JP5286520B2 - Liquid crystal display - Google Patents

Description

  The present invention relates to a liquid crystal display device that displays an image illuminated by a backlight.

  As a backlight of a liquid crystal display device, a cold cathode tube or a light emitting diode (hereinafter referred to as an LED) is used as a light source, and in the display state, the backlight always emits light at a constant luminance over time. Was composed. However, recently, as the use of LEDs in the backlight has rapidly progressed, it has become easy to adjust the backlight, and at the same time, it has become possible to realize high-speed variation that can be synchronized with an image.

  Therefore, in order to further improve the contrast by applying backlight brightness control, the backlight brightness can be varied according to the image, and the displayed γ characteristic can be maintained in an ideal state even when the brightness is switched. A driving method for switching the gradation has been proposed.

  In this driving method, when it is determined that the image is bright, the luminance of the backlight is increased and the γ characteristic is switched to a direction in which the gradient from dark to light is laid down. On the other hand, when it is determined that the image is dark, the luminance of the backlight is lowered and the γ characteristic is switched to a direction in which the gradient from dark to light is increased. As a result, an image having a wider dynamic range can be expressed. In other words, the brightness displayed from the liquid crystal could be varied in the wide range from the maximum brightness of the image, the maximum brightness of the backlight to the minimum brightness of the image, and the minimum brightness of the backlight. .

  However, since the backlight brightness is controlled collectively over the entire screen, the backlight brightness is limited for images that contain many highlights that cause a large brightness gradient in the screen. The conventional method and the dynamic range, which are constant, are not different, and the improvement effect cannot be exhibited.

  For this reason, recently, as proposed in Patent Document 1, there is a technique in which the backlight is divided into a plurality of illumination areas, and the luminance is controlled according to the input image signal. In this method, since the brightness of each illumination area is controlled based on the image signal, the brightness is high for a display portion including a lot of bright image information in the entire screen, and conversely dark image information. Therefore, the luminance of the display portion including a large amount of can be lowered, and the dynamic range of the entire screen is further expanded.

  Further, since this method changes the luminance of the backlight for each illumination area, when the image signal is supplied to the liquid crystal panel with the same gradation, the luminance of the display image is shifted between the illumination areas. . Therefore, in order to convert the image signal according to the brightness of the backlight for each illumination area, the brightness of the display image is shifted between each illumination area by the appropriate gradation converted according to the brightness of the backlight of each illumination area. Therefore, an appropriate display image can be obtained.

  In addition, in a liquid crystal display device using the OCB mode, a voltage for performing normal display is applied in order to take advantage of the high-speed response characteristic of the liquid crystal in the OCB mode and to approximate impulse-type driving such as a CRT. High contrast display can be performed by using a driving method called double speed conversion that alternately performs the operation of applying the black voltage and the operation of applying the black voltage. Hereinafter, this driving method is referred to as “black insertion driving method”.

  In such an OCB mode liquid crystal display device, a backlight driving method linked to the black insertion driving method has also been proposed. In order to obtain a higher contrast when performing the black insertion driving method using the OCB mode liquid crystal, the backlight is turned off at the timing of black insertion and turned on only at the timing at which the image signal is written.

Even if the backlight is turned off at the timing of this black insertion, the desired effect cannot be obtained because all the scanning lines can be turned off only when black is written. Therefore, the backlight is configured so that the light source is divided into a plurality of blocks in the vertical direction and the LEDs are sequentially turned off from the block in which black writing is performed in units of blocks. That is, at the time of black writing, only the backlight of that portion is turned off, thereby realizing contrast and low power.
JP 2002-99250 A JP 2007-199127 A

  By dividing and controlling the illumination light of the backlight and the gradation of the image for each region as described above, the dynamic range can be expanded and the image quality can be improved. Furthermore, it is possible to improve the contrast and moving image response by the black insertion driving method and to prevent reverse transition.

  However, as performance required from a product incorporated as a liquid crystal panel, if the improvement in contrast due to expansion of the dynamic range is sufficient, further improvement in performance due to black insertion may not be required. On the other hand, mobile phones and other products that use batteries, such as mobile-related products, and products that pursue energy conservation may require energy conservation rather than black insertion.

  Accordingly, in view of the above problems, the present invention provides a liquid crystal display device capable of saving energy while performing the black insertion driving method.

