BL _reduced duty.

As described above, the backlight enhancement unit 9 adjusts the light emission luminance level BL _reduced (here, Duty) selected so as to be close to the target CR by the distortion module 31 in a direction in which the duty is amplified in a bright image. . That is, in many cases, as BL _reduced , a value in a direction for reducing power consumption by reducing the light emission luminance level of the backlight is selected based on the distortion of the image. However, the backlight enhancement unit 9 uses this value. With respect to (BL _reduced ), the process of amplifying the light emission luminance level again to brighten the bright image.
At this time, the black level of the video signal is corrected by the black level correction unit 94 in order to suppress the floating of the black level due to amplification of the light emission luminance level of the backlight in a bright image.

  The black level correction unit 94 receives the backlight gain output from the temporary filter 92 and calculates an LUT for correcting the black level of the video signal based on the backlight gain. For example, if the light emission luminance of the backlight is doubled, the luminance of black on the screen is also doubled. At this time, in order to prevent the contrast feeling due to black floating from deteriorating, an LUT is created so that the signal on the low gradation side is crushed by the increase in black luminance.

  For example, the relationship between the gradation of the input video signal and the screen luminance is modeled as in the following equation (8).

  Here, Y is the display luminance on the liquid crystal panel, BL is the light emission luminance level (backlight duty) of the backlight, CV is the pixel value input to the liquid crystal panel, and CR is the contrast of the liquid crystal panel.

Here, when the backlight emits light at the reference light emission luminance level BL _ref , the display luminance when the light emission luminance level of the backlight is BL ′ with respect to the display luminance when gradation 0 is displayed. The following equation (9) is established, where Cv is the pixel value for keeping the constant.

  It becomes. BLGain is a backlight gain output from the temporary filter 92 of the backlight enhancement unit 9. For the gradations equal to or lower than CV in the equation (14), an LUT in which the output gradations are all 0 gradations is calculated.

FIG. 15 is a diagram illustrating an example of the LUT generated by the black level correction unit of the backlight enhancement unit. The black level correction unit 94 of the backlight enhancement unit 9 calculates CV using the backlight gain output from the temporary filter 92 and the panel contrast value according to the above equation (14). Then, as shown in FIG. 15, in an area where the input pixel (gradation) value is CV or less, an LUT having an output gradation of 0 (that is, black) is created. In the region of gradation higher than CV, the output gradation when the input gradation is the maximum gradation (255) is 255, and this point is an LUT having a characteristic in which the point of the input gradation CV is connected by a straight line. .
As a result, the signal on the low gradation side can be crushed by the increase in black luminance due to the backlight gain, and deterioration of the contrast in the low gradation portion when the backlight is brightened on a bright screen can be prevented.

  As described above, in the advanced luminance modulation processing in which the gain of the video signal is increased to compensate for the decrease in the emission luminance while the emission luminance of the backlight is reduced according to the feature amount of the input video signal, the feature amount The tone curve generated in conjunction with the light emission luminance level set using is corrected with the tone curve generated based on the geometric average value (GAve), which is an image feature amount close to human sense, and By correcting the black area of the video signal in order to increase the backlight emission brightness when the metric average value (GAve) is high (the screen is bright) and to prevent the contrast from decreasing due to the black side becoming bright at this time , Without increasing power consumption and without corrupting the image, degrading contrast even in bright environments It is possible to make the screen look bright without.

DESCRIPTION OF SYMBOLS 1 ... Histogram / APL detection part, 2 ... BL (backlight) brightness level setting part, 3 ... Advanced brightness modulation part, 4 ... Geometric correction part, 5 ... Image quality correction part, 6 ... RGB adjustment part, 7 ... FRC part , 8 ... Multiplier, 31 ... Distortion module, 32 ... Temporary filter, 33 ... Tone curve design unit, 41 ... Geometric Ave calculation unit, 42 ... Gain calculation unit, 43 ... Temporary filter, 44 ... Tone curve design unit, 45 ... Low gradation linearization processing unit.

