JP5002725B2 - Backlight device and backlight control method - Google Patents

Description

  The present invention relates to a liquid crystal display device that includes a backlight that irradiates a liquid crystal panel that displays an image from the back, and that adjusts the luminance of the backlight in accordance with a video signal to be displayed, and a backlight control method.

  Unlike a self-luminous display device such as a CRT (Cathode Ray Tube) or a plasma display panel, a liquid crystal display device irradiates the panel to a non-luminous liquid crystal panel (light transmissive light modulation element) and its back surface. It has a configuration with a backlight. Normally, the backlight emits a light source such as an LED with a constant brightness regardless of the video signal, and displays the video with the desired brightness by controlling the light transmittance of the liquid crystal panel according to the brightness of the video signal. To do. Therefore, even if it is a dark image, the power of the backlight light source does not decrease and is consumed at a constant level, and the power efficiency is not good. As a countermeasure, a technique is known in which the brightness of the backlight is variable and the power consumption is reduced by controlling the gradation level of the liquid crystal panel and the brightness of the backlight in accordance with the level of the input video signal. A technique is also known in which the backlight is divided into a plurality of areas (areas) and the brightness of the backlight is controlled for each area (referred to as area control or local dimming).

  For example, in the image display device described in Patent Document 1, the average luminance of a video signal is provided in order to provide a high-performance ACC (Automatic Contrast Circuit) circuit by controlling the luminance of an LED backlight in units of pixels. Screen information analysis means for detecting information, black level area detection, white level area detection, and LED backlight control means for controlling the brightness of each LED backlight by a control signal output from the screen information analysis means Yes.

JP 2006-30588 A

  According to the area control, power consumption can be optimized for each area, so that power consumption of the entire backlight can be minimized. However, depending on the picture on the screen, image quality may be deteriorated by performing area control.

  FIG. 6 is a diagram for explaining that the effect of area control depends on the picture on the screen. The illumination surface of the backlight that illuminates one screen (corresponding to the display surface of the liquid crystal panel) is two-dimensionally arranged in a plurality of areas. It is divided into In this example, it is divided into 45 areas.

  The screen 610 is a case where a small white area (white window) 612 exists on a black background 611. If area control is performed in this case, effects of power reduction and contrast improvement can be obtained. This is because the larger the black area, the larger the number of areas for reducing the backlight luminance, so that the total power reduction amount increases, and the black luminance is suppressed by lowering the backlight luminance of the black region 611, and the white window 612. This is to improve the contrast ratio. On the other hand, there is a problem that the luminance of the white window 612 leaks into the surrounding black region 611 and the halo 613 is easily generated by lowering the backlight luminance of the black region 611.

  The screen 620 is a case where a small black region (black window) 622 exists on the white background 621. In this case, the effects of power reduction and contrast improvement are small. This is because the number of areas for reducing the backlight luminance is small and the visual contrast is high in a picture in which the black window 622 is present in the large-area white background 621. In this case, a reduction in luminance of the white background area around the black window 622 becomes a problem. This is because by reducing the backlight luminance of the black window 622, there is no backlight light leaking from the black window area to the surrounding white background area. When the brightness is reduced for a bright image, a large image quality deterioration is visually caused.

  As described above, when area control is performed on a bright pattern, the adverse effect of image quality deterioration is increased although the obtained effect is small. Therefore, it is desirable that the area control is appropriately performed according to the picture pattern in consideration of the balance between power reduction and image quality improvement. In Patent Document 1, the LED backlight is controlled so as to lower the luminance of the signal below the mutation point according to the area of the black level region or to increase the luminance of the signal above the variation point according to the area of the white level region. However, it does not consider the balance between power reduction and image quality improvement.

  An object of the present invention is to provide a liquid crystal display device and a backlight control method that can improve power reduction and image quality in a well-balanced manner in backlight area control.

In order to solve the above-described object, for example, the configuration described in the claims is adopted.

According to the present invention, it is possible to provide a backlight device and a backlight control method that improve power reduction and image quality improvement in a balanced manner in backlight area control .

1 is a block configuration diagram showing an embodiment of a liquid crystal display device according to the present invention. 7 is a flowchart showing the operation of the backlight luminance correction unit 20. The figure which shows the specific example of the minimum light control value Kmin with respect to the black area S. FIG. The figure which shows the specific example of LED correction gain G1 with respect to the black area S. FIG. The figure which shows the general luminance characteristic in a liquid crystal panel. The figure explaining that the effect of area control depends on the picture of a screen. The figure which shows the example of 1 structure of the backlight block corresponding to each area of a backlight. The figure which shows an example of the light-guide plate used for a backlight block.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First, the configuration of the backlight block corresponding to each area of the backlight according to the present embodiment will be described with reference to FIGS. That is, the backlight according to the present embodiment is configured by two-dimensionally arranging a plurality of backlight blocks.

