The present application claims priority from Japanese patent application serial No. JP 2009-205844, filed on Sep. 7, 2009, the content of which is hereby incorporated by reference into this application.
(1) Field of the Invention
The present invention concerns a liquid crystal display device having a backlight for illuminating a liquid crystal panel that displays images at the back thereof and adjusting the luminance of the backlight in accordance with video signals to be displayed, and a backlight control method.
(2) Description of the Related Art
Different from an emissive type display device such as a CRT (cathode ray tube) or a plasma display panel, a liquid crystal display device has a non-light emitting liquid crystal panel (transparent light modulator) and a backlight for illuminating the panel at the back surface thereof. Usually, the backlight is caused to emit a light at a predetermined brightness irrespective of video signals and an image of a desired bright is displayed by controlling the light transmittance of a liquid crystal panel in accordance with the brightness of the video signals. Accordingly, the electric power of a backlight light source is not decreased even for dark images and consumed constantly to make the power efficiency poor. As a countermeasure, it has been proposed a technique of making the brightness of the backlight (hereinafter also expressed as luminance) variable and controlling the gradation level of the liquid crystal panel and the brightness of the backlight in accordance with the level of the input video signals, thereby decreasing the power consumption and improving the image quality.
For example, in a liquid crystal display device described in Example 1 of JP-A No. 2008-15430, a backlight is divided into a plurality of regions (light source blocks), a brightest gradation level in the frame is detected on every R, G, B of input video signals in each region, the gradation level of the input video signals is converted such that the gradation level is at a level identical with the upper limit value of the gradation level, and the backlight is turned on and off at a duty cycle corresponding to the ratio of the brightest gradation level to the upper limit value of the gradation level during lighting period of the backlight. Further, in Example 2 of JP-A No. 2008-15430, an average value for gradation levels within a predetermined range containing the brightest gradation level in the frame is detected, the gradation level of the input video signals is converted such that the average value is at a level identical with the upper limit value of the gradation level, and the backlight is turned on and off at a duty corresponding to the ratio of the average value to the upper limit value of the gradation level.
According to the technique described in Example 1 of JP-A No. 2008-15430, when the brightest gradation level Ppeak is lower than the upper limit value Pmax of the gradation level, and the brightness of the backlight is lowered to a value obtained by multiplying the ratio Ppeak/Pmax to decrease the consumption power by so much. Further, according to the technique described in Example 2 of the Patent Document 1 (referred to as an average value method), since the detected average value Pav is lowered to less than the brightest gradation level Ppeak, and the brightness of the backlight is lowered to a value obtained by multiplying the ratio Pav/Pmax of the average value Pav and the upper limit value Pmax of the gradation level, the power consumption is further decreased.
However, in a case of using the average value method described above, since the gradation level of the input video signals is converted such that the average value Pav is at a level identical with the upper limit value Pmax of the gradation level, the gradation level is clipped at the upper limit value Pmax for the input video signal at the gradation level brighter than the average value Pay. As a result, in the clipped pixel, an inherent brightness of the images can no more be expressed. The phenomenon is hereinafter referred to as “deterioration of gradation”.
The deterioration of gradation causes deterioration of the image quality and the prominence of degradation (visual recognizability) is different depending on the generation position within the screen. In a backlight divided into a plurality of regions, deterioration of gradation generated in a central portion of the region is not so visually remarkable. However, when the deterioration of gradation is generated in a region boundary portion, continuity of brightness with an adjacent region cannot be kept and this tends to be visually recognized as the luminance step.
The present invention intends to provide a liquid crystal display device for decreasing the power consumption of a backlight while suppressing generation of a luminance step in a region boundary and a method of controlling the backlight.
