JP2009058718A - Liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal display which can change the backlight brightness according to the scene changes in the images. <P>SOLUTION: When the gradation change rate between a frame period and the previous frame period is equal to the reference or larger, the average brightness is obtained by averaging the brightness of frame periods of more than one frame period past and having brightness less than the normal state, and the backlight is turned on in this average brightness. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a liquid crystal display device.

Conventionally, a TN type liquid crystal display device has been generally used. However, OCB type liquid crystal display devices characterized by high-speed response have been mounted on liquid crystal televisions, liquid crystal monitors, and the like in order to improve moving image visibility (see, for example, Patent Document 1).
JP 2004-185027 A

  In the liquid crystal display device as described above, in order to improve the contrast, a driving method for changing the luminance of the backlight according to the video is introduced. That is, when the video is determined to be bright, the backlight luminance is increased, and when the video is determined to be dark, the backlight luminance is decreased.

  In such a liquid crystal display device linked with backlight brightness control, the backlight brightness changes complicatedly according to the image, and therefore the backlight brightness can be varied in large increments or the image brightness is complicated. In repeated cases, the backlight brightness appears to flicker.

  Further, in the case where the liquid crystal display device does not have a frame memory, there is a problem that the timing at which an image is output and the timing at which the luminance of the backlight is switched are shifted. For example, when the video at the time of n frames is captured, the backlight brightness is obtained at (n + 1) frames, and the backlight brightness actually changes at (n + 2) frames, so that the backlight brightness is delayed by 2 frames from the displayed video. The brightness changes. For this reason, the flicker due to the backlight luminance appears to be larger.

  For this reason, a technique has been developed in which the backlight brightness value is delayed with a constant time constant to prevent flickering. However, since the gradation level of the image changes suddenly at the time of the scene change of the image, the backlight luminance delay processing cannot follow the change of the image and the delay of the luminance change is uncomfortable for the observer. There is a problem that it is observed as a certain image.

  Therefore, in view of the above problems, the present invention provides a liquid crystal display device that can change the luminance of a backlight according to a scene change of an image.

  The present invention obtains the luminance value of the backlight based on the video signal for each frame period, stores the luminance value, and stores the n frame period (provided from the first frame period). , N> 1, and an average luminance value calculation unit that averages the luminance values to obtain an average luminance value, and the backlight is turned on by the average luminance value, and the video signal of the first frame period is A control unit for displaying, and a gradation change calculation unit for obtaining a change rate of gradation of the video signal between adjacent frames based on the video signal of each frame period, and the average luminance value calculation unit When the rate of change between the first frame period and the frame period immediately before the first frame period is greater than or equal to a reference value, the average luminance value is traced back to the past from the first frame period. Frame periods (where, n> m> 1 a is) averages respective brightness values of obtaining an average luminance value, a liquid crystal display device.

  As described above, the present invention can change the luminance of the backlight according to the change rate of the gradation of the video signal.

  Hereinafter, an OCB type liquid crystal display device according to an embodiment of the present invention will be described with reference to the drawings.

(First embodiment)
An OCB type liquid crystal display device according to a first embodiment of the present invention will be described with reference to FIGS.

(1) Configuration of Liquid Crystal Display Device The configuration of the liquid crystal display device of this embodiment will be described with reference to FIG. FIG. 1 is a block diagram showing a configuration of an OCB type liquid crystal display device.

  The OCB type liquid crystal display device includes a 7 to 9 inch liquid crystal panel 10, a backlight 11, a source driver 12, a gate driver 13, a controller 14, an input power supply 16, and a liquid crystal drive voltage generation circuit 17.

  The liquid crystal panel 10 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 are provided at intersections thereof. An OCB mode liquid crystal is sandwiched between them. In the liquid crystal panel 10 of this embodiment, it is assumed that 1 to m signal lines are formed in the vertical direction and 1 to n scanning lines are formed in the horizontal direction. For example, in the case of WVGA, since it is 800 (× RGB) × 480, n = 480.

  The backlight 11 is disposed on the back surface of the liquid crystal panel 10. The backlight 11 uses an LED as a light source, and the luminance is controlled by a built-in LED driver.

  The input power supply 16 supplies power to the backlight 11, the controller 14, and the liquid crystal drive voltage generation circuit 17, and the liquid crystal drive voltage generation circuit 17 selects the source driver 12 according to the timing at which display data is displayed on the liquid crystal panel 10. The voltage supplied to the gate driver 13 is adjusted.

  The gate driver 13 supplies a gate signal to the scanning line of the liquid crystal panel 10, and the source driver 12 supplies a voltage corresponding to a video signal or the like to the signal line of the liquid crystal panel 10.

  The source driver 12 includes a D / A converter 23 and a shift register 24.

