Abstract:
Elements of the present invention relate to systems and methods for generating, modifying and applying backlight array driving values.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the present invention comprise methods and systems for generating, modifying and applying backlight driving values for an LED backlight array. 
       BACKGROUND 
       [0002]    Some displays, such as LCD displays, have backlight arrays with individual elements that can be individually addressed and modulated. The displayed image characteristics can be improved by systematically addressing backlight array elements. 
       SUMMARY 
       [0003]    Some embodiments of the present invention comprise methods and systems for generating, modifying and applying backlight driving values for an LED backlight array. 
         [0004]    The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS 
         [0005]      FIG. 1  is a diagram showing the elements of an exemplary LCD display; 
           [0006]      FIG. 2  is a chart showing a typical LCD response; 
           [0007]      FIG. 3  is a diagram showing a typical LCD with a CCFL backlight; 
           [0008]      FIG. 4  is a diagram showing a typical LCD with an LED backlight; 
           [0009]      FIG. 5  is a chart illustrating a ghosting effect; 
           [0010]      FIG. 6  is a plot showing an exemplary cluster screen function with backlight on times; 
           [0011]      FIG. 7  is a plot showing an exemplary disperse screen function with backlight on times; 
           [0012]      FIG. 8  is a plot showing a transition between disperse and cluster screen functions; 
           [0013]      FIG. 9  is a plot showing a transition between disperse and cluster screen functions using transition frames; 
           [0014]      FIG. 10  is a diagram showing a timing chart for a typical processor; 
           [0015]      FIG. 11  is a diagram showing an LED backlight array; 
           [0016]      FIG. 12  is a diagram showing offset blank signals; 
           [0017]      FIG. 13A  is a diagram showing pulse widths corresponding to a blank signal, wherein pulse widths are measured forward from the leading edge of the pulse; 
           [0018]      FIG. 13B  is a diagram showing pulse widths corresponding to a blank signal, wherein pulse widths are measured backward from the leading edge of the pulse; and 
           [0019]      FIG. 14  is a diagram showing an exemplary apparatus comprising PWM timing correlated with a blank signal. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0020]    Embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The figures listed above are expressly incorporated as part of this detailed description. 
         [0021]    It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the methods and systems of the present invention is not intended to limit the scope of the invention but it is merely representative of the presently preferred embodiments of the invention. 
         [0022]    Elements of embodiments of the present invention may be embodied in hardware, firmware and/or software. While exemplary embodiments revealed herein may only describe one of these forms, it is to be understood that one skilled in the art would be able to effectuate these elements in any of these forms while resting within the scope of the present invention. 
         [0023]    In a high dynamic range (HDR) display, comprising an LCD using an LED backlight, an algorithm may be used to convert the input image into a low resolution LED image, for modulating the backlight LED, and a high resolution LCD image. To achieve high contrast and save power, the backlight should contain as much contrast as possible. The higher contrast backlight image combined with the high resolution LCD image can produce much higher dynamic range image than a display using prior art methods. However, one issue with a high contrast backlight is motion-induced flickering. As a moving object crosses the LED boundaries, there is an abrupt change in the backlight: In this process, some LEDs reduce their light output and some increase their output; which causes the corresponding LCD to change rapidly to compensate for this abrupt change in the backlight. Due to the timing difference between the LED driving and LCD driving, or an error in compensation, fluctuation in the display output may occur causing noticeable flickering along the moving objects. The current solution is to use infinite impulse response (IIR) filtering to smooth the temporal transition, however, this is not accurate and also may cause highlight clipping. 
         [0024]    An LCD has limited dynamic range due the extinction ratio of polarizers and imperfections in the LC material. In order to display high-dynamic-range images, a low resolution LED backlight system may be used to modulate the light that feeds into the LCD. By the combination of modulated LED backlight and LCD, a very high dynamic range (HDR) display can be achieved. For cost reasons, the LED typically has a much lower spatial resolution than the LCD. Due to the lower resolution LED, the HDR display, based on this technology, can not display high dynamic pattern of high spatial resolution. But, it can display an image with both very bright areas (&gt;2000 cd/m 2 ) and very dark areas (&lt;0.5 cd/m 2 ) simultaneously. Because the human eye has limited dynamic range in a local area, this is not a significant problem in normal use. And, with visual masking, the eye can hardly perceive the limited dynamic range of high spatial frequency content. 
