Patent Publication Number: US-6661400-B2

Title: Liquid crystal panel drive and method of driving liquid crystal panel

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to a liquid crystal panel drive and a method of driving a liquid crystal panel. More particularly, it relates to a liquid crystal panel drive and a method of driving a liquid crystal panel designed to improve the hysteresis of the liquid crystal panel. 
     2. Related Background Art 
     Known attempts for improving the image displaying performance of a liquid crystal panel include devices for improving the response speed of the panel as disclosed in Japanese Patent Application Laid-Open No. 3-90993 and U.S. Pat. No. 5,119,084. 
     These known devices are designed to improve the response speed and consequently the performance of displaying moving images of the liquid crystal panel and hence not adapted to improve the hysteresis of the panel. Therefore, there still remains the problem that the gradation of still and moving images displayed on the screen can become degraded by the hysteresis of the liquid crystal panel. Thus, there is a demand for novel methods for improving the hysteresis of a liquid crystal panel. 
     Like the present invention, Japanese Patent Application Laid-Open No. 7-20828 discloses an attempt for improving the hysteresis of a liquid crystal panel. The patent document proposes to prepare a table on the hysteresis of a liquid crystal panel and store it in a memory so that the performance of the liquid crystal of the panel may be corrected by referring to the table whenever necessary during the operation of the liquid crystal panel. 
     Japanese Patent Application Laid-Open No. 3-96993 proposes a technique for compensating the performance of the liquid crystal of a liquid crystal panel by utilizing the difference between two image signals separated by a field. 
     However, the techniques disclosed in the above patent documents are accompanied by the problem of structural complexity, although neither of them can achieve a satisfactorily high operating speed. 
     SUMMARY OF THE INVENTION 
     In view of the above identified existing technological problems, it is therefore the object of the present invention to provide a liquid crystal panel drive and a method of driving a liquid crystal panel designed to improve the hysteresis of the liquid crystal panel that are free from the above problems. 
     According to an aspect of the invention, the above object is achieved by providing a liquid crystal panel drive for driving a liquid crystal panel according to an input image signal, characterized by comprising: 
     a revision means for revising the reference peak level on the basis of the current level of the image signal and the current reference peak level; and 
     a drive signal generation means for generating a drive signal for the liquid crystal panel on the basis of the level of the signal related to the reference peak level and the level of the current input image signal; 
     the liquid crystal panel being driven by the drive signal. 
     According to another aspect of the invention, there is provided a method of driving a liquid crystal panel according to an input image signal, characterized by comprising steps of: 
     driving the liquid crystal panel by a drive voltage determined on the basis of the voltage brightness correction characteristic of the liquid crystal panel selected according to a signal relating to the reference peak voltage currently retained by the input image signal and the current voltage of the input image signal; and 
     sequentially revising the reference peak voltage on the basis of the current voltage of the input image signal and the currently retained peak voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of an embodiment of liquid crystal panel drive according to the invention. 
     FIG. 2 is a schematic block diagram of the encoder that can be used for the embodiment of FIG.  1 . 
     FIGS. 3A and 3B are illustrations of the principle of hysteresis correction of a liquid crystal panel that can be used for the purpose of the invention. 
     FIG. 4 is a graph showing a hysteretic characteristic (dependency on the applied voltage) of a liquid crystal panel. 
     FIG. 5 is a graph showing another hysteretic characteristic (dependency on the elapse of time) of a liquid crystal panel. 
     FIG. 6 is a graph showing still another hysteretic characteristic (the dependency of the hysteresis generation level on the display time) of a liquid crystal panel. 
     FIG. 7 is another graph showing the dependency of the performance of a liquid crystal panel on the applied voltage. 
     FIG. 8 is still another graph showing the dependency of the performance of a liquid crystal panel on the applied voltage. 
     FIG. 9 shows a program that can be used to produce Table 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, the present invention will be described by referring to the accompanying drawings that illustrate preferred embodiments of the invention. 
     FIGS. 3A and 3B are illustrations of the principle of hysteresis correction of a liquid crystal panel that can be used for the purpose of the invention. Referring to FIG. 3A, input image signals S i1  to S i4  are signals for a same pixel input for every other frame. Signal S i1  represents the white level and signal S i2  represents the gray level, whereas signal S i3  represents the black level and signal S i4  represents also the gray level. Thus, both signals S i2  and S i4  represent a same level. S o1  to S o4  represent the levels of display brightness corresponding to the input image signals S i1  to S i4  respectively. As shown, the level of display brightness S o4  is lower than that of display brightness S o2  and hence a hysteretic quantity is generated between them. This is due to the hysteretic characteristics of the liquid crystal of the liquid crystal panel. 