  FIG. 7 is a diagram showing a configuration example of a backlight block corresponding to each area of the backlight, and corresponds to each of the 45 divided areas shown in FIG. 6, for example. In each of the backlight blocks, for example, as shown in FIG. 7, for example, a light emitting diode (LED) 901 as a primary light source is mounted on one surface (surface on the liquid crystal panel 906 side) of the LED drive substrate 902. . On the other hand, an LED driver 907 for supplying a drive current to the LED 901 is mounted on the other surface of the LED drive board 902, and the drive current supplied from the LED driver 907 to the LED 901 is transmitted by an LED driver IF30 described later. Be controlled. Here, the LED 901 emits white light, for example, and emits light in a horizontal direction with respect to the electrode surface of the LED (in this example, equal to a direction parallel to the main plane of the LED drive substrate 902). A so-called side view type LED is used. Of course, a top-view type LED that emits light in a direction perpendicular to the electrode surface of the LED may be used.

  A light guide plate 904 that guides the light emitted from the LED 901 (indicated by a dotted arrow in FIG. 7) to the front side (the liquid crystal panel 906 side) is disposed on the light emission side of the LED 901. Here, a plurality of (for example, three) LEDs 901 are used for one light guide plate 904. Here, it is assumed that the plurality of LEDs 901 are arranged in a row in a direction perpendicular to the paper surface. Further, on the back side of the light guide plate block 904, a reflection sheet 903 for efficiently reflecting the emitted light from the LED 901 incident on the light guide plate 904 to the front side is provided. A support member 909 that is white for light reflection is disposed in the space between the reflection sheet 903 and the LED drive substrate 902. The support member 909 allows the reflection sheet 903 and the light guide plate 904 to be connected to the space. Support from the back side.

  The cross section of the light guide plate 904 (the light guide plate 104) in the vertical direction of the liquid crystal panel 106 (left and right direction in FIG. 7) is from the incident end face where the light is incident as shown in FIG. It has a wedge-like shape that gradually decreases in thickness over the part. Further, a reflection sheet 903 is provided on the back side of the light guide plate 904. For this reason, the light emitted from the LED 901 incident on the light guide plate 904 is bent upward (in the direction toward the liquid crystal panel 906) inside the light guide plate 904 due to the wedge shape of the light guide plate 904 and the reflection action of the reflection sheet 903. Is done. Further, planar light with a substantially uniform incident light luminance level is obtained by the action of the diffuse reflection pattern provided on the bottom surface (surface on the reflection sheet 903 side) or the light emission surface (surface on the liquid crystal panel 906 side) of the light guide plate 904. For example, as indicated by the dotted arrow in FIG. 7, the light is emitted upward (in the direction toward the liquid crystal panel 906).

  The diffusion plate 905 diffuses the light emitted from the light guide plate 904 and emits it to the liquid crystal panel 906 as spatially uniform planar light. The liquid crystal panel 906 controls the light transmittance for each pixel based on the input video signal, and spatially modulates the light from the diffusion plate 905 to form an image. As a result, the image light indicated by the arrow heading upward in the drawing is output to the front side of the liquid crystal display device.

  In this example, an LED that emits white light is used as the LED 101. However, the present invention is not limited to this. For example, a plurality of sets of three LEDs each emitting light of three colors of R, G, and B are used. Also good.

  A plurality of backlight blocks having such a configuration are arranged two-dimensionally in the horizontal and vertical directions of the screen on the back surface of the liquid crystal panel. The brightness of each area can be controlled independently by individually controlling the LEDs 101 (here, a set of three) provided for each backlight block.

  FIG. 8 is a diagram illustrating an example of a light guide plate used in the backlight block. As the light guide plate, one light guide plate may be used for one light source block 202. For example, as shown in FIG. 8, a plurality of light guide plates (horizontal direction, equal to the depth direction in FIG. In FIG. 8, four light guide plates may be combined and integrated, and one integrated light guide plate 910 may be used for the four backlight blocks. A plurality of the integrated light guide plates 910 are arranged in the horizontal and vertical directions of the screen to constitute a light guide plate that covers the entire surface of the liquid crystal panel. At this time, the integrated light guide plate 910 is formed with a groove 911 extending in the vertical direction of the screen, whereby the integrated light guide plate 910 is divided into a plurality of light guide plate blocks 912 each corresponding to the light source block 202. . On the contrary, although not shown, a plurality of light guide plates may be combined and integrated in the vertical direction of the screen.

