Patent Publication Number: US-8537087-B2

Title: Method and apparatus for driving liquid crystal display

Description:
This application is a continuation of prior application Ser. No. 09/994,039, filed Nov. 27, 2001, now U.S. Pat No. 7,161,575 , which is hereby incorporated by reference. 
    
    
     This application claims the benefit of Korean Application No. P2001-54128 filed on Sep. 4, 2001, which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display, and more particularly, to a method and apparatus for driving a liquid crystal display. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for enhancing a picture quality. 
     2. Discussion of the Related Art 
     Generally, a liquid crystal display (LCD) controls a light transmittance of each liquid crystal cell in accordance with a video signal, thereby displaying a picture. An active matrix LCD including a switching device for each liquid crystal cell is suitable for displaying a dynamic picture. The active matrix LCD uses a thin film transistor (TFT) as a switching device. 
     The LCD has a disadvantage in that it has a slow response time due to inherent characteristics of a liquid crystal, such as a viscosity and an elasticity, etc. Such characteristics can be explained by using the following equations (1) and (2):
 
τ r ∝γd 2 /Δε|V a   2 −V F   2 |  (1)
 
where τ r  represents a rising time when a voltage is applied to a liquid crystal, V a  is an applied voltage, V F  represents a Freederick transition voltage at which liquid crystal molecules begin to perform an inclined motion, d is a cell gap of the liquid crystal cells, and γ represents a rotational viscosity of the liquid crystal molecules.
 
τ f   =γd   2   /K   (2)
 
where τ f  represents a falling time at which a liquid crystal is returned into the initial position by an elastic restoring force after a voltage applied to the liquid crystal was turned off, and K is an elastic constant.
 
     A twisted nematic (TN) mode liquid crystal has a different response time due to physical characteristics of the liquid crystal and a cell gap, etc. Typically, the TN mode liquid crystal has a rising time of 20 to 80 ms and a falling time of 20 to 30 ms. Since such a liquid crystal has a response time longer than one frame interval (i.e., 16.67 ms in the case of NTSC system) of a moving picture, a voltage charged in the liquid crystal cell is progressed into the next frame prior to arriving at a target voltage. Thus, due to a motion-blurring phenomenon a screen is blurred out at the moving picture. 
     Referring to  FIG. 1 , the conventional LCD cannot express desired color and brightness. Upon implementation of a moving picture, a display brightness BL fails to arrive at a target brightness corresponding to a change of the video data VD from one level to another level due to its slow response time. Accordingly, a motion-blurring phenomenon appears from the moving picture and a display quality is deteriorated in the LCD due to a reduction in a contrast ratio. 
     In order to overcome such a slow response time of the LCD, U.S. Pat. No. 5,495,265 and PCT International Publication No. W099/05567 have suggested to modulate data in accordance with a difference in the data by using a look-up table (hereinafter referred to as high-speed driving strategy). This high-speed driving method allows data to be modulated by a principle as shown in  FIG. 2 . 
     Referring to  FIG. 2 , a conventional high-speed driving method modulates input data VD and applies the modulated data MVD to the liquid crystal cell, thereby obtaining a desired brightness MBL. This high-speed driving method increases |V a   2 −V F   2 | from the above equation (1) on the basis of a difference of the data so that a desired brightness can be obtained in response to a brightness value of the input data within one frame interval, thereby rapidly reducing a response time of the liquid crystal. Accordingly, the LCD employing such a high-speed driving method compensates for a slow response time of the liquid crystal by modulating a data value in order to alleviate a motion-blurring phenomenon in a moving picture, thereby displaying a picture at desired color and brightness. 
     In other words, the high-speed driving method compares most significant bit data of a current frame Fn with most significant bit data of the previous frame Fn−1. If the variation in the most significant bit data MSB is detected, a modulated data corresponding to the variation is selected from a look-up table, thereby modulating the source data (or input data) into the modulated data as shown in  FIG. 3 . The high-speed driving method modulates only a part of the most significant bits among the input data for reducing a memory capacity. 
     Referring to  FIG. 4 , a conventional high-speed driving apparatus includes a frame memory  43  connected to a most significant bit output bus line  42  and a look-up table  44  connected to the most significant bit output bus line  42  and the memory  43 . 
     The frame memory  43  stores most significant bit data MSB during one frame period and supplies the stored data to the look-up table  44 . Herein, the most significant bit data MSB are higher order 4 bits among 8 bits of the source data RGB. 
     The look-up table  44  makes a mapping of the most significant bit data of the current frame Fn inputted from the most significant bit output bus line  42  and the most significant bit data of the previous frame Fn−1 inputted from the frame memory  43  into a modulation data table such as Table 1 to select modulated most significant data Mdata. Such modulated most significant bit data Mdata are added to a non-modulated least significant bit data LSB from a least significant bit output bus line  41  before outputting to a liquid crystal display. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
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                 1 
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                 2 
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                 3 
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                 4 
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                 5 
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                 6 
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                 7 
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                 8 
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                 9 
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                 15 
               