The present invention includes a liquid crystal panel that displays an image in a display area based on an image signal, and a plurality of illumination areas that are turned on by a plurality of point light sources arranged in a grid pattern in the vertical and horizontal directions. A backlight that illuminates the display area of the liquid crystal panel; a backlight control unit that calculates the luminance of each of the illumination areas based on the image signal; and a black display signal is inserted into the image signal for each frame. In the liquid crystal display device having a black insertion unit that performs black display in the display region, the black insertion unit detects a low gradation region having a gradation lower than a reference gradation in the image signal, and A backlight control unit that turns off the plurality of point light sources corresponding to the display region that is the low gradation region and performs the black display independently of illumination of the illumination region; region The brightness of the point light source of the illumination area corresponding to the outer region, are turned in accordance with the gradation other than the low gray scale region, a liquid crystal display device.

  According to the present invention, it is possible to save energy by turning on the luminance of the point light source in accordance with the gradation in the low gradation region while realizing the black insertion driving method.

  Hereinafter, a liquid crystal display device 1 according to an embodiment of the present invention will be described with reference to FIGS.

(1) Configuration of Liquid Crystal Display Device 1 The configuration of the liquid crystal display device 1 according to the present embodiment will be described. FIG. 1 is a block diagram of a liquid crystal display device 1 of the present embodiment.

  The liquid crystal display device 1 includes a controller 10, a liquid crystal panel 11, a backlight 12, a source driver circuit 22, and a gate driver circuit 23.

  The liquid crystal panel 11 uses OCB type liquid crystal and has a size of 20 to 30 inches. The liquid crystal panel 11 includes an array substrate and a counter substrate. The array substrate is wired so that signal lines and scanning lines are orthogonal to each other, and TFTs (thin film transistors) are provided at the intersections thereof. OCB mode liquid crystal is sandwiched between the substrate and the counter substrate.

  A source driver circuit 22 and a gate driver circuit 23 are connected to the liquid crystal panel 11, and a signal from the controller 10 is received by the source driver circuit 22 and the gate driver circuit 23 to display an image on the liquid crystal panel 11.

  The backlight 12 is disposed on the back surface of the liquid crystal panel 11, and the backlight 12 includes a plurality of white LEDs 24, which are point light sources, arranged in a lattice pattern.

  The controller 10 stores the RGB image signal in the frame memory 13 and then receives it, and controls the source driver circuit 22, the gate driver circuit 23, and the backlight 12 based on an external timing signal.

  As shown in FIG. 2, the liquid crystal panel 11 and the backlight 12 are each divided into a plurality of regions. Hereinafter, the divided areas are referred to as “display areas” in the liquid crystal panel 11 and “illumination areas” in the backlight 12.

(2) Configuration of Backlight 12 As shown in FIG. 8, the backlight 12 in the present embodiment has a plurality of white LEDs 28 arranged in a lattice pattern, and has four illumination areas 30 vertically and eight horizontally. A total of nine LEDs 28 are arranged in each illumination area 30, three vertically and three horizontally.

(3) Controller 10
Next, the controller 10 will be described with reference to FIG.

  As shown in FIG. 1, the controller 10 includes a black insertion unit 9, an image luminance calculation unit 14, an image luminance data holding unit 15, a backlight luminance calculation unit 16, a backlight luminance data holding unit 17, and a backlight luminance control unit 18. , A tone conversion unit 19, a tone correction lookup table (tone correction LUT) 20, and a tone correction unit 21.

  The controller 10 converts the image information input to the liquid crystal panel 11 based on the luminance distribution information of the backlight 12 so that correct gradation reproduction can be obtained in the screen.

  First, the input image signal is analyzed for each illumination area of the backlight 12, and the average or most frequent gradation, the maximum gradation, and the minimum gradation are extracted. Based on the image information, the luminance distribution of the backlight 12 is determined. That is, when the image information includes a lot of bright luminance information, the luminance of the backlight 12 is set high, and when the image information includes a lot of dark luminance information, the luminance of the backlight 12 is set low.

  Next, based on the control result of the backlight 12, the gradation shift amount is determined for each display area having a smaller area in the liquid crystal panel 11. At this time, even if the luminance control value of the backlight 12 that illuminates the display area is the same, the gradation shift amount is not the same. The reason is that even if the brightness control value that directly illuminates the display area is the same, the brightness control value around the illumination area changes depending on the surrounding image information, and the brightness value that illuminates the display area due to crosstalk between the illumination areas Because changes. Therefore, an appropriate gradation shift amount is determined based on matrix information of luminance control values that illuminate the image. The gradation shift amount is also generally not linear, and is nonlinearly shifted according to the γ characteristic that relates the gradation and the display luminance level.

