Abstract:
A method of displaying image data, which can mitigate a double-boundary problem and improve MPRT, includes the steps of: receiving a plurality of frame data of a pixel; correcting subframe data of two of the plurality frame data; and sequentially displaying each of the subframe data of the plurality frame data.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional of U.S. Ser. No. 11/784,943, filed Apr. 10, 2007 now U.S. Pat. No. 7,705,816, which claims the benefit of Taiwan Application No. 095112668, filed Apr. 10, 2006, which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to generating corrected gray-scale data to improve display quality. 
     BACKGROUND 
     With improvements in liquid crystal display (LCD) technology, LCD televisions including LCD panels are becoming increasingly popular. An LCD panel includes a matrix of pixels that are driven with pixel data values to display a desired image. 
     In attempts to improve display quality of such LCD panels, subframes are often inserted to form pulse-like image data according to the pulse-like LCD technology. An issue with using LCD panels in televisions is that the perceived image quality can suffer as a result of edge blurring. To address this, subframes are inserted to provide luminance similar to that of a CRT (cathode ray tube) television. With one conventional technique, a normally black subframe is often inserted in each frame, as shown in  FIG. 1 .  FIG. 1  shows two adjacent pixels  101  and  102  for respectively receiving gray-scale data A and B and displaying the gray-scale data A and B in a frame time T f . 
       FIG. 2  shows a first pulse-like liquid crystal display technology, in which a normally black subframe (a subframe having a gray-scale value of 0) is inserted into the pixels  101  and  102  along with the gray-scale data A and B, if an image doubled frame rate technology is used. The image doubled frame rate technology refers to using a doubled frame rate so that two subframes of data can be provided in each frame. Thus, the pixels  101  and  102  of  FIG. 2  respectively display the subframe with the gray-scale data A and B in the front half frame time (½ T f ), and display a black frame in the rear half frame time (½ T f ). According to the eye-tracking model, the conventional black frame inserting method can effectively halve the blurred width (or brightness edge width). However, the conventional black frame inserting method enables the pixel to display the gray-scale data correctly only during one half of the frame time, and to display the normally black frame of gray-scale data of 0 during the other half of the frame time. Thus, the frame luminance is reduced in half, thereby negatively influencing the image displaying effect. 
     To improve the problem of the halved pixel luminance caused by the black frame insertion technique, a second conventional subframe insertion technique does not influence the equivalent luminance of the frame. As shown in  FIG. 3 , when the pixels  101  and  102  receive the gray-scale data A and B, the second subframe insertion technique enables the pixel  101  to sequentially display subframes A′ and C and the pixel  102  to sequentially display subframes B′ and D. The average luminance of the pixel  101  for displaying the subframes A′ and C in the frame time T f  is the same as the luminance effect of directly displaying the gray-scale data A throughout the frame time T f  in  FIG. 1 . The average luminance of the pixel  102  for displaying the subframes B′ and D in the frame time T f  is the same as the luminance effect of directly displaying the gray-scale data B throughout the frame time T f  in  FIG. 1 . 
       FIG. 4  shows an example look-up table  40  used in the second subframe insertion technique of  FIG. 3  for generating the subframes. As shown in  FIGS. 3 and 4 , the second subframe insertion technique sequentially displays two subframes having the gray-scale values of 250 and 0 when the pixel receives an original gray-scale value of 150, and two subframes having the gray-scale values of 255 and 0 when the pixel receives an original gray-scale value of 151. In the look-up table  40  of  FIG. 4 , the original gray-scale value not greater than 151 is mapped to various gray-scale values for the first subframe and mapped to a black value for the second subframe. The gray-scale values of the first and second subframes together provide a synthesized luminance effect that is equal to the luminance corresponding to the original gray-scale value. In addition, the original gray-scale value greater than 152, is mapped to a gray-scale value of 255 for the first subframe, and mapped to various gray-scale values for the second subframe. The gray-scale values for the second subframe are adjusted to provide a synthesized luminance effect that is equal to the luminance of the original gray-scale value. 
     In typical image data, the gray-scale values of the adjacent pixels are very close to each other. Thus, if the original gray-scale values of the pixels  101  and  102  of  FIG. 3  are both smaller than 151, the gray-scale values C and D of the subframe are equal to 0. If the original gray-scale values of the pixels  101  and  102  are both greater than 152, the gray-scale values A′ and B′ of the subframe are equal to 255. The two conditions can effectively halve the blurred width of the motion picture image without influencing the image displaying luminance. 