               
                 10 
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                 11 
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                 12 
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                 13 
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                 14 
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                 15 
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     In the above Table 1, a left column is for a data voltage VDn−1 of the previous frame Fn−1 while an uppermost row is for a data voltage VDn of the current frame Fn. 
     Such a conventional high-speed driving method enhances a dynamic contrast ratio in comparison with a conventional normal driving method that does not modulate the source data. However, the conventional high-speed driving method gradually enhances brightness so that a desired brightness level is achieved at the end of one frame interval. Due to this, the conventional high-speed driving method cannot provide a desired picture quality. In other words, due to a data maintaining characteristic of the liquid crystal display device in the conventional high-speed driving method, a dynamic contrast ratio cannot be reached at a desired level. Furthermore, colors represented by combining red, green, and blue are distorted due to the data maintaining characteristic of liquid crystal display device. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a method and apparatus for driving a liquid crystal display that substantially obviates one or more of problems due to limitations and disadvantages of the related art. 
     Another object of the present invention is to provide a method and apparatus for driving a liquid crystal display enhancing a picture quality. 
     Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for driving a liquid crystal display includes modulating source data using registered data previously provided and supplying the modulated source data to a display panel at an initial period of one frame interval, and applying a black voltage as black data to the display panel for at least a portion of the rest period of the frame, the black voltage allowing a black picture to be displayed on the display panel. 
     The method further includes applying the source data to the display panel in such a manner that the source data is positioned between the modulated data and the black data. In this case, the display panel sequentially receives the modulated data, the source data, and the black data. The source data is delayed while applying the modulated data and the black data to the display panel. 
     In another aspect of the present invention, an apparatus for driving a liquid crystal display includes a modulator modulating source data using registered data previously provided and supplying the modulated source data to a display panel at an initial period of one frame interval, and a black voltage generator generating a black voltage as black data to apply to the display panel for at least a portion of the rest period of the one frame interval, the black voltage allowing a black picture to be displayed on the display panel. 
     The apparatus further includes a source data provider applying the source data to the display panel in such a manner that the source data is positioned between the modulated data and the black data. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention. 
       In the drawings: 
         FIG. 1  is a waveform diagram showing a brightness variation with respect to an applied voltage according to a conventional liquid crystal display; 
         FIG. 2  is a waveform diagram showing a brightness variation with respect to an applied voltage according to a conventional high-speed driving method; 
         FIG. 3  illustrates a modulation of most significant bit data in the conventional high-speed driving apparatus for 8 bits of data; 
         FIG. 4  is a block diagram showing a configuration of a conventional high-speed driving apparatus; 
         FIG. 5  is a block diagram showing a configuration of a driving apparatus for a liquid crystal display according to a first embodiment of the present invention; 
         FIG. 6  is a block diagram showing an embodiment of the data modulator shown in  FIG. 5 ; 
         FIG. 7  is a block diagram showing depicts another embodiment of the data modulator shown in  FIG. 5 ; 
         FIGS. 8A to 8C  are graphic diagrams showing modulated data and brightness in the first embodiment of the present invention to compare the conventional normal speed driving method with the present invention; 
         FIG. 9  is a block diagram showing a configuration of a driving apparatus for a liquid crystal display according to a second embodiment of the present invention; and 
         FIGS. 10A to 10C  are graphic diagrams showing modulated data and brightness in the second embodiment of the present invention to compare the conventional normal speed driving method with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Reference will now be made in detail to the illustrated embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     An apparatus for driving a liquid crystal display (LCD) according to a first embodiment of the present invention is shown in  FIG. 5 . 
     The LCD driving apparatus includes a liquid crystal display panel  57  having a plurality of data lines  55  and a plurality of gate lines  56  crossing each other and having TFT&#39;s provided at the intersections therebetween to drive liquid crystal cells Clc. A data driver  53  supplies data to the data lines  55 . A gate driver  54  applies a scanning pulse to the gate lines  56 . A timing controller  51  receives digital video data and horizontal and vertical synchronizing signals H and V. A data modulator  52  is connected between the timing controller  51  and the data driver  53  to modulate input data RGB. The LCD driving apparatus further includes a black voltage generator  60  generating black data BL, a switch  58  connected between the data modulator  52 , the black voltage generator  60  and the data driver  53  to select any one of the black data, modulated data AMdata and normal data, and a data delay circuit  59  connected between the timing controller  51  and the switch  58 . The normal data are data which are not modulated. 
