Abstract:
An exemplary method for driving a liquid crystal display includes: dividing a frame into a first sub-frame period and a second sub-frame period; displaying a normal image in the first sub-frame period; and displaying a gray image in the second sub-frame period. The gray image includes a plurality of pixels, and some of the pixels are black, and each of the pixels is black at least one time in a predefined minimum period, the minimum period being at least two consecutive frames.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to methods for driving liquid crystal displays (LCDs), and more particularly to a method for driving an LCD with insertion of gray images. 
       GENERAL BACKGROUND 
       [0002]    LCDs are widely used in various modern information products, such as notebooks, personal digital assistants, video cameras and the like. 
         [0003]    When motion pictures are displayed on an LCD, a so-called residual image phenomenon may occur. A motion picture is a series of images displayed one after another in rapid succession. In general, the displaying of each image lasts for a period of time known as a frame. Typically, each frame lasts a small fraction of a second. When a viewer is viewing an image of a current frame, the viewer may still be perceiving the image of the previous frame. That is the image of the previous frame remains in the viewer&#39;s perception as a so-called afterimage. The afterimage overlaps with the image of the current frame being viewed, and this causes the residual image phenomenon. From the standpoint of the viewer, the display quality of the LCD is impaired. To overcome the above-described problem, a method known as black insertion driving has been developed to drive an LCD. 
         [0004]      FIG. 7  is an abbreviated circuit diagram of a conventional LCD. The LCD  100  includes a liquid crystal panel  101 , a scanning circuit  102 , and a data circuit  103 . The scanning circuit  102  and the data circuit  103  are configured for driving the liquid crystal panel  101 . 
         [0005]    The liquid crystal panel  101  includes a plurality of parallel scanning lines  110 , a plurality of parallel data lines  120  orthogonal to the plurality of parallel scanning lines  110 , and a plurality of pixel regions  130  cooperatively defined by the crossing scanning lines  110  and data lines  120 . The scanning lines  110  are electrically coupled to the scanning circuit  102 . The data lines  120  are electrically coupled to the data circuit  103 . 
         [0006]    Each pixel region  130  includes a thin film transistor (TFT)  131 , a pixel electrode  132 , a common electrode  134 , and liquid crystal molecules (not shown) interposed between the pixel electrode  132  and the common electrode  134 . The TFT  131  is disposed near an intersection of a corresponding one of the scanning lines  110  and a corresponding one of the data lines  120 . A gate electrode of the TFT  131  is electrically coupled to the corresponding scanning line  110 , and a source electrode of the TFT  131  is electrically coupled to the corresponding data line  120 . Further, a drain electrode of the TFT  131  is electrically coupled to the pixel electrode  132 . The common electrode  203  is electrically coupled to a common voltage generating circuit (not shown) that is configured to provide common voltages. Moreover, each pixel electrode  132 , the corresponding common electrode  134 , and the liquid crystal molecules therebetween cooperatively form a liquid crystal capacitor  133 . 
         [0007]    Referring to  FIG. 8  and  FIG. 9 , when the LCD  100  is driven by the black insertion driving method, each frame period is divided into a first sub-frame period T 1  and a second sub-frame period T 2 . In particular, the first sub-frame period T 1  serves as a normal display period, and the second sub-frame period T 2  serves as a black frame insertion period. 
         [0008]    During the first sub-frame period T 1 , a plurality of first scanning signals  150  are generated by the scanning circuit  102 , and are sequentially supplied to the scanning lines  110 , so as to scan the corresponding pixel regions  130  row by row. When the corresponding row of pixel regions  130  are scanned by the first scanning signal  150 , the TFTs  131  of the pixel regions  130  are switched on. The data circuit  103  then supplies a plurality of first driving voltages to the pixel electrodes  132  of the pixel regions  130  via the data lines  120  and the TFTs  131 . Thus, during the first sub-frame period T 1 , the LCD  100  displays a normal image  201 . 