       FIG. 5  is a graph for mapping first and second subframe gray-scale values to original gray-scale values, according to the look-up table  40  of  FIG. 4 . According to  FIG. 5 , the gray-scale value of the first subframe is 255 when the original gray-scale value is greater than g 51 , and the gray-scale value of the second subframe is 0 when the original gray-scale value is smaller than g 51 . The value of g 51  of  FIG. 5  may be any reasonable design value. For example, the value of g 51  may be 151 for an 8-bit gray-scale display system. 
     An LCD panel is limited by the response speed of liquid crystal cells. When the gray-scale value displayed by a pixel is changed, the corresponding liquid crystal cell requires a certain response time to reach the target gray-scale value. In some cases, an over-drive technique is used to enable the pixel to switch between low and high gray-scale levels. 
       FIG. 6  shows a graph illustrating application of the second subframe insertion technique in conjunction with an over-drive technique. The example of  FIG. 6  is for an 8-bit gray-scale display system, which has a gray-scale display range from 0 to 255. The pixel sequentially receives the pixel data of four frames f 61 , f 62 , f 63  and f 64  in time periods from t 61  to t 63 , from t 63  to t 65 , from t 65  to t 67  and from t 67  to t 69 , respectively. The original gray-scale values of the four frames are successively 32, 32, 64 and 64. Thus, the liquid crystal cell sequentially receives the control voltages of V(L 2 ), V(L 0 ), V(L 2 ), V(L 0 ), V(L 4 ), V(L 0 ), V(L 3 ) and V(L 0 ) provided to the pixel according to the second subframe insertion technique. The corresponding luminances of the pixel are represented as L 2 , L 0 , L 2 , L 0 , L 3 , L 1 , L 3  and L 1 , respectively. Note that the luminances are represented as triangular waves where increases and decreases in luminance slope upwardly or downwardly according to response times of the corresponding liquid crystal cell. However, if the response speed of the liquid crystal cell is not high enough, the liquid crystal cell cannot be charged to the voltage value for correctly displaying the gray-scale luminance L 3  (for frame f 63 ) if the liquid crystal cell is directly driven by the pixel control voltage V(L 3 ) corresponding to the gray-scale luminance L 3  after the gray-scale luminance L 0  (in the previous frame f 62 ). Thus, as shown in  FIG. 6 , an over-drive voltage is applied to drive the liquid crystal cell in frame f 63 . That is, a new pixel data voltage higher than the original pixel control voltage is applied to the liquid crystal cell from the time instant t 65  to the time instant t 66 . For example, the control voltage V(L 4 ) corresponding to the gray-scale luminance L 4  (L 4 &gt;L 3 ) of  FIG. 6  is applied so that the pixel can display the gray-scale luminance L 3  immediately and correctly. Similarly, if the response speed of the liquid crystal cell is not high enough, the pixel still can only display the gray-scale luminance L 1  rather than the full black at the time instant t 67  although the control voltage is dropped to 0 from the time instant t 66  to the time instant t 67 . Because the pixel is not fully black at the time instant t 67 , no over-drive voltage has to be applied from the time instant t 67  to the time instant t 68 , and only the control voltage V(L 3 ) correctly corresponding to the gray-scale luminance L 3  needs to be applied for the pixel to correctly display the gray-scale luminance L 3 . 
     However, the conventional pulse-like liquid crystal display adopting the driving technique of  FIG. 6  usually has the problems of double-boundary (or double image) and poor MPRT (Motion Picture Response Time), which degrades motion picture quality. For example, the double-boundary problem results from the integration areas of the frame times between t 63  and t 65  and between t 65  and t 67  being significantly different from each other. 
       FIG. 7  shows an eye stimuli integration curve corresponding to the technique of  FIG. 6 , wherein the horizontal axis represents the time, the vertical axis represents the normalized intensity, and the turning portion of A is where the double-boundary occurs. Thus, although the driving technique of  FIG. 6  can be used for the purpose of correcting the image by re-adjusting the single subframe data of a single frame, the technique cannot improve the double-boundary problem completely, and even induces the condition of boundary overshooting or boundary undershooting. 