     In this embodiment, the input signals are transformed into corrected signals by means of arithmetic operations for correcting display signals and the liquid crystal panel is driven by the corrected signals to eliminate any hysteretic quantity that can be generated in the liquid crystal panel as shown in FIG.  3 B. Referring to FIG. 3B, the level of the display brightness S o4 ′ is made to agree with that of the display brightness S o2 ′ by transforming the input image signal S i4  into corrected signal S c4 . 
     Before describing the embodiments in detail, some of the properties of liquid crystal panels found by the inventors of the present invention will be discussed. 
     FIG. 4 is a graph showing a hysteretic characteristic (of the dependency on the applied voltage) of the display brightness of a liquid crystal panel obtained when a varying voltage that shifts from the black level to the white level is applied to the panel. In FIG. 4, the curve a is the hysteretic characteristic (of the dependency on the applied voltage) of the display brightness after applying a voltage of the white level and the curve b is the corresponding hysteretic characteristic (of the dependency on the applied voltage) of the display brightness after applying a voltage of the black level. ΔR in FIG. 4 represents the hysteretic quantity generated between the display brightness obtained when a voltage corresponding to gradation  30  image signal is applied after applying a voltage of the white level and the display brightness obtained when a same voltage corresponding to gradation  30  image signal is applied after the application of a voltage of the black level. 
     This hysteretic characteristic also depends on the peak level (white level or black level) used as starting point. FIG. 7 shows the dependency of the performance of a liquid crystal panel on the peak level. More specifically, FIG. 7 shows the relationship between the applied voltage and the display brightness for three instances observed respectively after applying a voltage of the white level with a 100% brightness, after applying a voltage of the black level with a 0% brightness and after applying a voltage of the black level with a 5% brightness. Note that FIG. 4 shows the relationship between the applied voltage and the display brightness for two instances observed respectively after applying a voltage of the white level and after applying a voltage of the black level with a 0% brightness. As clearly seen from FIG. 7, the characteristic curve for a peak level corresponding to the black level with a 0% brightness differs from the curve for a peak level corresponding to the black level with a 5% brightness, although the peak levels are considerably close to each other. FIG. 8 is a graph showing the initial stages of the curves of FIG. 7 with an enlarged scale. This characteristic will be referred to as peak level dependency. 
     It will also be seen that, while the above hysteretic characteristic depend on the peak level, the characteristic curves appear similar to each other. For example, the difference between the characteristic curve for a peak level corresponding to the black level with a 0% brightness and the characteristic curve for a peak level corresponding to the white level with a 100% brightness for each applied voltage, which is a hysteretic quantity, converges to 0 as the black level and the white level approach each other near the gray level. This is also true for the characteristic curve for a peak level corresponding to the black level with a 5% brightness. Therefore, the hysteretic quantity can be determined by mapping a value (coefficient) representing the characteristic property on the level of the current image signal and multiplying a quantity representing the absolute value of the hysteretic quantity corresponding to the peak level by the coefficient. Then, the hysteresis of the display brightness can be corrected by applying the voltage that has been corrected for the obtained hysteretic quantity. 
     Each of the curves for the hysteretic characteristic of FIGS. 4 and 7 shows the display brightness when image signals for different levels of gradation are written 1 second after writing the white or black level. The hysteretic characteristic changes if the 1 second is replaced by some other time span. In short, the hysteresis depends on time. 
     FIG. 5 shows the hysteretic characteristic of time dependency. More specifically, FIG. 5 shows the transitional stages of the display brightness when a signal for gradation  30  is applied stepwise after applying the black level and that of the display brightness when a signal for gradation  30  is applied stepwise after applying the white level. As clear from FIG. 5, while the responsiveness of the display brightness reflects the LPF characteristics and depends on the liquid crystal panel, the curves for the two occasions converge to a same level typically after 30 seconds. 
     The time dependency of hysteresis has another aspect. The hysteretic quantity also depends on the time during which the voltage of the peak level (hysteresis generation level) is applied. FIG. 6 shows that the hysteretic quantity AR varies depending on the time during which the voltage of the peak level is applied. 
     In this embodiment, the hysteresis is corrected by utilizing the above described peak level dependency and two different time dependencies. Additionally, the similarity of the hysteretic behaviors of liquid crystal panels is also utilized. More specifically, in this embodiment, image signals whose peak level is the black level and those whose peak level is the white level are corrected. 
     FIG. 1 is a schematic block diagram of the embodiment of liquid crystal panel drive according to the invention. 