         [0025]    Another problem with modulated-LED-backlight LCDs is flickering along the motion trajectory, i.e. the fluctuation of display output. This can be due to the mismatch in LCD and LED temporal response as well as errors in the LED point spread function (PSF). Some embodiments may comprise temporal low-pass filtering to reduce the flickering artifact. 
         [0026]    Aspects of some embodiments of the present invention may be described with reference to  FIG. 1 , which shows a block diagram of a data path in an LCD panel. Video data  2  from different sources are input to the scanning timing generator circuit  4  where video data is converted to a format that can be displayed on an LCD  14 . Each line is sent to the overdrive circuit  8  to compensate for the LCD&#39;s slow temporal response. The overdriven signal is converted to a voltage in the data driver  12  and output to data electrodes on the LCD  14 . The scanning timing generator  4  also outputs a clock to the gate driver  10 , selects one row at a time, and stores the voltage data on the data electrode on the storage capacitor of each pixel. Scanning timing generator  4  also generates backlight control signal controlling timing for backlight flashes and sends these signals to the backlight controller  16 . The overdrive circuit  8  may also store video image data in a frame buffer  6  to detect various changes or trends between video frames. 
         [0000]    Motion Blur Reduction with Flashing Backlight 
         [0027]    Typical overdrive processes can reduce the motion blur due to an LCD&#39;s slow temporal response, but generally do not eliminate the motion blur completely. This is due to the fact that the image displayed on the LCD is always on during the entire frame time. The fact that the eye tracks the motion while the image is held during the frame time causes a relative motion on the retina. The average effect of this relative motion on the retina is perceived as motion blur. 
         [0028]    One way to reduce this motion blur is to reduce the time that an image frame is displayed.  FIG. 2  illustrates a flashing backlight approach. The backlight is off after LCD driving voltage is applied and then turned on near the end of the frame period  20  when the LCD transmission approaches the target level. 
         [0029]      FIG. 3  illustrates an LCD display comprising an LCD layer  30 , which comprises a plurality of addressable LCD “cells”  38 , which act as light valves that can be individually modulated. This display also comprises a diffusion layer or diffuser  32 , which acts to diffuse light emitted from a backlight  34 . The backlight  34  of this exemplary display comprises multiple cold-cathode fluorescent (CCFL) tubes  36 . The diffusion layer  32  functions, at least in part, to diffuse the light from the tubes  36  so that the light is transmitted evenly onto the LCD layer  30 . In some embodiments of the present invention, the backlight  34  can be modulated, such as by flashing, to effect motion-blur-related and flicker-related characteristics as well as brightness and other characteristics. 
         [0030]      FIG. 4  illustrates an LCD display comprising an LCD layer  40 , which comprises a plurality of addressable LCD “cells”  48 , which act as light valves that can be individually modulated. This display also comprises a diffusion layer or diffuser  42 , which acts to diffuse light emitted from a backlight  44 . The backlight  44  of this exemplary display comprises multiple light-emitting diode (LED) elements  46 . The diffusion layer  42  functions, at least in part, to diffuse the light from the LEDs  46  so that the light is transmitted evenly onto the LCD layer  40 . In some embodiments of the present invention, the backlight  44  can be modulated, such as by flashing, to affect motion-blur-related and flicker-related characteristics as well as brightness and other characteristics. 
         [0031]    Backlight flashing can reduce motion blur, but, flickering, which is normally associated with a cathode ray tube (CRT) display, is visible due to the impulse backlight. One way to reduce the flickering artifacts is to increase the refresh rate. CRT monitors used in computer display are commonly set to a refresh rate of 75 Hz to reduce flickering. For an LCD, with a fixed frame rate, it is possible to flash the backlight multiple times per frame to increase the refresh rate. However, for motion images, multiple flashes in a single frame can cause ghosting images. 
         [0032]      FIG. 5  illustrates the path of an object with constant motion on a display with double flashing. With the first flashing of each frame period  50   a - 50   d,  we can see the object moving along the solid line  54 . With the second flashing at half of a frame period later  52   a - 52   c,  the same image is shown again, but shifted in the time axis by half of the frame period. The perceived object motion is along the dashed line  56  (ghosting object). 
         [0033]    One way to solve this ghosting problem is to drive the LCD at the same rate as the backlight flashing rate, e.g. 120 Hz, and using motion compensated frame interpolation. However, the costs associated with motion estimation and a high frame rate driver in LCD is generally prohibitive. 