Claims (7)

In a display device comprising a liquid crystal panel for displaying an image based on an input video signal and a light source for illuminating the liquid crystal panel,
A geometric average calculation unit for calculating a geometric average value by raising a logarithmic average of pixel values of an input video signal, and a light emission luminance of the light source based on the geometric average value calculated by the geometric average calculation unit. A light source luminance control unit that dynamically controls, and a luminance level selection unit that selects a light emission luminance level used for controlling the light emission luminance of the light source based on the feature amount of the input video signal,
The light source luminance control unit dynamically controls the light emission luminance of the light source by correcting the light emission luminance level selected by the luminance level selection unit based on the geometric average value calculated by the geometric average calculation unit. A display device characterized by: 2. The display device according to claim 1, wherein the light source luminance control unit is based on a relationship between a predetermined geometric average value and a correction value for correcting the light emission luminance of the light source. This is to correct the emission brightness level selected in
The relationship between the geometric average value and the correction value is such that the emission luminance is increased when the geometric average value is greater than a predetermined value, and the luminance level selection unit when the geometric average value is less than or equal to the predetermined value. A display device characterized by maintaining a light emission luminance level selected in (1).   3. The display device according to claim 1, wherein a tone curve generating unit for generating a tone curve to be applied to a video signal in conjunction with light emission luminance of the light source, and light emission of the light source based on the geometric average value. A black level correction unit for converting a low gradation region video signal in a range determined based on the luminance correction value into a black gradation value of zero gradation, and the black level correction unit A display device, wherein the tone curve generated by the tone curve generation unit is corrected by the generated black correction data. The display device according to claim 3,
Based on a predetermined geometric average value and gain, a gain calculation unit that calculates a gain corresponding to the geometric average value calculated by the geometric average calculation unit, and the gain calculation unit calculates A correction tone curve generation unit that generates a correction tone curve for correcting the tone curve generated by the tone curve generation unit using the gain;
The tone curve generated by the correction tone curve generator has a convex shape when the input gradation is plotted on the horizontal axis and the output gradation is plotted on the vertical axis, and the convex peak is formed in the intermediate gradation range. A convex toe curve having
For the convex tone curve, the gain calculated by the gain calculation unit is applied to a specific area of low gradation, and the applied gain is reduced as the gradation becomes higher. A display device characterized in that the tone curve is modified so that a gain of a specific region in a low gradation portion is unity.   5. The display device according to claim 4, wherein when the geometric average value is equal to or less than a specific value, a gain greater than 1 is set as a relationship between the predetermined geometric average value and the gain. A display device characterized in that when the average value is larger than the specific value, the gain is 1. 6. The display device according to claim 3, wherein a luminance level setting for setting a reference light emission luminance level based on a luminance control characteristic that defines a light emission luminance of the light source with respect to a feature amount of an input video signal. Part
The luminance level selection unit uses a histogram of the input video signal, and on a histogram that changes in accordance with a light emission luminance level of the light source in a video luminance range that can be displayed at a target contrast set in the liquid crystal panel. Evaluate the frequency of images that cannot be expressed, and based on the evaluated value, select a light emission luminance level used to control the light emission luminance of the light source,
The tone curve generation unit determines a gain to be applied to the tone curve using the light emission luminance level selected by the luminance level selection unit and the reference light emission luminance level output from the luminance level setting unit. A display device that applies the determined gain to a low gradation portion and generates a tone curve in which a gain smaller than the determined gain is applied to a high gradation portion. In Viewing apparatus according to claim 6, wherein the brightness level selection unit, in the histogram, the in the video luminance range which can be displayed when the target contrast, image brightness can not be expressed in certain light emission luminance level of the light source The frequency of the image in the range is evaluated by multiplying by a predetermined weighting factor, the evaluation is executed for all the light emission luminance levels of the selectable light source, and the light emission luminance level having the lowest evaluation value obtained by the evaluation is obtained. The display device is selected in a range that does not exceed the reference emission luminance level set by the luminance level setting unit.

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