         [0009]    During the second sub-frame period T 2 , the scanning circuit  102  supplies a plurality of second scanning signals  160  to switch on the TFTs  131  of pixel regions  130  row by row. The data circuit  103  supplies a plurality of second driving voltages having values the same as that of the corresponding common voltages supplied to the pixel electrodes  132  of the pixel regions  130 . Thus, during the second sub-frame period T 2 , the LCD  100  displays a black image  202 . The black image  202  includes a plurality of pixels (not labeled) arranged in a matrix, and all the pixels are black. Each of the pixels corresponds to one of the pixel regions  130  of the LCD  100 . 
         [0010]    By employing the black insertion driving method, normal images  201  and black images  202  are displayed alternately. In a complete frame period, a viewer perceives the normal image  201  during the first sub-frame period T 1 , and perceives the black image  202  during the second sub-frame period T 2 . Thus, an afterimage of the normal image  201  displayed in the first sub-frame period T 1  is removed from the viewer&#39;s perception during the second sub-frame period T 2 . This means that the problem of the residual image phenomenon can be solved. 
         [0011]    However, the black image  202  has the least brightness among all images displayed by the LCD  100 . For example, in a continuous four frame periods, the LCD displays four normal images  201  and four black images  202 . A time of displaying the four black images  202  is equal to that of displaying the four normal images  201 . Thus, a brightness of images displayed by the LCD  100  is seriously reduced. 
         [0012]    It is, therefore, desired to provide a method for driving an LCD which can overcome the above-described deficiencies. 
       SUMMARY 
       [0013]    In one aspect, a method for driving a liquid crystal display includes: dividing a frame into a first sub-frame period and a second sub-frame period; displaying a normal image in the first sub-frame period; and displaying a gray image in the second sub-frame period. The gray image includes a plurality of pixels, and some of the pixels are black, and each of the pixels is black at least one time in a predefined minimum period, the minimum period being at least two consecutive frames. 
         [0014]    In another aspect, a method for driving a liquid crystal display includes: providing a driving circuit; the driving circuit generating a plurality of first signals corresponding to displaying a normal image; and the driving circuit generating a plurality of second signals corresponding to displaying a gray image between each two sequential normal images. Any four sequential gray images dither into M black image(s) as perceived by the human eye, M being a positive integer less than four. 
         [0015]    Other novel features and advantages will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is an abbreviated circuit diagram of an LCD that employs a driving method according to an exemplary embodiment of the present invention, the LCD including a plurality of scanning lines. 
           [0017]      FIG. 2  is a waveform diagram of scan signals of the scanning lines of  FIG. 1 . 
           [0018]      FIG. 3  is a diagram illustrating an operation principle of displaying normal images and gray images on the LCD of  FIG. 1 . 
           [0019]      FIG. 4  is a diagram of a first example pattern of a gray image for display according to  FIG. 3 . 
           [0020]      FIG. 5  is a diagram of a second example pattern of a gray image for display according to  FIG. 3 . 
           [0021]      FIG. 6  is a diagram of a third example pattern of a gray image for display according to  FIG. 3 . 
           [0022]      FIG. 7  is an abbreviated circuit diagram of a conventional LCD, the LCD including a plurality of scanning lines. 
           [0023]      FIG. 8  is a waveform diagram of scan signals of the scanning lines of  FIG. 7 . 
           [0024]      FIG. 9  is a diagram illustrating an operation principle of displaying normal images and black images on the LCD of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]    Reference will now be made to the drawings to describe preferred and exemplary embodiments of the present invention in detail. 
         [0026]      FIG. 1  is an abbreviated circuit diagram of an LCD that employs a driving method according to an exemplary embodiment of the present invention. The LCD  300  includes a liquid crystal panel  301  and a driving circuit (not labeled). The driving circuit includes a scanning circuit  302 , a data circuit  303 , and a timing control circuit  304 . The scanning circuit  302  is configured for providing a plurality of scan signals. The data circuit  303  is configured for providing a plurality of data voltages. The timing control circuit  304  is configured for controlling driving timing of the scanning circuit  302  and the data circuit  303 . 