     The liquid crystal display panel  57  has a liquid crystal formed between two glass substrates, and has the data lines  55  and the gate lines  56  provided on the lower glass substrate in such a manner to perpendicularly cross each other. The TFT&#39;s provided at each intersection between the data lines  55  and the gate lines  56  respond to a scanning pulse to apply data on the data lines  55  to the liquid crystal cells Clc. To this end, gate electrodes of the TFT&#39;s are connected to the gate lines  56  while source electrodes are connected to the data lines  55 . The drain electrodes of the TFT&#39;s are connected to pixel electrodes of the liquid crystal cells Clc. 
     The timing controller  51  rearranges digital video data supplied from a digital video card (not shown). The RGB data rearranged by the timing controller  51  are supplied to the data modulator  52  and the data delay circuit  59 . Further, the timing controller  51  creates timing control signals, such as a dot clock Dclk, a gate start pulse GSP, a gate shift clock GSC (not shown), an output enable/disable signal, and a polarity control signal using horizontal and vertical synchronizing signals H and V to control the data driver  53  and the gate driver  54 . The dot clock Dclk and the polarity control signal are applied to the data driver  53  while the gate start pulse GSP and the gate shift clock GSC are applied to the gate driver  54 . Herein, the timing control signals and the polarity control signal generated in the timing controller  51  have frequencies three times greater than those of the conventional timing control signals and a prior polarity control signal. The timing controller  51  also provides a switching control signal SW allowing the switch  58  to switch three times within one frame interval. To this end, the switching control signal SW varies to have a different logical value within one frame interval. In detail, the logical value of the switching control signal SW varies at each ⅓ period unlike the conventional vertical synchronous signal V. The switching control signal consists of at least two bit data so that the switch  58  selects any one of at least three signals such as modulated data Mdata, normal data RGB, black data BL, and so on. 
     The gate driver  54  includes a shift register sequentially generating a scanning pulse, that is, a gate high pulse in response to the gate start pulse GSP and the gate shift clock GSC applied from the timing controller  51 , and a level shifter shifting a voltage of the scanning pulse into a level suitable for driving the liquid crystal cell Clc. The TFT is turned on in response to the scanning pulse to apply video data to the data line  55  to the pixel electrode of the liquid crystal cell Clc. Each gate start pulse GSP and gate shift clock GSC has a frequency three times greater than that of the conventional gate start pulse and the gate shift clock and allows all scanning lines  56  on the liquid crystal display panel  57  to be scanned three times within one frame interval. 
     The data driver  53  is sequentially supplied with the modulated data AMdata, the normal data RGB and the black data BL from the switch  58  within one frame interval, as well as a dot clock Dclk from the timing controller  51 . The data driver  53  continuously selects each of the modulated data Mdata, the normal data RGB and the black data BL in synchronization with the dot clock Dclk and then latches the selected data by one line. The latched data for one line by the data driver  53  is converted into analog data and applied to the data lines  55  in each scanning period. Further, the data driver  53  may apply a gamma voltage corresponding to the modulated data to the data line  55 . The dot clock Dclk has a frequency three times greater than that of the conventional dot clock, so that each of the modulated data Mdata, the normal data RGB and the black data BL is applied to each liquid crystal cell Clc within one frame interval. 
     The data modulator  52  includes a look-up table, as shown in  FIGS. 6 and 7 , described with the modulated data AMdata opposing to each gray scale value of the normal data RGB. The data modulator  52  modulates the normal data RGB into the modulated data AMdata on the look-up table. The data modulator  52  modulates 8 bits of the source data into 8 bits of the modulated data, as shown in  FIG. 6 . Alternatively, the data modulator  52  modulates only 4 most significant bits MSB among the 8 bits of the source data into 4 bits of the modulated data in order to reduce a capacity of a memory which is used for the look-up table, as shown in  FIG. 7 . 
     The black voltage generator  60  (shown in  FIG. 5 ) generates black data having a voltage which enables the liquid crystal panel  57  to entirely shield light emitted from the back light unit (not shown) to display in black. The black data BL is applied to the switch  58 . 
     In case of modulating the most significant bit data MSB having 4 bits, the modulated data on the look-up table can be mapped as the following Table 2. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Source data 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
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                 Modulated 
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                 data 
               