     In addition, an NBET parameter is widely used to represent the motion picture quality. The NBET parameter is defined as follows:
 
 NBEW=BEW /velocity,  (Eq. 1)
 
 NBET=NBEW /frame rate,  (Eq. 2)
 
where BEW is the blurred boundary width of the motion picture image. A smaller NBET value represents less blurred boundary of the motion picture image and thus better motion picture quality. A greater NBET value is obtained when the phenomenon illustrated by the turning portion of A in  FIG. 7  occurs, increasing the blurred boundary and decreasing the motion picture quality.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration showing two pixels for respectively receiving gray-scale data, according to a conventional technique; 
         FIG. 2  is a schematic illustration showing two pixels, which receive the gray-scale data at doubled frame rates according to a first conventional technique; 
         FIG. 3  is a schematic illustration showing two pixels, which receive the gray-scale data at doubled frame rates according to a second conventional technique; 
         FIG. 4  shows a look-up table used by the second conventional technique; 
         FIG. 5  is a graph mapping subframe gray-scale values to original gray-scale values according to the lookup table of  FIG. 4 ; 
         FIG. 6  illustrates timing charts corresponding to a technique of using the second conventional technique in conjunction with an over-drive technique; 
         FIG. 7  shows an eye stimuli integration curve corresponding to the driving technique of  FIG. 6 ; 
         FIG. 8  illustrates timing charts corresponding to a driving technique according to a first embodiment of the invention; 
         FIG. 9  illustrates timing charts corresponding to a driving technique according to a second embodiment of the invention; 
         FIG. 10  is a block diagram of a circuit architecture to provide a driving technique according to some embodiments; 
         FIG. 11  is an overall functional block diagram showing the circuit architecture of  FIG. 10 ; 
         FIG. 12  is a timing chart showing a simulated result according to a driving technique according to some embodiments; 
         FIG. 13  is a timing chart showing another simulated result according to a driving technique according to some embodiments; and 
         FIG. 14  is a schematic diagram of a display device incorporating an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments are possible. 
     To reduce or eliminate excessively long boundary blur of a motion picture image caused by the inadequate response speed of liquid crystal cells in a liquid crystal display (LCD) panel, a conventional driving technique simply adjusts the control voltage of a particular frame at the portion where the input gray-scale signal changes (i.e., the portion where the luminance changes) so as to change (lift or lower) the triangular wave of the luminance with respect to the time axis (see, e.g.,  FIG. 6 ). However, the conventional driving technique is unable to adequately solve the double-boundary problem or may even cause boundary overshooting or boundary undershooting. 
     In contrast, a driving technique according to some embodiments adjusts the control voltage of a particular frame where the luminance changes (i.e., when the input gray-scale data changes), based on frame data of the particular frame as well as frame data of the next frame, to address the double-boundary problem and to effectively reduce the blurred boundary problem. 
       FIG. 8  shows timing diagrams of frames as a function of time, corresponding control voltages as a function of time, and corresponding luminances as a function of time. In one example, the display system is assumed to be an 8-bit gray-scale display system, which has a gray-scale display range from 0 to 255. Control voltages represent pixel voltages applied to a pixel in a matrix of pixels of an LCD panel. As shown in  FIG. 8 , the pixel successively receives the pixel data of four frames f 81 , f 82 , f 83  and f 84  in the time periods from t 81  to t 83 , from t 83  to t 85 , from t 85  to t 87  and from t 87  to t 89 , respectively. The gray-scale values of the four frames are successively 32, 32, 64 and 64. In accordance with an embodiment, the control voltages of the pixel of the second subframe of the frame f 82  and the first subframe of the next frame f 83  (control voltages OD 81  and OD 82 , respectively, in  FIG. 8 ) are adjusted. The adjusted control voltages OD 81  and OD 82  correspond to time periods (t 84 , t 85 ) and (t 85 , t 86 ), respectively, during which the luminance changes (i.e., the time where the input gray scale signal changes) by a relatively large amount (greater than some threshold). The driving technique according to an embodiment increases the control voltage of the second subframe of the frame f 82  from the original control voltage V(L 0 ) corresponding to the gray-scale luminance L 0 , to a higher control voltage V(L 1 ), which is OD 81 , corresponding to the gray-scale luminance L 1 . Moreover, the driving technique decreases the control voltage of the first subframe of the frame f 83  from the over-drive control voltage V(L 4 ) of the original gray-scale luminance L 4  to the over-drive control voltage V(L 5 ), which is OD 82 , corresponding to the gray-scale luminance L 5  (where L 3 &lt;L 5 &lt;L 4 ). 