  The controller 14 has a backlight control unit 14. The controller 14 will be described later.

  The liquid crystal display device does not have a frame memory.

(2) Display Operation of Controller 14 Next, a display operation based on the controller 14 in the liquid crystal display device will be described with reference to FIG.

  The video signal which is RGB data is input to the controller 14. The controller 14 performs gamma correction processing on the video signal. Then, in accordance with the start pulse, the display data subjected to the correction processing is transferred to the shift register 24 of the source driver 12 for each row of pixels.

  The controller 14 also transfers black image data for preventing reverse transition to the shift register 24 for each row of pixels in accordance with the start pulse.

  Then, the controller 14 outputs a load pulse to the D / A converter 23 of the source driver 12. The D / A converter 23 simultaneously acquires the data stored in the shift register 24 for one row of pixels at the timing when the load pulse is input, performs D / A conversion, and outputs the voltage corresponding to each display data. Output to the signal line of panel 10.

  At this time, the controller 14 controls the output timing of the pulsed gate signal output from the gate driver 13 to each scanning line, and at the timing when each TFT of the liquid crystal panel 10 responds to the display data. The LEDs 112 are turned on to display the display data on the liquid crystal panel 10.

  Further, the controller 14 has a backlight control unit 15, which will be described next.

(3) Configuration of Backlight Control Unit 15 Next, the configuration of the backlight control unit 15 provided in the controller 14 will be described with reference to FIG.

  The backlight control unit 15 includes a histogram detection unit 30, a frame luminance calculation unit 32, a storage unit 34, an average luminance calculation unit 36, a change rate calculation unit 38, and a backlight drive unit 40.

  The histogram detection unit 30 counts the number of pixels included in each gradation range for each predetermined gradation from the video signal in the input video signal for one frame period, and represents the gradation representing each gradation range and each gradation range. A histogram is generated that is associated with the number of pixels included in (the number of pixels is an example of the pixel frequency). The detected histogram is output to the frame luminance calculation unit 32 and the change rate calculation unit 38, respectively.

  The frame luminance calculation unit 32 calculates the luminance of each frame of the backlight 11 based on the input histogram. For example, when the distribution of low gradation values in the histogram is large, the luminance for one frame period is reduced, and when there are many high gradation values, the luminance value is increased. The calculated luminance value of the backlight for each frame period is output to the storage unit 34.

  The storage unit 34 includes a shift register, and stores a backlight luminance value for each frame period. This storage state will be described in detail later.

  The average luminance calculation unit 36 calls the luminance value for each frame period stored in the storage unit 34, and calculates the average luminance value by averaging the luminance values for each frame period. The detailed operation of the average luminance calculation unit 36 will also be described later.

  The rate-of-change calculating unit 38 calculates the rate of change in gradation between adjacent frames based on the histogram output from the histogram detecting unit 30, and if there is a change in tone that exceeds the reference value, that is, the rate of change is If it is equal to or greater than the reference value, it is assumed that there is a cut point in the adjacent frame period, and the cut point is output to the storage unit 34 and the average luminance calculation unit 36.

  The backlight driving unit 40 adjusts the luminance of the backlight 11 based on the average luminance value output from the average luminance calculating unit 36. The adjustment uses a PWM (Pulse Width Modulation) control method in which luminance is modulated by switching between a light emission period and a non-light emission period at high speed. Therefore, the backlight driving unit 40 generates a PWM signal based on the average luminance value signal and outputs the PWM signal to the backlight 11. The backlight 11 has a built-in LED driver, and causes the LED to emit light based on the PWM control signal.

(4) Operation of Backlight Control Unit 15 Next, the operation of the backlight control unit 15 will be described in detail based on FIG.

  First, as described above, when the video signal is input to the controller 14, processing such as gamma correction is performed and the video signal is displayed on the liquid crystal panel 10. This display timing is defined as a first frame period.

  On the other hand, the backlight control unit 15 takes in the video signal for one frame as described above, detects the histogram, stores it, and then obtains the average luminance value. There will be a delay of 2 frames. For example, when a video signal is input in the 29th frame period, this video signal is displayed as it is in the 29th frame period, whereas the backlight luminance value calculated from the video signal is used in the 32nd frame period. It will be. For this reason, as described in the section for solving the problem, when the video scene is changed, the gradation level of the video changes suddenly. Will be observed. Therefore, in the present embodiment, the following processing is performed to prevent an uncomfortable video display.

  This prevention method will be described with reference to FIG.

  FIG. 3 is an explanatory diagram for obtaining the average luminance value of the backlight 11 from the 32nd frame to the 42nd frame.