         [0027]    The liquid crystal panel  301  includes a plurality of parallel scanning lines  310 , a plurality of parallel data lines  320  orthogonal to the plurality of parallel scanning lines  310 , and a plurality of pixel regions  330  cooperatively defined by the crossing scanning lines  310  and data lines  320 . The scanning lines  310  are electrically coupled to the scanning circuit  302 . The data lines  320  are electrically coupled to the data circuit  303 . Moreover, the plurality of pixel regions  330  are arrayed in a matrix, such as that the LCD  300  is an active matrix LCD. 
         [0028]    Each pixel region  330  includes a TFT  331 , a pixel electrode  332 , a common electrode  334 , and liquid crystal molecules (not shown) interposed between the pixel electrode  332  and the common electrode  334 . The TFT  331  is disposed near an intersection of a corresponding one of the scanning lines  310  and a corresponding one of the data lines  320 . A gate electrode of the TFT  331  is electrically coupled to the corresponding scanning line  310 , and a source electrode of the TFT  331  is electrically coupled to the corresponding data line  320 . Further, a drain electrode of the TFT  331  is electrically coupled to the pixel electrode  332 . The common electrode  334  is electrically coupled to a common voltage generating circuit (not shown). The common voltage generating circuit is configured to provide common voltages. When a value of the common voltage is equal to a minimum value of the data voltages, the pixel region  330  displays black. Moreover, each pixel electrode  332 , the corresponding common electrode  334 , and the liquid crystal molecules therebetween cooperatively form a liquid crystal capacitor  333 . 
         [0029]    Referring to  FIG. 2  and  FIG. 3 , each frame period is divided into a first sub-frame period Ta and a second sub-frame period Tb. In this embodiment, Ta=Tb. In other embodiments, Ta may be greater than Tb, or Tb may be greater than Ta. 
         [0030]    During the first sub-frame period Ta, a plurality of first scanning signals  350  are generated by the scanning circuit  302 , and are sequentially supplied to the scanning lines  310 , so as to scan the corresponding pixel regions  330  row by row. When the corresponding row of pixel regions  330  are scanned by the first scanning signal  350 , the TFTs  331  of the pixel regions  330  are switched on. The data circuit  303  then supplies a plurality of first data voltages to the pixel electrodes  332  of the pixel regions  330  via the data lines  320  and the TFTs  331 . The first data voltages correspond to a normal image  401 . Thus, during the first sub-frame period Ta, the LCD  300  displays the normal image  401 . 
         [0031]    During the second sub-frame period Tb, the scanning circuit  302  supplies a plurality of second scanning signals  360  to switch on the TFTs  331  of pixel regions  330  row by row. The data circuit  303  supplies a plurality of second data voltages to the pixel electrodes  332  of the pixel regions  330 . Some of the data voltages have the minimum value, and other data voltages have the same values as those of the first sub-frame period Ta. Thus, during the second sub-frame period Tb, the LCD  300  displays a gray image  402 . 
         [0032]    The gray image  402  includes a plurality of pixels (not labeled) arranged in a matrix, and each of the pixels corresponds to one of the pixel regions  330  of the LCD  300 . The gray image  402  can have any one of many different possible patterns. Three example patterns are shown in  FIG. 4 ,  FIG. 5 , and  FIG. 6 , respectively. The example pattern of  FIG. 5  is the same as that of the gray image  402  as illustrated in  FIG. 3 . 
         [0033]      FIG. 4  is a diagram of a first example pattern for the gray image  402 . In a second sub-frame period Tb 1  of the frame N+1 (N is a natural number), pixels at crossings of all the odd rows and all the odd columns are black, and other pixels keep the same colors as those in a first sub-frame Ta 1  of the frame N+1. In a second sub-frame period Tb 2  of the frame N+2, pixels at crossings of all the odd rows and all the even columns are black, and other pixels keep the same colors as those in a first sub-frame Ta 2  of the frame N+2. In a second sub-frame period Tb 3  of the frame N+3, pixels at crossings of all the even rows and all the even columns are black, and other pixels keep the same colors as those in a first sub-frame Ta 3  of the frame N+3. In a second sub-frame period Tb 4  of the frame N+4, pixels at crossings of all the even rows and all the odd columns are black, and other pixels keep the same colors as those in a first sub-frame Ta 4  of the frame N+4. Frame N+1, frame N+2, frame N+3, and frame N+4 together define a minimum period. The gray images  402  in the following second sub-frame periods repeat the above-described patterns of the frame N+1, frame N+2, frame N+3, and frame N+4. The gray images  402  in any four continuous frames dither into a black image as perceived by a human observer. A brightness of any gray image  402  is higher than that of the conventional black image  202  of the above-described conventional black insertion driving method. 