     Note that in time period (t 85 , t 86 ), the control voltage is over-driven to V(L 5 ), which is above V(L 3 ) corresponding to the original luminance L 3 . However, V(L 5 ) is less than V(L 4 ), which is the over-drive voltage used in the conventional driving technique of  FIG. 6  (in time period t 65 ). Consequently, the displayed luminance at the time instant t 85  (the initial time point of the first subframe of the frame f 83 ) is not the original gray-scale luminance L 0  but is the gray-scale luminance L 1  of the second subframe of the frame f 82 . In this manner, the double-boundary problem can be addressed, and the blurring of the boundary can be reduced, such that the display quality of the motion picture can be effectively enhanced. 
     The adjusted control voltages OD 81  and OD 82  are determined according to the stable frame data after the frame f 84  (as well as frame data in frames f 82  and f 83 ). The corrected subframe data of the first frame (e.g., f 82 ) and the second frame (e.g., f 83 ) are determined according to the data of the third frame (e.g., f 84 ). In order to achieve a superior display quality, the adjustment of the control voltage OD 81  may follow the principle for adjusting the control voltage OD 81  to make the displayed luminance of the first subframe (time instant t 85 ) of the frame f 83  equal to 50% to 100% of the displayed luminance of the first subframe (time instant t 87 ) of the frame f 84 . The control voltage OD 82  is adjusted to make the displayed luminance of the second subframe of the frame f 83  (time instant t 86 ) equal to 90% to 110% of the displayed luminance of the second subframe of the frame f 84  (time instant t 88 ). 
     The doubled frame rate technique may first generate and display, within each corresponding frame, a high-luminance subframe followed by a low-luminance subframe (see  FIG. 8 ) with respect to each frame, or may alternatively first generate and display the low-luminance subframe followed by the high-luminance subframe. Driving techniques according to some embodiments may be adapted to either of the two types of frame inserting and doubled frame rate technology. 
       FIG. 9  illustrates timing diagrams (frames, control voltages, and luminances) for the driving technique that initially generates and displays a low-luminance subframe followed by a high-luminance subframe in an example 8-bit gray-scale display system. As shown in  FIG. 9 , a pixel successively receives the pixel data of the four frames f 91 , f 92 , f 93  and f 94  in the time periods from t 91  to t 93 , from t 93  to t 95 , from t 95  to t 97  and from t 97  to t 99 , respectively. The gray-scale values of the four frames are successively 32, 32, 64 and 64. With this driving technique, the control voltages OD 91  and OD 92  in the first subframe and the second subframe of the frame f 93 , where the luminance changes by greater than a threshold, are adjusted. The driving technique increases the control voltage (OD 91 ) of the first subframe of the frame f 93  to be V(L 1 ) instead of the control voltage V(L 0 ) corresponding to the original gray-scale luminance L 0 , and reduces the over-drive control voltage (OD 92 ) of the second subframe of the frame f 93  to V(L 5 ), which is less than V(L 4 ). Note that the over-drive voltage V(L 5 ) is used in place of V(L 3 ) that corresponds to the original gray-scale L 3 . With this technique, when the liquid crystal display technology is for initially displaying the low gray-scale subframe and then subsequently the corresponding high gray-scale subframe, the MPRT response curve can also be improved. 
     The control voltage OD 91  is determined according to the stable frame data after the frame f 94  (as well as frame data in frame f 93 ). In other words, the corrected subframe data of the second frame (e.g., f 93 ) is determined according to the data of the third frame (e.g., f 94 ) and of the second frame (e.g., f 93 ). To achieve a superior display quality, the control voltage OD 91  can be adjusted according to the principle for adjusting the control voltage OD 91  to make the displayed luminance of the second subframe (time instant t 96 ) of the frame f 93  equal to 50% to 100% of the displayed luminance of the first subframe (time instant t 98 ) of the frame f 94 . Moreover, the control voltage OD 92  is determined to make the displayed luminance of the first subframe of the frame f 94  (time instant t 97 ) equal to 90% to 110% of the displayed luminance of the first subframe of the frame after frame f 94  (time instant t 99 ). 