The present invention provides a liquid crystal display device having a liquid crystal panel for controlling the transmittance of pixels in accordance with the gradation level of input video signals and a backlight for illuminating the liquid crystal panel at the back thereto, in which the liquid crystal panel is divided for the pixels on the panel into sub-regions comprising a plurality of pixel groups, and the backlight includes a plurality of light source blocks corresponding to the sub-regions and has; an in-region gradation value detection section which detects the gradation level of input video signals on every sub-region, removes gradation levels of pixels within a predetermined upper range (m %) from the brightest gradation level in the sub-region and detects the maximum in-region gradation level Pa, a boundary gradation value detection section which removes gradation levels of pixels within a predetermined upper range (n %) from the brightest gradation level and detects the maximum boundary gradation level Pb for the pixel groups belonging to the region boundary portion in the sub-region, a backlight control value deciding section which selects higher one of the in-region maximum gradation level Pa and the maximum boundary gradation level Pb as a maximum gradation level Pc for the sub-region, and decides the backlight control value K based on the ratio between the maximum gradation level Pc and the upper limit value Pmax of the gradation level, and a backlight control section which controls lighting at a backlight luminance based on the backlight control value K to the light source block for illuminating sub-region.
The present invention provides a method of controlling a backlight of a liquid crystal display device having a backlight for illuminating the liquid crystal panel at the back thereof, in which the liquid crystal panel is divided for the pixels on the panel is divided for the pixels on the panel into sub-regions comprising a plurality of pixel groups, and the backlight has a plurality of light source blocks corresponding to the sub-regions, the method including; detecting gradation levels of input video signals on every sub-region, removing the gradation levels for pixels within a predetermined upper range (m %) from the brightest gradation level in the sub-regions, and detecting the in-region maximum gradation level Pa, removing gradation levels for the pixels within a predetermined upper range (n %) from the brightest gradation level for the pixel groups belonging to the region boundary portion in the sub-region and detecting the maximum boundary gradation level Pb, selecting a higher one of the maximum in-region gradation level Pa and the maximum boundary gradation level Pb as the maximum gradation level Pc in the region, deciding the backlight control value K based on the ratio between the maximum gradation level Pc and the upper limit value Pmax of the gradation level, and controlling lighting at a backlight luminance based on the backlight control value K to the light source block for illuminates the sub-region.
The present invention can provide a liquid crystal display device capable of greatly reducing the power consumption of a backlight while suppressing the deterioration of image quality caused by the luminance step, and a method of controlling the backlight.
These and other features, objects and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings wherein:
Preferred embodiments of the invention are to be described with reference to the drawings.
At first, division of the region on the screen of the liquid crystal panel 108 is to be described.
An optical guide plate 904 for guiding an emission light from the LED 901 (shown by dotted arrows in
The cross section of the optical guide plate 904 in the direction vertical to the liquid crystal panel 906 (left to right direction of the sheet in
The diffusion plate 905 diffuses the light emitted from the optical guide plate 904 into a further uniform planar light in view of the space and emits the same to the liquid crystal panel 906. The liquid crystal panel 906 is controlled for the light transmittance on every pixel based on the input video signals and modifies the light from the diffusion plate 905 in view of the space to form images. Thus, an image light shown by arrows directing upward of the sheet in the drawing is outputted to the frontal side of the liquid crystal display device.
While LED that emits a white light is used as the LED 101 in this embodiment, LED is not restricted thereto and, for example, a set of three LEDs emitting lights of red, blue, green three colors respectively may be used in plurality.
The optical source blocks 202 having the structure as described above are arranged in plurality in a 2-dimensional manner, i.e., in the horizontal and vertical directions of the screen at the back of the liquid crystal panel. Then, the brightness of the sub-regions 201 can be controlled independently by individually controlling LEDs 101 (a set of three in this embodiment) disposed to each of the optical source blocks 202.
Then, operation for each of the portions is to be described. The in-region gradation value detection section 102 detects the gradation level of input video signals for the pixel groups belonging to the in-region 303 on every plural sub-regions 201 of the liquid crystal panel, and prepares an in-region histogram of the gradation level (frequency table) based on the result of the detection. Then, gradation levels for the pixels within a predetermined upper range (m %) from the brightest gradation level in the sub-region 201 are removed and the brightest gradation level among the remaining gradation levels is determined as the maximum in-region gradation level Pa.
The boundary gradation value detection section 103 detects the gradation level of input video signals for the pixel groups belonging to the boundary portion 305 in the sub-region on every plurality of sub-regions 201 of the liquid crystal panel and prepares a boundary portion histogram (frequency table) of the gradation level based on the result of detection. Then, gradation levels for the pixels within a predetermined upper range (n %) from brightest gradation levels in the boundary portion 305 are removed and the brightest gradation level among the remaining gradation levels is determined as the maximum boundary gradation level Pb.