               
                   
               
            
           
         
       
     
     In the above Table 2, each modulated data is determined to have a gray scale level voltage higher than that of the respective source data (the normal data) except for the lowest and highest gray scale level voltage of the data. 
     The liquid crystal display driving apparatus of the present invention may not require a frame memory because a comparison of the data between the frames is not necessary. Furthermore, since the modulated data to be stored is determined to oppose to each gray scale level of the normal data RGB input as the source data, the liquid crystal display driving apparatus of the present invention reduces a capacity of a memory used for the look-up table, as shown in Table 2. 
     The switch  58  responds to the switching control signal SW from the timing controller  51  and sequentially applies the modulated data AMdata, the normal data RGB and the black data BL to the data driver  53  within one frame period. 
     The data delay circuit  59  delays the normal data RGB while the modulated data AMdata and the black data BL are applied to the data driver  53 . 
       FIGS. 8A to 8C  illustrate a variation in brightness with respect to a voltage applied to the liquid crystal panel  57  in the liquid crystal display driving apparatus and method according to the first embodiment of the present invention. As shown in  FIG. 8C , one frame interval is divided into a first to third sub-fields SF 1  to SF 3 . The period of each sub-field SF 1  to SF 3  is appropriately adjusted within one frame interval. For example, the period of each sub-field SF 1  to SF 3  may be ⅓ of one frame interval. 
     In  FIG. 8A , “VD” is a normal data voltage and “BL” is a brightness varying with the normal data voltage VD. “MVD” is a modulated data voltage modulated by the conventional high-speed driving system and “MBL” is a brightness varying with the modulated data voltage MVD. In  FIG. 8B , “AMVD” is a modulated data voltage modulated by the liquid crystal display driving apparatus and method according to the present invention and “AMBL” is a brightness varying with the modulated data voltage AMVD. 
     In the first sub-field SF 1 , the modulated data AMdata modulated by the data modulator  52  is applied to the liquid crystal panel  57 . The normal data RGB, which is not modulated, is supplied to the liquid crystal panel  57  during the second sub-field SF 2  continued from the first sub-field SF 1 . The third sub-field SF 3  arranged at the end of the frame is used for a pause interval. In the third sub-field SF 3 , the black data BL is applied to the liquid crystal panel  57 . Due to the pause interval of the third sub-field SF 3 , the data voltage is not required to be maintained as a conventional cathode ray tube, so that a motion blurring does not appear from the moving picture. 
     Since the modulated data voltage AMVD in the first sub-field SF 1  is higher than the normal data voltage VD, an effective voltage applied to the liquid crystal panel  57  of the modulated data voltage AMVD is higher than that of the normal data VD. Accordingly, the brightness of the liquid crystal cell in the primary period of each frame reaches to a desired level. The brightness reached to the desired level is maintained until the second sub-field SF 2 . The brightness is gradually dropped down to the lowest level by applying the black data voltage within the period of the third sub-field SF 3 . 
     As shown in  FIGS. 8B and 8C , the liquid crystal display driving apparatus and method according to the present invention allow a data voltage to be shifted always from a black level to a white level or an arbitrary gray scale level of the normal data or the modulated data. To this end, the voltage level of the modulated data AMdata must be determined higher than that of the normal data RGB on the basis of the data modulating algorithm of the high-speed driving method. 