  The storage unit 34 includes the shift register as described above, and has a first storage position to a 32nd storage position, and sequentially assigns the luminance value for each frame period calculated by the frame luminance calculation unit 32 to each storage position. Store while shifting.

  The rate-of-change calculating unit 38 obtains the rate of change of the gradation value in the adjacent frame period as described above, and outputs a signal indicating that it is a cut point when this rate of change is higher than the reference value. When it is lower than the reference value, a signal indicating that the cut point is not output is output to the storage unit 34 and the average luminance calculation unit 36, respectively. Hereinafter, a state where a signal indicating that it is not a cut point is output is referred to as a “normal state”, and a case where a signal indicating a cut point is output is referred to as a “scene change state”.

  A case where the change rate calculation unit 38 detects that there is a cut point between the 33rd frame period and the 34th frame period and outputs the fact to the storage unit 34 and the average luminance calculation unit 36 will be described.

(4-1) 32nd frame period Since the 32nd frame period is in a normal state, the luminance value of the 32nd frame period is stored in the first storage position of the storage unit 34, and the second storage position is stored in the second storage position. The luminance value of the 31st frame period is stored, and stored in order, and the luminance value of the first frame period is stored in the 32nd storage position.

  Then, the average luminance calculation unit 36 sums the luminance values for these 36 frames, obtains an average luminance value value divided by 32, and outputs the average luminance value value to the backlight driving unit 40.

(4-2) 33rd frame period Since the 33rd frame period is also in the normal state, the brightness value is stored in each storage position by one shift from the 32nd frame period. That is, the luminance value of the 33rd frame period is stored in the first storage position, the luminance value of the 32nd frame period is stored in the second storage position, the following is stored in order, and the second storage position is stored in the second storage position. Stores the luminance value of the frame period.

  Then, the average luminance calculation unit 36 obtains an average value of the luminance values for these 32 frames, and outputs the average value to the backlight driving unit 40 as an average luminance value.

(4-3) 34th Frame Period In the 34th frame period, since the signal indicating that it is a cut point is input as described above, the storage unit 34 stores a scene change state different from the normal state. That is, the luminance value for the 34th frame period is stored in the first storage position, the luminance value for the 33rd frame period is calculated in the second storage position, and stored in order, and the 27th frame position is stored in the eighth storage position. Stores the luminance value of the period. In the ninth storage position, the luminance value of the currently input 34th frame period is stored, and thereafter all the luminance values of the 34th frame period are stored up to the 32nd storage position.

  Then, the average luminance calculation unit 36 sums the luminance values stored from the first storage position to the eighth storage position, divides them by 8, obtains an average luminance value, and outputs this to the backlight drive unit 40. . In this way, when the cut point, that is, the gradation change rate is large, the average luminance value is obtained by averaging the luminance values of eight frames, so that the conventional feeling of discomfort due to the luminance delay of the backlight is reduced.

(4-4) 35th to 41st Frame Period Next, in the 35th frame period, the storage unit 34 stores the luminance value of the 35th frame period in the first storage position, and in turn in the 8th storage position. The luminance value of the 28th frame period is stored, and the luminance value of the 34th frame period is stored from the ninth storage position to the 32nd storage position.

  Then, the average luminance calculation unit 36 averages the luminance values for eight frames in the same manner as described above to obtain the average luminance value.

  Thereafter, similarly up to the 41st frame period, when obtaining the average luminance value for eight times, the average luminance value is calculated using the luminance at the first storage position to the eighth storage position.

(4-5) After the 42nd frame period In the 42nd frame period, the influence of the cut point is eliminated, and the luminance value of the 42nd frame period is stored in the first storage position of the storage unit 34, and then stored in order. After the ninth storage position, the luminance value for the 34th frame period is stored.

  Then, the average luminance calculation unit 36 obtains the average value of the luminance values for 32 frames and outputs the average value to the backlight driving unit 40.

  When such a shifting operation is repeated 24 times after the 42nd frame period, the luminance values are sequentially stored in the respective storage positions as in the 32nd frame period.

(5) Effect Only when the cut point is detected as described above, the luminance value of the eight frames is averaged to obtain the average luminance value. On the other hand, flickering due to fine changes in the brightness of the backlight can be prevented during normal video display.

(Second Embodiment)
Next, a liquid crystal display device according to a second embodiment will be described.

  In the first embodiment, the average luminance value of the luminance values of the backlight 11 is obtained based on one reference value. However, with only one such reference value, the gradation level changes greatly even with a scene change. If the gradation level changes greatly even if it is not a scene change or a scene change, the change speed and the video change may not always mesh well. Therefore, in the present embodiment, two types of cut points are detected using a plurality of reference values (for example, two types of reference values), and the calculation method of the average luminance value is changed based on this.