The backlight control value deciding section 104 is a control data calculation section that gives a control data corresponding to each of the light source blocks 202 to the backlight control section 105, and compares the maximum in-region gradation level Pa detected by the in-region gradation value detection section 102 with the maximum boundary gradation level Pd detected by the boundary gradation value detection section 103, and decides a higher gradation level of them as the maximum gradation level Pc of the sub-region 201. Then, the backlight control value K to the sub-region 201 is determined based on the ratio between the decided maximum gradation level Pc and the upper limit value Pmax of the gradation level.
K=Pc/Pmax (K≦1)
Further, the luminance B of the backlight is lowered based on the backlight control value K as:
B=K×Bmax (Bmax is the maximum luminance of the backlight)
to decrease the power consumption of the backlight.
The backlight control section 105 receives the backlight control value K to each sub-region decided by the backlight control value deciding section 104 and controls the backlight 109 (light source block) belonging to the sub-region for lighting up the light source (LED). For adjusting the luminance of the light source, control is conducted, for example, by PWM (pulse width modulation) or amplitude modulation. In a case of PWM, the duty ratio is set to 100% for the maximum luminance and the duty ratio is changed in accordance with the backlight control value K. Further, it is preferred that the PWM frequency is higher than the frame frequency of the liquid crystal display device.
The backlight luminance calculation section 106 calculates the backlight luminance on the screen based on the backlight control value K to each of the sub-regions sent from the backlight control value deciding section 104. The backlight luminance Bsum at an arbitrary point A on the screen is determined by determining the luminance at the point A when the backlight (light source block) for each of sub-regions is lit one by one at the backlight control value K and taking the sum thereof.
The image correction section 107 corrects the video signals (gradation value) for each pixel based on the backlight luminance Bsum calculated by the backlight luminance calculation section 106. Assuming the backlight luminance when lit at 100% as Bmax, the backlight luminance as Bmax and the gradation value of the input video signal before correction as Pin, correction is conducted such that the gradation value Pout after the correction is:
Pout=Pin×Bmax/Bsum
The corrected video signals are sent to the liquid crystal panel 108.
In the liquid crystal panel 108, a gradation voltage control signal and a driving control signal are generated based on the input video signals after the correction, a gradation voltage is applied to the pixel circuit on the panel, and the transmittance of liquid crystals in the pixel region is controlled.
Respective elements of the in-region gradation detection section 102, the boundary gradation value detection section 103, the backlight control value deciding section 104, the backlight control section 105, the backlight luminance calculation section 106, and the image correction section 107 may, for example, be integrated into a unit backlight control circuit. For example, the backlight control circuit may be assembled into a central processing unit for controlling the entire liquid crystal display device in response to a user's instruction from a remote controller, or may be constructed with an IC or LSI used exclusively for backlight control separate from the CPU.
While not illustrated in
Further, the temporal filter is used for preventing flickering. The backlight control value K for each sub-region is held by several frames and compared with the backlight control value K′ of each region corrected by the spatial filter. In a case where the difference is larger than a predetermined threshold value, the control value is replaced with a value by slightly increasing or decreasing the held control value K instead of the corrected control value K′ as the backlight control value.
In this embodiment, the maximum gradation level Pb in the boundary portion 305 in the sub-region is determined together with the maximum gradation level Pa in the sub-region 201 and a higher gradation level is adopted as the maximum gradation level Pc. Accordingly, in a case where a pixel of high gradation level is present being localized to the boundary portion 305, the maximum gradation level Pc can be decided with preference. As a result, the deterioration of gradation in the boundary portion 305 can be decreased to suppress the accompanying luminance step.
At S402, pixel values after the gamma conversion are divided into a plurality of predetermined gradation levels. For example, when the maximum gradation value is 255, levels of 16 steps are set at the gradation width 16 (H1 to H16 in
At S404, it is judged whether the object pixel belongs to the boundary portion 305 or not. If it belongs to the boundary portion 305, the process proceeds to S405 and, if it does not belong (that is, if it is the central portion 304), the process proceeds to S406. At S405, the pixel is counted as the frequency value to the gradation level obtained at S402. At S406, it is judged if the processing for all pixels in the in-region 303 has been completed or not. It has not yet been completed, the process returns to S401 and the processing for remaining object pixels is repeated. If it has been completed, the process proceeds to S407.