       FIG. 9  illustrates a driving apparatus for a liquid crystal display (LCD) according to a second embodiment of the present invention. 
     The LCD driving apparatus in the second embodiment includes a liquid crystal display panel  97  having a plurality of data lines  95  and a plurality of gate lines  96  crossing each other and having TFT&#39;s provided at the intersections therebetween to drive liquid crystal cells Clc. A data driver  93  supplies data to the data lines  95  of the liquid crystal panel  97 . A gate driver  94  applies a scanning pulse to the gate lines  96  of the liquid crystal panel  97 . A timing controller  91  receives digital video data and synchronizing signals H and V. The LCD driving apparatus of the second embodiment further includes a data modulator  92  connected between the timing controller  91  and the data driver  93  to modulate an input data RGB, a black voltage generator  99  generating a black data BL, and a switch  98  connected between the data modulator  92 , the black voltage generator  99  and the data driver  93  to select any one of the black data and the modulated data AMdata. 
     The liquid crystal panel  97  has the same configuration as the liquid crystal panel  57  of the first embodiment, as shown in  FIG. 5 . 
     The timing controller  91  rearranges a digital video data supplied from a digital video card (not shown). The RGB data rearranged by the timing controller  91  is supplied to the data modulator  92 . 
     The timing controller  91  also creates timing control signals, such as a dot clock Dclk, a gate start pulse GSP, a gate shift clock GSC (not shown), an output enable/disable signal, and a polarity control signal using horizontal and vertical synchronizing signals H and V inputted thereto to control the data driver  93  and the gate driver  94 . The dot clock Dclk and the polarity control signal are applied to the data driver  93  while the gate start pulse GSP and the gate shift clock GSC are applied to the gate driver  94 . Herein, the timing control signals and the polarity control signal generated from the timing controller  91  have frequencies twice greater than those of the conventional timing control signals and a conventional prior polarity control signal, respectively. The timing controller  91  also provides a switching control signal SW allowing the switch  98  to switch the output data twice within one frame interval. To this end, the switching control signal SW is inverted in logical value within one frame interval. In detail, the logical value of the switching control signal SW is inverted at each ½ period unlike the conventional vertical synchronous signal V. The switching control signal consists of only one bit data. 
     The gate driver  94  includes a shift register sequentially generating a scanning pulse, that is, a gate high pulse in response to the gate start pulse GSP and the gate shift clock GSC applied from the timing controller  91 , and a level shifter shifting a voltage of the scanning pulse into a level suitable for driving the liquid crystal cell Clc. The TFT is turned on in response to the scanning pulse to apply video data to the data line  95  to the pixel electrode of the liquid crystal cell Clc. Each gate start pulse GSP and gate shift clock GSC has a frequency twice greater than that of the conventional gate start pulse and the gate shift clock and allows all scanning lines  96  on the liquid crystal panel  97  to be scanned twice within one frame interval. 
     The data driver  93  is sequentially supplied with the modulated data AMdata and the black data BL from the switch  98  within one frame interval, as well as a dot clock Dclk from the timing controller  91 . The data driver  93  continuously selects each of the modulated data AMdata and the black data BL in synchronization with the dot clock Dclk and thereafter latches the selected data by one line. The latched data for one line by the data driver  93  is converted into an analog data and applied to the data lines  95  in each scanning period. Further, the data driver  93  may apply a gamma voltage corresponding to the modulated data to the data line  95 . The dot clock Dclk has a frequency three times greater than that of the conventional dot clock, so that each of the modulated data Mdata and the black data BL is applied to each liquid crystal cell Clc within one frame interval. 