         [0034]    Some embodiments of the present invention comprise a motion-detection-based temporal dithering algorithm that can adapt to the video content. Each frame in a video sequence may be divided into multiple blocks. Each block corresponds to a backlight element, such as a CCFL tube or an LED. The backlight (e.g., CCFL tube or LED) may be operated in either “on” or “off” mode. Temporal dithering may be used to have the desired backlight output for each block. In temporal dithering, the desired backlight level is compared to a preset value called the screen function. If the backlight level is greater than the screen function, the backlight is turned on; otherwise, the backlight is off. 
         [0035]    In some embodiments, motion detection may be performed to classify each block as a motion block or a still block. The motion blocks may be temporally dithered with a “cluster” screen that is optimized for rendering a motion image. The still blocks may be dithered with a “dispersed” screen that is optimized for reducing flickering. The cluster screen can prevent motion blur, and since these blocks contain motion, flickering is typically not visible in these blocks. The dispersed screen can increase the backlight frequency to above the human visual system&#39;s flickering perception threshold. 
         [0036]      FIG. 6  shows an exemplary temporal dithering using a cluster screen. An exemplary screen function for a cluster screen is given by 
         [0000]        S   c ( t )= A (1−( t −floor( t ))) 
         [0000]    where t is the time in frames, and A is the screen amplitude, which determines the flashing duty cycle. Larger A reduces the duty cycle, which leads to lower motion blur. 
         [0037]      FIG. 7  shows an exemplary dispersed screen. An exemplary screen function for the dispersed screen is given by 
         [0000]        S   d   =A (1−(2 t −floor(2 t ))). 
         [0038]    The desired backlight level  60 ,  70 (dashed line in the figures) is compared to the screen function  62 ,  72  (solid line). If the desired backlight level  60 ,  70  is greater than the screen function  62 ,  72 , the backlight is on as indicated with the thick solid line on top of the  FIGS. 64 ,  74 . In this exemplary embodiment, the backlight on a dispersed screen  74  has twice the temporal frequency as the backlight with the cluster screen  64 , which can eliminate the perception of flickering. In other embodiments, other functions and mathematical relationships may be used to define cluster and dispersed screen functions. For example, sinusoidal functions, step functions, triangular functions and other functions and relationships may be used in some embodiments. It should be noted, however, in this exemplary embodiment, that the backlight is turned on at the later end of each backlight period. This may occur at the end of the frame period, as in the cluster screen with only one backlight period per frame or at the end of each backlight period of a frame, as in the dispersed screen where a backlight period may end at the midpoint of a frame period as well as the end of the frame period. Configuring the backlight o go on at the end of each backlight period gives the LCD more time to respond to its signal and reach its desired output. 
         [0039]    One problem with the two-screen approach is the boundary effect. Switching from one screen (e.g., disperse) to another screen (e.g., cluster) causes a temporal discontinuity as shown in  FIG. 8 . This discontinuity coupled with motion tracking of the eye causes flickering. Although this flickering is at a lower amplitude, it is also at a lower frequency and is therefore more objectionable to a typical viewer. 
         [0040]    To remove this flickering effect, some embodiments of the present invention create a transition region that may last one or more frames to gradually transition from one dither screen to another.  FIG. 9  illustrates an exemplary scheme using three transition screens to reduce the flickering effect. The screens in the transition frames are given by: 
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         [0000]    where N is the total number of transition frames, and i denotes the i th  transition frame. The transition from cluster to disperse may be the reverse of the transition from disperse to cluster. 
         [0041]    The concept of dithering using disperse and cluster screens can be implemented using an LED driver with programmable “on” timing and “off” timing. 
         [0042]      FIG. 10  shows the grayscale PWM timing chart of a typical processor. This processor controls  16  LEDs and all  16  LEDs share the same “on” timing, which is the falling edge of the BLANK signal. Since each LED&#39;s “on” timing and “off” timing are adaptive based on image content as well as motion. Some embodiments of the present invention are adapted to be implemented using this driver. 
         [0043]      FIG. 11  shows a typical arrangement of LED drivers  110  and LED backlight elements  112  in a display. Each driver  110  controls LEDs  112  in the same vertical position. The PWM “on” time is controlled by the BLANK signal. To compensate for the time difference between LCD driving from top to bottom, the BLANK signal may be shifted in synchronization with the LCD driving as shown in  FIG. 12 . In this exemplary embodiment, VBR n    120  and VBR n+1    121  are two vertical blanking retracing signals, which define an LCD frame time  122 . For each LCD frame, there may be two (or more) LED PWM pulses. In some embodiments, the time between the two PWM pulses  125  (T offset2 −T offset1 ) is exactly half of the LCD frame time  122  in this exemplary embodiment. T offset1    123  and T offset2    124  are adjusted based on their vertical position to synchronize with the LCD driving. For shorter duty cycles (i.e., duty cycle less than 100%), T offset1    123  and T offset2    124  should be shifted to the right so that PWM on occurs at the flat part of the LCD temporal response curve  20  as shown in  FIG. 2 . 