At S407, the maximum in-region gradation level Pa is decided with reference to the in-region histogram. In the in-region histogram, gradation levels for the pixels within the predetermined upper range (m %) from the brightest gradation level are removed and the brightest gradation level among the remaining gradation levels is determined as the maximum in-region gradation level Pa. At S408, the maximum boundary gradation level Pb is decided with reference to the boundary portion histogram. In the boundary portion histogram, the gradation levels for the pixels within a range (n %) of the predetermined upper range are removed from the brightest gradation level, and the brightest gradation level among the remaining gradation levels is determined as the maximum boundary gradation level Pb.
At S409, the maximum in-region gradation level Pa and the maximum boundary gradation level Pb are compared and a higher gradation level is defined as the maximum gradation level Pc in the sub-region. At S410, the ratio Pc/Pmax between the maximum gradation level Pc and the upper limit value Pmax of the gradation value is determined as the backlight control value K to the sub-region 201.
On the other hand,
In this case, the maximum in-region gradation level Pa is H12 and the maximum boundary gradation level Pb is H13, and H13 of the higher gradation level is adopted as the maximum gradation level Pc for the sub-region. In accordance therewith, the backlight control value K is decided as: K=Pc/Pmax=H13/H16=0.81.
The range (m %) for the pixels to be removed in the in-region histogram and the range (n %) for the pixels to be removed in the boundary portion histogram may be set properly in accordance with the permissible value for degradation of image quality, or the m value and the n value may be set in different ranges.
According to this embodiment, since the maximum gradation level Pc is decided by removing the levels within the predetermined range from the brightest gradation level, the power consumption of the backlight can be decreased remarkably. In this case, when a pixel of high gradation level is present in the boundary portion, the maximum gradation level Pc is decided with a priority being given thereto. Accordingly, the deterioration of gradation 614 in the boundary portion 603 as shown in
An in-region gradation value detection section 102 detects the gradation level of input video signals and prepares an in-level histogram for gradation levels based on the result of detection. Then, gradation levels for the pixels present in two predetermined ranges (m1%, m2%) are removed from the brightest gradation level in the sub-region 201 and the brightest gradation levels among the remaining gradation levels are determined as the two types of maximum in-region gradation levels Pa1, Pa2. When it is set as m1<m2, Pa1≧Pa2. It is set, for example, as m1=1% and m2=3%.
A boundary gradation value detection section 103 determines the maximum boundary gradation level Pb in the same manner as in
According to this embodiment, when a pixel of high gradation level is present in the boundary portion, since the range to be removed from the upper portion of the histogram is narrowed (m1% is adopted), deterioration of gradation in the boundary portion can be decreased to suppress the accompanying generation of the luminance step.
An in-region gradation detection section 102 determines the maximum in-region gradation level Pa in the same manner as in
That is, with reference to
According to this embodiment, the boundary gradation value detection section 103 conducts counting to the boundary portion histogram only for the pixels on both sides of the boundary line in which the gradation levels are close to each other.
Since the luminance step tends to be conspicuous in a case where the luminance on both sides of the boundary are close to each other, this can provide an effect of suppressing the generation of the luminance step more reliably.
In the description for the preferred embodiments of the invention described above, the maximum gradation level Pa for the entire inside of the sub-region and the maximum gradation level for at the boundary portion of the sub-region with the sub-region adjacent therewith, the invention is not restricted thereto. For example, the gradation level for the boundary portion of the sub-region and the maximum gradation level for the central portion except for the boundary portion may be determined. Further, a histogram for the entire inside of the sub-region may be determined and the maximum gradation level for the boundary portion may be determined by removing the histogram for the central portion therefrom. That is, any method may be used so long as it is possible to determine the maximum gradation level when a sub-region is taken entirely (that is, central portion except for the boundary portion or the entire sub-region including the central portion) and the maximum gradation level in the boundary portion of the sub-region.
While we have shown and described several embodiments in accordance with our invention, it should be understood that disclosed embodiments are susceptible of changes and modifications without departing from the scope of the invention. Therefore, we do not intend to be bound by the details shown and described herein but intend to cover all such charges and modifications that fall within the ambit of the appended claims.
Number | Date | Country | Kind |
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2009-205844 | Sep 2009 | JP | national |