     Referring to FIG. 1, reference numeral  201  denotes an image signal input terminal for receiving image signal g(l, j, k), where l represents the position of the image signal in terms of the horizontal direction of the screen and j represents the position of the signal in terms of the vertical direction of the screen, whereas k represents the frame number. Reference numeral  202  denotes encoder  1  for dividing the level of the image signal g(l, j, k) into 16 sections and encoding it into a 4-bit code. Reference numeral  203  denotes a coefficient generation circuit for generating a coefficient on the basis of the output of the encoder  1  (reference numeral  202 ) and reference numeral  204  denotes encoder  2  for dividing the level of the image signal g(l, j, k) into 8 sections and encoding it into a 3-bit code before outputting the encoded value nrb. Reference numeral  205  denotes an arithmetic unit which may typically be a CPU. Reference numeral  206  denotes a frame memory for delaying the output of the arithmetic unit  205  by a frame. The arithmetic unit  205  receives the output nrb of the encoder  2  (reference numeral  204 ) and the outputs rb(l, j, k−1), tbw(l, j, k−1) and tbp(l, j, k−1) of the frame memory  206  and outputs the outcome of arithmetic operations, or rb(l, j, k), tbw(l, j, k) and tbp(l, j, k). Signal rb(l, j, k) represents the retained peak level. As described above, the peak level is directed to the black level in this embodiment. The signal tbw(l, j, k) represents the duration of time (number of frames) of an image signal with the peak level. The signal tbp(l, j, k) represents the duration of time (number of frames) of an image signal with a level higher than the peak level. Reference numeral  207  denotes a look-up table (hereinafter referred to as LUT) that receives outputs rb(l, j, k), tbw(l, j, k), tbp(l, j, k) of the arithmetic unit  205  and by turn outputs, for example, a corrected 4-bit quantity. Reference numeral  208  denotes a multiplier for multiplying the corrected quantity output from the LUT  207  by the coefficient output from the coefficient generation circuit  203 . Reference numeral  209  denotes an adder/subtracter for adding the product of multiplication of the corrected quantity from the multiplier  208  and the coefficient to the image signal g(l, J, k) and outputting the sum as liquid crystal panel drive signal output(l, j, k). 
     FIG. 2 a schematic block diagram of the encoder  1  (reference numeral  202 ). The encoder  1  (reference numeral  202 ) comprises a total of fifteen comparators  1  to  15  (reference numerals  302  to  316 ) for comparing the input image signal with respective comparison levels  1  to  15  (reference numerals  322  to  336 ) and an encoder  317  for coding the 15-bit output of the comparators  1  to  15  (reference numerals  302  to  316 ) into a 4-bit code. The output can be non-linearly coded by selecting appropriate comparison levels. Thus, finely differentiated comparison levels can be selected for a region where the hysteresis varies remarkably to minimize the coefficient error, while using a relatively small number of bits for encoding. 
     The encoder  2  (reference numeral  204 ) has a configuration similar to the encoder  1  (reference numeral  202 ). Output nrb of the encoder  2  (reference numeral  204 ) takes a value found within a range between “000b” and “111b”. The range between “000b” and “101b” of the output nrb refers to six steps of the black level, where step “000b” is remotest from the white level and step “101b” is closest to the white level. In this embodiment, the range between “000b” and “101b” is referred to as black side hysteresis generation level. Step “111b” is referred to as white side hysteresis generation level. Step “110b” is referred to as intermediary level, where the display characteristic (the relationship between the applied voltage and the brightness) of the liquid crystal panel does not change. Thus, when the black side hysteresis generation level (“000b” to “101b”) is displayed before displaying the intermediary level, the display characteristic is lopsided to the black side hysteresis, whereas, when the white side hysteresis generation level (“111b”) is displayed before displaying the intermediary level, the display characteristic is lopsided to the white side hysteresis. To repeat the above description, tbw represents the time during which the black level (“000b” to “101b”) or the white level (“111b”) is displayed and tbp represents the time elapse of time since the transition to the intermediary level (“110b”) after displaying the black level (“000b” to “101b”) or the white level (“111b”). 
     The level of 60% of the image signal may typically be selected for the boundary line separating value “110b” and the value “111b” for output nrb. On the other hand, the level of 10% of the image signal may typically be selected for the boundary line separating value “101b” and value “110b”. 
     The arithmetic unit  205  performs arithmetic operations for determining the display characteristic (the relationship between the applied voltage and the brightness) retained by a displaying pixel for the hysteresis due to the white level or the black level. In other words, it determines the elapse of time tbp since the time when a hysteresis generating region was displayed on the basis of the level rb of the hysteresis generating region and the duration of time tbw of displaying the hysteresis generating region and then the LUT  208  determines the corrected quantity for the hysteresis on the basis of the determined elapse of time. 
     Now, the operation of the arithmetic unit  205  will be described by referring to Table 1 below showing the correspondence between the input and the output of the arithmetic unit  205 . 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 input 
                   