         [0044]    The use of two PWM pulses in one LCD frame enables motion adaptive backlight flashing. If there is no detected motion, the two PWM pulses may have the same width, but may be offset in time by half of an LCD frame time. If the LCD frame rate is 60 Hz, the perceived image is actually 120 Hz, thus eliminating the perception of flickering. If motion is detected, the first PWM pulse may be reduced or eliminated, while the width of the second PWM pulse in that frame may be increased to maintain the overall brightness. Elimination of the first PWM pulse may significantly reduce the temporal aperture thereby reducing motion blur. 
         [0045]      FIG. 13A  shows the PWM pulses in LED driving in a traditional LED driver. Assume the LED intensity is I {0,1} and duty cycle is λ{0,100% }, the PWM “on” time in terms of fractions of an LCD frame time is given by 
         [0000]      ΔT=λI 
         [0000]      Δ T   1   +ΔT   2   =ΔT    
         [0046]    An alternative approach in the LED driver is to set the PWM “off” signal at the blank signal, and the PWM “on” to be sometime before the blank signal as shown in  FIG. 13B . This enables the backlight to be on when LCD reaches the target value, thus reducing ghosting. 
         [0047]      FIG. 14  shows an exemplary flow diagram comprising aspects of the present invention, which convert input image/video to be displayed on a display with an area adaptive backlight comprising a lower resolution LED backlight and higher resolution LCD. In these exemplary embodiments, an input image frame  140  is low-pass filtered and then sub-sampled  141  to the backlight resolution. The backlight resolution may be determined by the number of backlight units, e.g. the number of LEDs in the backlight. Each pixel in the low resolution backlight image corresponds to a block in the input HDR image  140 . 
         [0048]    For each backlight element or HDR block, motion detection  144  is performed to determine whether it is a motion block or still block. For motion detection purposes, each backlight block may be subdivided into sub-blocks. In some embodiments, each sub-block may consist of 8×8 pixels in the high resolution HDR image  140 . 
         [0049]    In an exemplary embodiment, the process of motion detection  144 , resulting in a motion map  145  and the determination of pulse timing  143 , are as follows:
   For each frame,   1. Calculate the average of each sub-block in the HDR image for the current frame.   2. If the difference between the average in this frame and the sub-block average of the previous frame is greater than a threshold (e.g., 5% of total range), then the backlight block that contains the sub-block is a motion block. Thus a first motion map is formed.   3. Perform a morphological dilation operation on the first motion map (change the still blocks neighboring a motion block to motion blocks) to form a second motion map.   4. Perform a logical “OR” operation on the second motion map of the current frame with the second motion map of a previous frame to form a third motion map.   5. For each backlight block,
       if it is motion block,
           mMap(i,j)=max(N, mMap (i,j)+1); where N is number of transition frames else (still block)   mMap (ij)=min(0, mMap (i,j)−1);   
           
       6. The PWM pulse “on” widths are given by   
 
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         [0000]        ΔT   1   =ΔT −0.5 
         [0000]      ΔT 2 =0.5 
         [0061]    The sub-sampled and low-pass filtered image  141  may be used to determine LED driving values  142 , which may be sent to the LED backlight driver  146  after combination with the pulse timing data  143 . Pulse timing data  143  may also be sent to a backlight prediction process  149 . The actual backlight image that will be used to illuminate the full resolution input image  140 , may be predicted by convolving the backlight signal with the point spread function of the display, which comprises the diffusion layer. This image may then be up-sampled  150  to the full LCD image resolution. The input image  140  may then be divided  152  by the up-sampled backlight image to create a display image that will have the proper image characteristics when displayed with the pulsed backlight determined for the image. This display image data may then be sent to the overdrive circuit  151 , which may also access a frame buffer to determine overdrive image values. The overdriven image values may then be sent to the LCD driver  148 , where a blank signal may be derived  147  and sent to the backlight driver  146  to synchronize LED flashing with LCD driving. The pulsed backlight may then be used to display the overdriven display image. 
         [0062]    The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalence of the features shown and described or portions thereof.