(4) Description of Liquid Crystal Panel 11 and Backlight 12 FIG. 2 is an explanatory diagram of the liquid crystal panel 11 and the backlight 12.

  In the liquid crystal panel 11, the RGB image signal is modulated and controlled for each pixel based on the RGB gradation conversion data for each region.

  In the backlight 12, brightness control is performed based on image brightness information for each display area of the liquid crystal panel 11.

  In the present embodiment, as shown in FIG. 2, the liquid crystal panel 11 is divided into a 6 × 8 display area (FIG. 2A), and the backlight 12 is divided into a 3 × 4 illumination area (FIG. 2B). For convenience, the display area on the liquid crystal panel 11 is indicated by (i, j), and the illumination area on the backlight 12 is indicated by [i, j].

(5) Operation of Liquid Crystal Display Device 1 The operation of the liquid crystal display device 1 will be described with reference to FIG.

  The RGB image signals are stored in the frame memory 13 and then read out for each display area.

  The image luminance calculation unit 14 calculates a luminance value for each display region based on the read image signal, and sends the calculated luminance data for each display region to the image luminance data holding unit 15.

  The backlight luminance calculation unit 16 calculates the backlight luminance level for each illumination region based on the luminance data for each display region from the image luminance data holding unit 15. The calculated backlight luminance data for each illumination area is sent to the backlight luminance data holding unit 17.

  The backlight luminance control unit 18 calculates the backlight luminance for each illumination area based on the calculation result of the backlight luminance calculation unit 16.

  The gradation converting unit 19 sequentially reads out image signals stored in the frame memory 13 for each pixel, and performs gradation modulation based on the luminance data of the backlight 12 that illuminates the display area.

  The gradation correction unit 21 performs appropriate gradation correction using the data of the gradation correction LUT 20 based on the backlight luminance data around the display area, and finally the RGB input to the source driver circuit 22 Image signal (R "G" B ").

(6) Gradation Correction Method Next, the gradation correction method performed after the gradation conversion process will be specifically described together with the backlight luminance level determination method described so far.

FIG. 4 is a diagram showing an example of the result of calculating the average luminance gradation for each display area in FIG. 2A for the RGB image signal levels corresponding to the images stored in the frame memory. The relationship between the RGB image signal and the signal luminance level Y takes into account the visibility of each RGB image signal.

Y = 0.30R + 0.59G + 0.11B (1)

It can be expressed as The coefficient of equation (1) is determined by the RGB chromaticity points and the white point, that is, the specifications of the display system.

  In this embodiment, the 2 × 2 display area corresponds to the illumination area (see FIG. 2), and the average luminance can be calculated for each illumination area from FIG. 4 as shown in FIG. Similarly, the maximum value and the minimum value of the RGB signal on the illumination area can be calculated from the maximum value and the minimum value of the signal level for each display area.

(7) Determination procedure of luminance level The luminance level of each illumination area is determined from the average, maximum and minimum luminance signal levels in the procedure as shown in FIG.

  That is, for example, an appropriate backlight luminance level is selected based on the average luminance signal level, and other parameters (here, maximum value and minimum value) fall within the displayable signal level range at the selected backlight luminance level. Judge whether.

  If any parameter is out of range, the backlight brightness level is selected by iterative processing.

  When all the parameters are not included in the displayable signal level range at any backlight luminance level, a backlight luminance level that includes either the minimum value or the maximum value is selected.

(8) Gradation correction processing In this embodiment, the backlight luminance level as shown in FIG. 6B is selected from the RGB display average luminance signal level for each illumination area as shown in FIG. To do.

  The RGB image signal levels sequentially read from the frame memory 13 are subjected to gradation conversion from the backlight luminance level information (see FIG. 6B) of each illumination area.

  However, it is necessary to perform tone correction processing for the following reasons.

  FIG. 7 shows a region [2, 2] in which the luminance level 1 is selected in FIG. 6B and a region [3, 2] in which the luminance level 3 is selected, which is located on the lower side of the screen of [2, 2]. It is the figure which showed typically the spatial distribution of the brightness | luminance at the time of white display.