     In addition, to prevent the average luminance displayed by every frame (especially the frame representing a single gray-scale) from changing due to the polarity change of the subframe data, the high gray-scale subframe data and the low gray-scale subframe data of each frame data should have the same polarity and two continuous adjacent frame data should have different polarities. Alternatively, the high gray-scale subframe data and the low gray-scale subframe data of each frame data have different polarities, when the subframe data of successive two adjacent frame data have opposite polarity arrangements. The two principles mentioned above are suitable for the typical doubled frame rate technology for initially generating and displaying the high-luminance subframe and subsequently the low-luminance subframe, or alternatively, initially generating and displaying the low-luminance subframe and subsequently the high-luminance subframe. 
     In addition, the low-luminance subframe may be a normally black subframe or a subframe with a lower gray-scale luminance. 
     To implement the above-mentioned driving techniques, a circuit architecture  1000  according to  FIG. 10  can be employed. As shown in  FIG. 10 , the circuit architecture  1000  receives a first frame signal f n-1  and a second frame signal f n , which are generated by an image signal generator according to a timing sequence. The circuit architecture  1000  includes an image signal generator  1001 , a buffer register  1010 , a look-up table  1020 , a comparator  1030  and two look-up tables  1040  and  1050 . The buffer register  1010  stores the first frame signal f n-1 . The look-up table  1020  is electrically coupled to the buffer register  1010  and generates a first over-drive voltage OD 1  and a second over-drive voltage OD 2  according to the first frame signal and the second frame signal, f n-1 , f n , respectively (which are generated by the image signal generator  1001 ). The comparator  1030  is electrically connected to the first look-up table  1020  to compare the first over-drive voltage OD 1  with the second over-drive voltage OD 2  to determine whether the first over-drive voltage OD 1  and the second over-drive voltage OD 2  are substantially the same (within a predefined threshold). The two look-up tables  1040  and  1050  are electrically connected to the comparator  1030  and respectively determine a corrected first over-drive voltage and a corrected second over-drive voltage according to the comparison result of the comparator regarding whether the first over-drive voltage OD 1  and the second over-drive voltage OD 2  are substantially the same (e.g., OD 1  and OD 2  differ by less than the predefined threshold). Next, the corrected first over-drive voltage and the corrected second over-drive voltage are sequentially output through a buffer register  1060 . If OD 1  and OD 2  are substantially the same, then the lookup tables  1040  and  1050  are used to correct OD 1  and OD 2 . However, if OD 1  and OD 2  are not substantially the same, then correction using the lookup tables OD 1  and OD 2  is bypassed. 
     OD 1  and OD 2  correspond to OD 81  and OD 82 , respectively, in  FIG. 8 , and to OD 91  and OD 92 , respectively, in  FIG. 9 . Using the circuit of  FIG. 10 , the correction of OD 1  and OD 2  is performed based on the comparison of the original OD 1  and OD 2  values. 
       FIG. 11  is an overall functional block diagram showing the circuit architecture  1000  of  FIG. 10 . As shown in  FIG. 11 , the buffer register stores the first frame signal f n-1 . The look-up table generates the corresponding output signal according to the first frame signal f n-1  and the second frame signal f n . That is, the look-up tables  1020 ,  1040  and  1050  of  FIG. 10  are integrated to form a look-up table  1050  of  FIG. 11 . 
       FIG. 14  illustrates a display device that has a backlight module  1100  to generate light directed through an LCD panel  1102 . The LCD panel  1102  has a timing controller  1104  that includes the circuit of  FIG. 10 , as well as other circuitry to provide data signals to the matrix of pixels of the LCD panel  1102 . 
       FIGS. 12 and 13  illustrate simulated results derived based on a driving technique according to an embodiment.  FIG. 12  illustrates the luminance obtained using the driving technique, and  FIG. 13  illustrates the MPRT according to  FIG. 12 . Referring to  FIG. 13 , the NBET value based on the driving technique according to an embodiment is greatly reduced so that the blurring of boundaries can be reduced. Compared with  FIG. 7 , the normalized intensity curve of  FIG. 13  is smoother. 
     In summary, some embodiments of the invention provide an image data driving technique capable of optimizing MPRT to reduce the double-boundary problem and blurring phenomenon. The driving technique according to an embodiment may apply the doubled frame rate technology for initially displaying the high gray-scale subframe and subsequently the low gray-scale subframe, or alternatively, for initially displaying the low gray-scale subframe and subsequently the high gray-scale subframe. The improvement is most significant when the displayed frame changes from low gray-scale to high gray-scale. Thus, the efficiency of the display is simply and effectively enhanced. 
     While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.