Next, area control according to the present embodiment will be described.
FIG. 1 is a block diagram showing an embodiment of a liquid crystal display device for performing area control according to the present invention. As described above, such a liquid crystal display device includes an LED light source that divides a liquid crystal panel that displays an image into a plurality of areas and independently controls the brightness of each area as a backlight. In the figure, a control portion of backlight brightness (LED gain) and a correction portion of a video signal supplied to the liquid crystal panel are shown.

  The configuration of the apparatus includes an initial dimming value calculation unit 11, a spatial filter 12, a time filter 15, and an LED driver interface (I / F) 30 as a system for controlling the backlight luminance of each area. In addition, as a system for correcting the video signal of each pixel displayed on the liquid crystal panel, a video correction magnification calculating unit 33 and a video correction processing unit 34 are provided. Furthermore, in this embodiment, a backlight luminance correction unit 20 that analyzes the pattern of the input video and optimally corrects the luminance of the backlight according to the black area is provided.

The operation of each part will be described. The initial dimming value calculation unit 11 detects the signal luminance (intensity) of each area of the video signal (RGB) input to the input terminal 10 (for example, detects the maximum luminance (intensity) in the area) and detects it. The initial dimming value K0 of the backlight in the area is calculated according to the luminance. As a result, in areas where the brightness of the video signal is low, the backlight light intensity (dimming value) is lowered, and the transmittance of the liquid crystal panel is increased accordingly, reducing the power without changing the display brightness on the liquid crystal panel. To do. The spatial filter 12 applies a low-pass filter to the spatial distribution of the initial dimming value K0 for each area (processing unit 13) to obtain a dimming value K0 ′ in which a steep change is reduced. The selection unit 14 compares the dimming value K0 ′ after the filter processing with the lowest dimming value Kmin output from the backlight luminance correction unit 20 described later, selects the larger dimming value, and selects this as K1. To do. The time filter 15 sets a dimming value K1 ′ obtained by applying a low-pass filter to the change in the dimming value K1 between frames to reduce the steep time change.

  The LED driver I / F 30 includes an LED control signal calculation unit 31, which calculates an LED control signal from the dimming value K1 'after the time filter 15 processing and the LED maximum level setting value G2, and outputs the LED control signal to the LED driver. Here, the LED maximum level setting value G2 is obtained by multiplying the value G0 of the LED gain setting unit 32 interlocked with the screen brightness adjustment by the user by the LED correction gain G1 output from the backlight luminance correction unit 20 described later. It has been corrected.

  The backlight luminance correction unit 20 is a particularly characteristic part in the present embodiment, and analyzes the pattern (black area) of the input image for one screen (one frame or one field), and according to the black area. It controls the dimming value and LED gain of the backlight. The Y conversion unit 21 converts an input RGB signal into a luminance signal Y. The black area measuring unit 22 compares the luminance signal level Y of each pixel in the screen with the black level threshold value Y0, and determines “black” if the threshold value Y0 or less. Then, as the degree of pixels determined to be black in the video signal in one screen, the ratio (black area) of the pixels determined to be black to the video signal in one screen is calculated or measured. The black level threshold Y0 is set by the black level threshold setting unit 23.

  The lowest dimming value output unit 25 compares the black area S measured by the black area measuring unit 22 with the black area threshold value S0 to determine the lowest dimming value Kmin. Specifically, when the black area S is equal to or less than the threshold value S0, the maximum value that can be taken as the dimming value is given as Kmin (case A), and when the black area S is the entire screen, Kmin is used for all black. A dimming value is given (case B), and when the black area S is an intermediate value thereof, an intermediate dimming value that is larger than the minimum value that the dimming value can take and that is predetermined according to the black area S is set. Give (Case C). That is, in the present embodiment, in the case A, in the video in which black and other gradations are mixed, the above-described area control (operation for controlling the brightness of the backlight for each divided area) has a small black area. ), The area control is turned off. In case C, the area control is turned on because the black area is large and the effect of the area control is likely to appear.

  Here, the black area threshold value S0 is set by the black area threshold value setting unit 24, but a different value is set in conjunction with the mode switching unit 27. Also, different values are set for the intermediate dimming value used in case C in conjunction with the mode switching unit 27. The minimum dimming value Kmin is output to the selection unit 14 of the spatial filter 12, and the larger value is selected. That is, the dimming value K1 output from the selection unit 14 does not fall below the minimum dimming value Kmin.