  When the backlight brightness is modulated for each illumination area, the backlight brightness that illuminates a certain display area is superposed not only on the illumination area directly below but also on the adjacent illumination area due to crosstalk between the illumination areas. . That is, a phenomenon occurs in which the actual backlight luminance is deviated from the backlight luminance used for the gradation conversion due to the illuminating light from the adjacent area. That is, an illumination error phenomenon occurs.

  In FIG. 7, for the sake of simplification, the slot loss talk between the two regions is shown, but actually, as shown in FIG. 6B, the region around the [2, 2] region, that is, [1, 1], [1 , 2], [1,3], [2,1], [2,3], [3,1], [3,2], and [3,3] Is determined. Since the crosstalk is determined by the number of divisions of the illumination area, the area of the area, the design of the backlight, and the like, depending on the conditions, it may be affected by a change in the luminance level in the illumination area other than the adjacent area. For example, if the actual brightness of the backlight with brightness level 1 selected for a certain display area includes an error of 20% with respect to the brightness data at the time of gradation conversion (20% brightness is high), the second to fifth floors A gradation conversion error of about a tone occurs, and is visually recognized as an obstacle such as a pseudo contour between the display areas and gradation inversion.

  In this embodiment, as shown in FIG. 3, in order to compensate for the gradation conversion error due to the illumination error phenomenon, the gradation correction unit 21 uses the gradation correction LUT 20 and the final image signal to the source driver circuit 22 is supplied. Is output. The gradation correction LUT 20 stores gradation correction data corresponding to a combination of the luminance level of a certain illumination area and the luminance level of the adjacent illumination area for each display area, and the luminance level of the backlight 12 is stored. The gradation correction amount is determined while referring to it.

  The controller 10 outputs a black display signal before outputting an image signal for each frame, and controls to alternately display a black display image and a normal image within one frame.

  The operation of the black insertion portion 9 will be described in detail later.

(9) Black Insertion Control Next, the operation of the black insertion unit 9 will be described.

  The black insertion unit 9 inserts a black display image for each frame of the RGB image signal input from the frame memory 13 and alternately displays a black display image and a normal image. In this case, the black insertion unit 9 performs special control on the black display image. Hereinafter, the control will be described.

  Generally, as a condition for the occurrence of reverse transition in the OCB liquid crystal, a low gradation is required so that a voltage (white voltage) of 10 to 20% or less of the maximum voltage (black display voltage) to the OCB liquid crystal is applied. It has been experimentally confirmed that the reverse transition occurs when the image signal is continued.

  Therefore, it is possible to eliminate the black insertion driving method only in a period in which the low gradation (white voltage) in which the reverse transition occurs in the entire screen does not exist in the image signal. However, such a white-side voltage image signal is rare in a moving image.

  Therefore, in the present embodiment, for the purpose of expanding the dynamic range, for each illumination area, a low gradation (a white-side voltage that causes reverse transition of the gradation of the image signal, which is referred to as a “reference voltage” below) If the voltage is lower than the reference voltage, the black insertion drive method is performed only for the illumination area segment, and the lighting period during which there is no state lower than the reference voltage continues. For the area, the black insertion driving method is stopped only during that period.

  Specifically, based on the luminance data for each illumination area from the image luminance data holding unit 15, the black insertion driving method described below is performed only when the luminance data is equal to or lower than the reference voltage described above, and the reference voltage For the upper region, the backlight 12 is turned on with the luminance for which the gradation correction processing described above has been performed.

  For example, in FIG. 9, it is assumed that an elliptical portion in the approximate center on the screen is a gray image in which reverse transition does not occur, and an outer image is a white image in which reverse transition can occur.

In this case, as shown in FIG. 11, black insertion is performed for portions other than the gray image where the reverse transition does not occur, and the luminance of the backlight 12 is set in the same manner as described above for the corresponding illumination area of the gray image. Determine and light up. When black insertion is performed, all the white LEDs 28 are turned off as in the conventional case.

  On the other hand, when there is no low gradation region as described above, as shown in FIG. 10, the black insertion unit 9 turns off all the white LEDs 28 including the scanning lines of the black display image. All the white LEDs 28 in the horizontal direction belonging to the scanning lines corresponding to the black display image are turned off. However, since one white LED 28 corresponds to a plurality of scanning lines, for example, 10 scanning lines correspond to each other. When the black display image in the ten scanning lines is displayed, all the corresponding white LEDs 28 in the horizontal direction are turned off. Then, after the white LED 28 at the top is turned off, the white LED 28 of the next line is turned off in order to perform black display.