  For example, in the first embodiment, when it is higher than the reference value, 32 frames are changed to 8 frames, but in this embodiment, the first level when the degree of change is small, the first level in the case of the standard. Divided into 3 levels, 2 levels and 3rd level in the early case.

  At the first level, instead of the average of the luminance values for 32 frames, the average luminance value of the luminance values for 12 frames is used. At the second level, the average luminance value is obtained using the luminance values for 8 frames instead of the luminance values for 12 frames. At the third level, the average value of the luminance values for four frames is used instead of the average luminance value for eight frames.

  As a result, the luminance value of the backlight 11 changes quickly in the case of an image accompanied by a large luminance change in the video scene, and slowly in the case of an image accompanied by a slight luminance change in the video scene. By changing the luminance value, it is possible to realize a change in the luminance of the backlight that is more linked to the video image, so that a change in the backlight 11 does not cause a sense of incongruity, and flickering can be prevented.

(Third embodiment)
Next, a liquid crystal display device according to a third embodiment will be described with reference to FIGS.

  In the first embodiment and the second embodiment, the same brightness control is performed in both the bright state from the dark state and the dark state to the bright state, but from this dark state to the bright state and the bright state. When the gradation changes from a dark state to a dark state and when the gradation changes to a reference value or more, different control may be required.

  As shown in FIG. 4, when changing from a dark state to a bright state, the luminance of the backlight 11 gradually decreases even if control is performed by the control method of the first embodiment and the second embodiment. Flushing can be prevented. 4 to 6, the upper diagram shows the luminance of the backlight 11 when the delay process is performed at the cut point, and the lower diagram shows the gamma conversion state of the video.

  However, if the same processing as in the first and second embodiments is performed when the state changes from a bright state to a dark state, image quality degradation occurs due to a long transition period as shown in FIG. There is a case. For example, when the average luminance value for 8 frames is obtained and changed as in the first embodiment, image quality degradation may occur due to the transient period for 8 frames.

  Therefore, as shown in FIG. 6, when changing from a bright state to a dark state to a reference value or more, instead of an average luminance value value for eight frames, an average luminance value value of luminance values with fewer frames than four frames. And the luminance of the backlight 11 is changed based on the average luminance value.

  As a result, as shown in FIG. 6, even when the gamma conversion is changed from the bright state to the dark state and the image is dark, the amplitude is small and unnoticeable because the image is dark, and the image can be displayed without a sense of incongruity. .

  In addition, when calculating | requiring the average luminance value value of the luminance value of the number of frames less than 4 frames, you may use only the luminance value of the last frame.

(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.

  Although the OCB type liquid crystal display device has been described in the above embodiment, the change in luminance of the backlight according to the present invention can be applied to other TN type liquid crystal display devices.

It is a block diagram of the liquid crystal display device in a 1st embodiment. It is a block diagram of a backlight control unit. It is drawing which shows the content of the luminance value stored in each storage position in a storage part. It is drawing which shows the state which can suppress the flushing for the backlight brightness fall in 2nd Embodiment, an upper figure shows a backlight brightness | luminance, and a lower figure is a figure which shows the state of a gamma conversion. It is a figure which shows the state which the image quality degradation generate | occur | produced by the cut period becoming long. It is a figure in 3rd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Liquid crystal panel 11 Backlight 12 Source driver 13 Gate driver 14 Controller 15 Backlight control part 30 Histogram detection part 32 Frame brightness calculation part 34 Storage part 36 Average brightness calculation part 38 Change rate calculation part 40 Backlight drive part

Claims (3)

A storage unit for obtaining the luminance value of the backlight based on the video signal for each frame period, and storing the luminance value;
An average luminance value calculation unit that averages the luminance values of the stored n frame periods (where n> 1) from the first frame period to obtain an average luminance value;
A controller that turns on the backlight according to the average luminance value and displays the video signal of the first frame period;
A gradation change calculation unit for obtaining a change rate of gradation of the video signal between adjacent frames based on the video signal of each frame period;
Have
The average luminance value calculation unit
When the rate of change between the first frame period and the frame period immediately before the first frame period is greater than or equal to a reference value, the average luminance value is traced back from the first frame period to the m frame period. (Where n>m> 1) is averaged to obtain an average luminance value.
Liquid crystal display device. The m frame period is shortened as the change rate is large, and the m frame period is lengthened as the change rate is small.
The liquid crystal display device according to claim 1. When the change rate is equal to or higher than a reference value and the gradation changes from a dark state to a bright state, an average luminance value is obtained by averaging the luminance values of the m frame periods,
When the change rate is equal to or higher than a reference value and the gradation changes from a bright state to a dark state, the luminance values of s frame periods (where m> s => 1) are averaged. Find the average brightness value,
The liquid crystal display device according to claim 1.

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