  The LED gain correction unit 26 calculates an LED correction gain signal G1 according to the black area S measured by the black area measurement unit 22, and outputs the LED correction gain signal G1 to the LED driver I / F 30.

  The image correction magnification calculator 33 calculates an image correction magnification for compensating for the amount of change in the dimming value in accordance with the dimming value K1 'from the time filter 15. The video correction processing unit 34 supplies a video signal obtained by multiplying the input video signal by the video correction magnification to the liquid crystal panel. As a result, the liquid crystal panel can display an image with the original luminance.

  The mode switching unit 27 selectively switches between two control modes according to the user's preference. One is “high image quality setting” (mode 1) that reduces power while preventing image quality deterioration, and the other is “low power setting” (mode that uses area control as much as possible to maximize power reduction). 2).

FIG. 2 is a flowchart showing the operation of the backlight luminance correction unit 20. This will be described below in the order of steps.
In ST101, the black area S of the screen is measured. The content is that the luminance level Y of the input pixel is compared with the black level threshold Y0, and the number of pixels (the number of black pixels) satisfying Y ≦ Y0 is counted. The black area S is measured from the ratio of the number of black pixels to the total number of pixels in the screen.

  In ST102, the measured black area S is compared with the black area threshold value S0. In the case of S ≦ S0, the process proceeds to ST103, and the maximum value (for example, gradation level 1023) that the light control value can take is given as the minimum light control value Kmin (case A). That is, even if a black area exists in the screen, the corresponding backlight dimming value is maximized and the area control is turned off (stopped).

  When S> S0, the process proceeds to ST104, and it is determined whether or not the black area S covers the entire screen (S = 100%). In this case, it may be considered that the entire screen is not completely 100% but, for example, 95% or more. When the black area S is the entire screen, the process proceeds to ST105, where a predetermined dimming value for black (for example, gradation level 10) is given as the minimum dimming value Kmin (Case B).

  When the black area S is neither of the above cases A and B but So <S <100%, the process proceeds to ST106, and an intermediate dimming value corresponding to the black area S is given (case C). In the example of this figure, Kmin = 50, and even if a black area exists in the screen, the dimming value does not fall below the minimum value 50. The intermediate dimming value is determined in advance for each value of the black area S, and when there is no corresponding value, it is determined by linear interpolation as appropriate. A specific example of the minimum dimming value Kmin will be described later.

On the other hand, in ST107, the LED correction gain G1 is output according to the measured value of the black area S. In ST108, the output LED correction gain G1 is multiplied by the dimming value determined in the above step to determine the LED control signal. In the example of this figure, G1 = 0.5 is applied to the dimming value of case C, and an LED control signal (case C ′) in which the dimming value of each area is uniformly halved is generated. The LED correction gain G1 is determined in advance for each value of the black area S, and when there is no corresponding value, it is determined by linear interpolation as appropriate. A specific example of the LED correction gain G1 will be described later.

Here, “black float” which is a problem in the liquid crystal panel will be described.
FIG. 5 is a diagram showing general luminance characteristics in the liquid crystal panel. The relationship between the input level P and the display luminance L in the liquid crystal panel is approximated by L = P ^ γ, and when this is expressed by a logarithmic axis, logL and logP are indicated by straight lines having a slope γ. In an actual panel, the area where the input P is small is brighter than this straight line. This is called “black float” and causes deterioration of contrast. Therefore, in a region where black floating occurs, it is possible to suppress black floating and reduce the contrast by reducing the backlight luminance.

  Even in the case of the present embodiment, the initial dimming value calculation unit 11 performs correction for narrowing the dimming value in a range where the input level is small in order to prevent black floating. Further, as the black level threshold value Y0 used in the black area measuring unit 22, a boundary value of a region where black floating occurs (in this case, input gradation level = 32) can be used.

  FIG. 3 is a diagram illustrating a specific example of the minimum dimming value Kmin with respect to the black area S. This figure also shows how the minimum dimming value Kmin is set for the two modes.

  When the black area S is equal to or less than the black area threshold value S0, the maximum dimming level (Kmin = 1023) is set, and the area control is turned off (case A). When the black area S is the entire screen (95% or more), the dimming value for all black (Kmin = 10) is set (case B). In the middle of them, the minimum dimming value Kmin is changed and set using an intermediate dimming value predetermined according to the black area S (case C).