         [0034]      FIG. 5  is a diagram of a second pattern of the gray image of  FIG. 3 . In a second sub-frame period Tb 1  of the frame N+1, pixels at crossings of all the odd rows and all the odd columns are black, pixels at crossings of all the even rows and all the even columns are black, and other pixels keep the same color as that in a first sub-frame Ta 1  of the frame N+1. In a second sub-frame period Tb 2  of the frame N+2, pixels at crossings of all the odd rows and all the even columns are black, pixels at crossings of all the even rows and all the odd columns are black, and other pixels keep the same color as that in a first sub-frame Ta 2  of the frame N+2. In a second sub-frame period Tb 3  of the frame N+3, pixels at crossings of all the odd rows and all the odd columns are black, pixels at crossings of all the even rows and all the even columns are black, and other pixels keep the same color as that in a first sub-frame Ta 3  of the frame N+3. In a second sub-frame period Tb 4  of the frame N+4, pixels at crossings of all the odd rows and all the even columns are black, pixels at crossings of all the even rows and all the odd columns are black, and other pixels keep the same color as that in a first sub-frame Ta 4  of the frame N+4. Frame N+1, frame N+2, frame N+3, and frame N+4 define a minimum period. The gray image  402  in the following second sub-frame periods repeat that in one of the frame N+1, frame N+2, frame N+3, and frame N+4. The gray images  402  in any four continuous frames dither into two black images by human eyes. That is, a brightness of any gray image  402  in  FIG. 4  is higher than that of the black image  202  of the above-described conventional black insertion driving method. 
         [0035]      FIG. 6  is a diagram of a third pattern of the gray image of  FIG. 3 . In a second sub-frame period Tb 1  of the frame N+1, pixels at crossings of all the odd rows and all the odd columns keep the same color as that in a first sub-frame Ta 1  of the frame N+1, and other pixels are black. In a second sub-frame period Tb 2  of the frame N+2, pixels at crossings of all the odd rows and all the even columns keep the same color as that in a first sub-frame Ta 2  of the frame N+2, and other pixels are black. In a second sub-frame period Tb 3  of the frame N+3, pixels at crossings of all the even rows and all the even columns keep the same color as that in a first sub-frame Ta 3  of the frame N+3, and other pixels are black. In a second sub-frame period Tb 4  of the frame N+4, pixels at crossings of all the even rows and all the odd columns keep the same color as that in a first sub-frame Ta 4  of the frame N+4, and other pixels are black. Frame N+1, frame N+2, frame N+3, and frame N+4 define a minimum period. The gray images  402  in the following second sub-frame periods repeat that in one of the frame N+1, frame N+2, frame N+3, and frame N+4. The gray images  402  in any four continuous frames dither into three black images by human eyes. That is, a brightness of any gray image  402  in  FIG. 4  is higher than that of the black image  202  of the above-described conventional black insertion driving method. 
         [0036]    In the three above-described example patterns for the gray image  402 , the gray images  402  in any four continuous frames dither into one, two, or three black image(s) by human eyes, respectively. Thus, the driving method of the above-described embodiments can solve the residual image phenomenon. Furthermore, the brightness of any gray image  402  is higher than that of the black image  202  of the above-described conventional black insertion driving method. Thus, the brightness of the LCD  300  is higher than that of the LCD  100  employing the above-described conventional black insertion driving method. 
         [0037]    It is to be further understood that even though numerous characteristics and advantages of preferred and exemplary embodiments have been set out in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only; and that changes may be made in detail within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.