     The data modulator  92  includes a look-up table, as shown in  FIGS. 6 and 7 , described with the modulated data AMdata opposing to each gray scale value of the normal data RGB and modulates the normal data RGB into the modulated data AMdata on the look-up table. The data modulator  92  modulates 8 bits of the source data into 8 bits of the modulated data, as shown in  FIG. 6 . Alternatively, the data modulator  92  modulates only 4 most significant bits MSB among the 8 bits of the source data into 4 bits of the modulated data in order to reduce a capacity of a memory which is used for the look-up table, as shown in  FIG. 7 . 
     In case of modulating the most significant bit data MSB having 4 bits, the modulated data on the look-up table can be mapped as shown in Table 2. 
     The black voltage generator  99  generates the black data having a voltage which enables the liquid crystal panel  97  to entirely shield lights from the back light unit (not shown) to display in black. The black data BL is applied to the switch  98 . 
     The switch  98  responds to the switching control signal SW from the timing controller  91  and sequentially applies the modulated data AMdata and the black data BL to the data driver  93  within one frame. 
       FIGS. 10A to 10C  illustrate a variation in brightness with respect to a voltage applied to the liquid crystal panel  97  in the liquid crystal display driving apparatus and method according to the second embodiment of the present invention. 
     Referring to  FIGS. 10B and 10C , one frame interval is divided into a first and second sub-fields SF 1  and SF 2 . The period of each sub-field SF 1  and SF 2  is appropriately adjusted within one frame interval. For example, the period of each sub-field SF 1  and SF 2  may be ½ of one frame interval. 
     In the first sub-field SF 1 , the modulated data AMdata modulated by the data modulator  92  is applied to the liquid crystal panel  97 . 
     The second sub-field SF 2  continued from the first sub-field SF 1  is used for a pause interval. In the second sub-field SF 2 , the black data BL is applied to the liquid crystal panel  97 . Due to the second sub-field SF 2 , a motion blurring does not occur in the moving picture. 
     As described above, the LCD driving apparatus and method according to the present invention apply the normal data and the black data to the liquid crystal panel after supplying of the modulated data to the liquid crystal panel. Alternatively, the LCD driving apparatus and method according to the present invention can sequentially supply the modulated data and the black data to the liquid crystal panel. Accordingly, the LCD drive apparatus and method allow a motion blurring to be minimized. As a result, the LCD drive apparatus and method provide with a high quality moving picture. 
     The data modulator may be implemented by other means, such as a program and a microprocessor for carrying out this program, rather than the look-up table. The present invention may be applied to a digital flat display device, which requires the data modulation, such as a plasma display panel, a electro-luminescence display device, an electric field emitting device and so on. Furthermore, the switch, the data delay circuit and the black voltage generator may be combined in one unit together with the timing controller or the data driver. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the method and apparatus for driving the liquid crystal display of the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.