  The black area threshold S0 is a large value (about 20%) in mode 1 (high image quality setting) and a small value (about 5%) in mode 2 (low power setting). As a result, in the case of mode 2, the area control can be turned on in a wide range, and the power can be further reduced. Further, the minimum dimming value Kmin in case C is set to a larger value in mode 1 than in mode 2. This is because image quality deterioration due to the occurrence of halo is suppressed by giving the minimum dimming value Kmin large. Further, the minimum dimming value Kmin is decreased as the black area S increases, but this is to suppress the conspicuous image quality degradation due to black floating due to the black area S increasing. Accordingly, in mode 1, it is possible to reduce power while preventing image quality deterioration due to area control.

  FIG. 4 is a diagram illustrating a specific example of the LED correction gain G1 with respect to the black area S. In FIG. The LED correction gain G1 is set similarly in the two control modes.

  When the black area S is medium (30-50%), the gain G1 is made larger than the reference value 1, and as the black area S increases (70-100%), the gain G1 is made smaller than the reference value 1. This is because the luminance is improved when the black area is medium, and the difference in luminance is compressed when the black area is increased to make the occurrence of halo inconspicuous.

  Thus, by changing not only the dimming value but also the LED correction gain G1 with respect to the black area S, it is possible to improve power reduction and image quality improvement in a balanced manner. It should be noted that the control conditions (numerical values and the like) described in the above embodiments are merely examples, and it is needless to say that the control conditions can be appropriately changed according to the performance of the target liquid crystal display device.

11 ... Initial light control value calculation unit,
20 ... Backlight brightness correction unit,
22 ... black area measuring section,
25: Minimum dimming value output section,
26 ... LED gain correction unit,
30 ... LED driver interface (I / F),
31 ... LED control signal calculation unit,
33: Image correction magnification calculator,
34 ... image correction processing unit,
Kmin: Minimum dimming value,
G1: LED correction gain.

Claims (12)

For each area of the liquid crystal panel that displays video based on the video signal, a plurality of light sources that are installed for each area and irradiate light based on a dimming value set for each area;
In the video signal , a detection unit that detects a ratio of pixels having a predetermined luminance or less among pixels in one screen ;
Based on the luminance of the video signal displayed in each area of the liquid crystal panel, area control is performed to set a dimming value for each area. When the ratio detected by the detection unit is equal to or less than a predetermined value, the area control is performed. A control unit to stop;
A backlight device comprising: The backlight device according to claim 1,
When the control unit stops the area control, light having a predetermined light control value is emitted from the plurality of light sources. The backlight device according to claim 1 or 2,
The said control part calculates the minimum light control value based on the said ratio , and sets the light control value of each area of the said light source so that it may become more than the said minimum light control value. The backlight device according to any one of claims 1 to 3,
The said control part sets the dimming value of each area of the said light source to the predetermined all-black dimming value, when the said ratio is more than predetermined value, The backlight apparatus characterized by the above-mentioned. The backlight device according to any one of claims 1 to 4,
The control unit includes a plurality of control modes,
The backlight unit according to claim 1, wherein the control unit changes a dimming value of each area according to the control mode. The backlight device according to any one of claims 1 to 5,
The control unit adjusts the dimming value so that the light intensity of the light source that irradiates the predetermined area with light is larger than 0 when the luminance of the image displayed on the predetermined area of the liquid crystal panel is zero. A backlight device characterized by setting. Irradiate light to each area of the liquid crystal panel that displays video based on the video signal based on the dimming value set for each area,
In the video signal , a ratio of pixels having a predetermined luminance or less among pixels in one screen is detected,
Performing area control for setting a dimming value for each area based on the luminance of the video signal displayed by each area of the liquid crystal panel, and stopping the area control when the detected ratio is equal to or less than a predetermined value;
The backlight control method characterized by the above-mentioned. The backlight control method according to claim 7,
When the area control is stopped, light having a predetermined light control value is irradiated. The backlight control method according to claim 7 or 8,
A backlight control method comprising: calculating a minimum dimming value based on the ratio and setting the dimming value of each area to be equal to or greater than the minimum dimming value. The backlight control method according to any one of claims 7 to 9,
When the ratio is equal to or greater than a predetermined value, the dimming value of each area is set to a predetermined all-black dimming value. The backlight control method according to any one of claims 7 to 11,
With multiple control modes,
A backlight control method, wherein a dimming value of each area is changed according to the control mode. The backlight control method according to any one of claims 7 to 11,
The dimming value is set so that the intensity of light that irradiates the predetermined area with light is larger than 0 when the luminance of the image displayed on the predetermined area of the liquid crystal panel is zero. To control the backlight.

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