(10) Effect 1
According to this embodiment, it is possible to determine and determine the presence or absence of the black insertion driving method for each illumination area while performing luminance control according to the image for each illumination area for the purpose of expanding the dynamic range. If there is only a part of the low gradation on the white side, the black insertion drive method is implemented only in that illumination area. In this case, the black insertion driving method can be performed on all illumination areas except the black-side gradation area.

  Therefore, it is possible to reduce the case where the luminance drop due to the black insertion driving method is captured by the illumination light.

(11) Effect 2
Next, the reason why energy saving can be realized by the black insertion driving method according to the present embodiment will be described with reference to FIG.

  FIG. 12A is a graph of black-side gradation without the black insertion driving method, and FIG. 12B is a graph showing a change in luminance of the backlight 12 that performs the black insertion driving method. is there.

  As shown in FIG. 12, when the black insertion driving method is not performed, the luminance of the backlight 12 is substantially constant according to the gradation in the region of the image. On the other hand, when the black insertion driving method is performed, the luminance of the backlight 12 changes from lighting to extinguishing. Therefore, when the light is turned on next time, it is necessary to increase the luminance of the backlight 12 more than before the light is turned off in order to maintain the illuminance of the screen even in the same image gradation.

  Since the overall amount of power from the backlight 12 is proportional to this luminance, the power consumption is higher when the black insertion driving method is performed than when the black insertion driving method shown in FIG. 12A is not performed. It becomes. However, by implementing the black insertion driving method only in a part of the low gradation as in the present embodiment, it is possible to suppress the amount of power due to the increase in luminance when the backlight 12 is turned on after the backlight 12 is turned off, thereby saving energy. Can be realized.

(Example of change)
The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist thereof.

  In the above embodiment, nine white LEDs 28 are arranged in each illumination region 30. However, the number of white LEDs 28 is not limited to this, and the white LEDs 28 can be turned off more finely in accordance with the black insertion driving method. Can do.

It is the block diagram which showed the structural example of the liquid crystal display device of one Embodiment of this invention. It is the figure which showed an example of the area | region division of a screen and a backlight. It is the figure which showed the correspondence of the brightness | luminance level of a backlight, a brightness | luminance, and a gradation signal. It is the figure which showed an example of the result of having calculated the average luminance gradation of the input signal level for every display area. It is the figure shown about the selection method of a backlight luminance level. It is the figure which showed an example of the result of having calculated the average luminance gradation of the input signal level for every illumination area. It is the figure which showed the illumination error by the crosstalk between illumination areas. It is a block diagram of a backlight. It is explanatory drawing when there exists a low gradation area | region on an image. It is explanatory drawing in the case of implementing a black insertion drive method. It is explanatory drawing which implements the black insertion drive method only in a part with a low gradation area | region. It is a graph which shows the change of the brightness | luminance of a backlight when not implementing the black insertion drive method and when implementing

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Liquid crystal display device 9 Black insertion part 10 Controller 11 Liquid crystal panel 12 Backlight 13 Frame memory 22 Source driver circuit 23 Gate driver circuit 28 White LED
30 Illumination area

Claims (2)

A liquid crystal panel for displaying an image in a display area based on an image signal;
A plurality of illumination areas that are turned on by a plurality of point light sources arranged in a grid in the vertical and horizontal directions, and a backlight that illuminates the display area of the liquid crystal panel from each of the illumination areas;
A backlight control unit that calculates the brightness of each illumination area based on the image signal;
A black insertion unit that inserts a black display signal into the image signal for each frame and performs black display in the display region;
In a liquid crystal display device having
The black insertion portion detects a low gradation region having a gradation lower than a reference gradation in the image signal;
The backlight control unit
A plurality of the point light sources corresponding to the display area that performs the black display in the low gradation area, and are turned off independently of illumination of the illumination area;
The brightness of the point light source in the illumination area corresponding to the area other than the low gradation area is turned on according to the gradation other than the low gradation area,
Liquid crystal display device. The liquid crystal panel is an OCB mode liquid crystal panel,
The reference gradation is a gradation that causes reverse transition,
The liquid crystal display device according to claim 1.

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