               
            
           
           
               
               
               
               
            
               
                 current 
                 preceding 
                   
                 output 
               
            
           
           
               
               
               
               
               
               
            
               
                 pixel 
                 pixel 
                 tbp 
                 rb 
                 tbw 
                 tbp 
               
               
                 nrb 
                 rb (l,j,k-1) 
                 (l,j,k-1) 
                 (l,j,k) 
                 (l,j,k) 
                 (l,j,k) 
               
               
                   
               
               
                 &lt;“110” 
                 ≠“111” 
                 =0 
                 smaller 
                 +1 
                 previous 
               
               
                   
                   
                   
                   
                   
                 value 
               
               
                   
                   
                   
                   
                   
                 (1) 
               
               
                   
                   
                 ≠0 
                 current 
                 reset 
                 reset 
               
               
                   
                   
                   
                 pixel nrb 
                   
                 (2) 
               
               
                   
                 =“111” 
                 don&#39;t 
                 current 
                 reset 
                 reset 
               
               
                   
                   
                 care 
                 pixel nrb 
                   
                 (3) 
               
               
                 =“110” 
                 don&#39;t 
                 don&#39;t 
                 current 
                 previous 
                 +1 
               
               
                   
                 care 
                 care 
                 pixel rb  
                 value 
                 (4) 
               
               
                 =“111” 
                 =“111” 
                 don&#39;t 
                 current 
                 +1 
                 previous 
               
               
                   
                   
                 care 
                 pixel nrb 
                   
                 value 
               
               
                   
                   
                   
                   
                   
                 (5) 
               
               
                   
                 ≠“111” 
                 don&#39;t 
                 current 
                 reset 
                 reset 
               
               
                   
                   
                 care 
                 pixel rb  
                   
                 (6) 
               
               
                   
               
            
           
         
       
     
     Firstly, the arithmetic unit  205  determines the history of the currently displaying pixel in terms of the display characteristics due to its hysteresis on the basis of nrb, rb and tbp. Then, it calculates the values of rb, tbw and tbp to be output to the LUT  208  and the frame memory  206  on the basis of the outcome of its determining operation. 
     The arithmetic operations of the arithmetic unit  205  can be classified into six categories (1) to (6) of Table 1, which will be described below. 
     (1) When the signal level nrb of the image signal for the current frame is “000b” to “101b”, it represents the black side hysteresis region so that the arithmetic unit  205  compares the signal level with the value rb (=rb(l,j,k−1)) for the previous frame. If nrb is not greater than “101b”, it indicates the hysteresis generated on the black side. Then, the arithmetic unit  205  determines if the black level is continuing from the previous pixel or if the previous pixel is at the intermediary level on the basis of tbp. If tbp=0, it indicates that the previous frame was on the black level so that the retained black level is revised. The revised black level will be the black level rb because the smaller of nrb and rb is rb and hence rb is written in the frame memory  206  so that a greater hysteretic quantity may be retained. Again, if tbp=0, 1 is added to tbw because the display of the black level continues. On the other hand, tbp retains the current value. 
     (2) When tbp≠0, the pixel was at the intermediary level at the previous frame. Then, rb=nrb is realized and both tbw and tbp are reset to 0 in order to revise the hysteresis generation level. 
     (3) When the previous hysteresis generation level was rb=“111b”, it indicates the pixel was on the white level at the previous frame. Then, rb=nrb is realized and both tbw and tbp are reset to 0 in order to revise the hysteresis generation level. 
     (4) When the current pixel signal level is nrb=“110b”, the display characteristics determined on the hysteretic history are retained so that both rb and tbw retain the respective current values and 1 is added to tbp that represents the elapse of time after passing to the hysteresis generating region. 
     (5) When the current pixel signal level is at “111b” and the previous hysteresis generation level rb was equal to “111b”, it indicates that the value used for the display at the previous frame was that of the white level. Then, 1 is added to tbw that represents the period of time for writing the hysteresis generation level. 
     (6) When the current pixel signal level is at “111b”, it indicates that the value used for the display at the previous frame was not that of the white level. Then, rb=nrb is realized and both tbw and tbp are reset to 0 in order to revise the hysteresis generation level to the white level. 
     FIG. 9 shows a program that can be used to produce Table 1. 
     The duration of time tbw of displaying the hysteresis generation level and the duration of time tbp of displaying the intermediary gradation level can be revised not on a frame by frame basis but every several frames, the number of which can be arbitrarily selected, by using a reference signal coming from the counter for counting the number of frames or a technique of receiving a reference signal regularly and cyclically from a CPU. With such an arrangement, the relationship between the number of bits for the duration of time tbw of displaying the hysteresis generation level and the duration of time tbp of displaying the intermediary gradation level and the period of cyclic revision can be selected appropriately by taking the relationship between the level of accuracy required for arithmetic operations and the quantity of hardware into consideration. 
     The encoder  1  (reference numeral  202 ), the coefficient generation circuit  203 , the LUT  207 , the multiplier  208  and the adder  209  may be realized as a single LUT. 
     While the hysteretic quantity is determined from the duration of time of displaying the hysteresis generation level, the duration of time of displaying the intermediary gradation level and the current image signal level in this embodiment, this embodiment may be so modified as to omit one or more than one of the above listed factors if the performance of the liquid crystal panel is improved and such factor or factors may be disregarded. 
     [Advantages of the Invention] 
     As described above, the gradation of still and moving images displayed on the screen of a liquid crystal panel according to the invention can be improved if degraded by hysteresis. Thus, a liquid crystal panel according to the invention can maintain its operation of displaying high quality images regardless of hysteresis.