Patent Publication Number: US-2006007080-A1

Title: Apparatus and method for improving image-sticking effect of liquid crystal display

Description:
FIELD OF THE INVENTION  
      The present invention relates to a method and an apparatus for improving the image-sticking effect of a display, and more particularly, to a method and an apparatus for improving the image-sticking effect of a liquid crystal display.  
     BACKGROUND OF THE INVENTION  
      Opto-electronics technologies have recently progressed by leaps and bounds due to the coming of the digital era, which also has stimulated the market for liquid crystal displays (LCDs). The liquid crystal displays possess many advantages, such as, for example, high display quality, small volume occupation, light weight, low driving voltage, and low power consumption. Therefore, the liquid crystal displays are gradually replacing conventional cathode ray tube (CRT) displays and are applied widely to 3C (computers, communications, and consumer electronic) products, for example, personal digital assistants (PDAs), cellular phones, video recording units, notebook computers, desktop monitors, vehicular monitors, and projective televisions.  
      So far, the liquid crystal displays are usually used by alternating current (AC) driving method. The liquid crystal displays further reverse the polarity of a voltage applied to a liquid crystal cell every frame time, in order to reduce the driving voltage of the liquid crystal displays.  FIG. 1  illustrates a conventional pixel cell of a thin film transistor (TFT) liquid crystal display. The conventional pixel cell includes a thin film transistor  120  having a gate connected to a scanning line  110  and a source connected to a data line  130 , as well as a liquid crystal cell  160  connected among a drain of the thin film transistor  120 , an auxiliary capacitor  180  and a common voltage source (V com ), as shown in  FIG. 1 . The thin film transistor  120  is selectively turned on by controlling the scanning line  110  to connect the data line  130 . When the thin film transistor  120  is turned on, the liquid crystal cell  160  and the auxiliary capacitor  180  accumulate a signal voltage from the data line  130  and maintain the accumulated signal voltage until the thin film transistor  120  is turned on again. However, the voltages of liquid crystal cells in a liquid crystal display are not identical, which incurs different gate voltages for each scanning line. As a result, the polarities of the positive voltage and the negative voltage applied to the liquid crystal cell  160  have different absolute values with respect to each other. Hence, a certain amount of direct-current (DC) charges accumulate in the liquid crystal cell  160 . Consequently, a residual image caused by the accumulated DC charges is displayed on a panel of the liquid crystal display when changing a displayed picture; especially the displayed picture is at a still status for a long time.  
     SUMMARY OF THE INVENTION  
      It is the objective of the present invention to provide a method and an apparatus for improving the image-sticking effect of a liquid crystal display resulting from a still image appearing on a panel of the liquid crystal display for a period of time.  
      According to the aforementioned objective of the present invention, an apparatus for improving the image-sticking effect of a liquid crystal display is provided. The apparatus recognizes the state of an image displayed on a panel of the liquid crystal display and then forces image to vary when detecting a still image state.  
      According to the aforementioned objective of the present invention, on the other hand, a method for improving the image-sticking effect of a liquid crystal display is provided. The image-sticking effect caused by DC charges is diminished through altering an image displayed on a panel, especially when the displayed image is still on the panel for a long period of time.  
      In accordance with a preferred embodiment of the present invention, a comparator first analyzes an image datum and determines whether it&#39;s a still image or not after receiving it. If a still image is detected, a value of a counter is then increased. An inset image is next produced by an inset device and is transmitted to a panel of a liquid crystal display when the value of the counter successively accumulates to a threshold value. Therefore, a still image appearing on the panel of the liquid crystal display is changed by displaying the inset image, so as to reduce the image-sticking effect of the liquid crystal display. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing aspects, as well as many of the attendant advantages and features of this invention will become more apparent by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
       FIG. 1  illustrates a conventional pixel cell of a thin film transistor liquid crystal display;  
       FIG. 2  illustrates a block diagram of an apparatus for improving the image-sticking effect of a liquid crystal display in accordance with a preferred embodiment of the present invention; and  
       FIG. 3  illustrates a flow chart of a method for improving the image-sticking effect of a liquid crystal display in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A preferred embodiment in accordance with the present invention is disclosed in details as follows, taken in conjunction with the accompanying drawings.  
       FIG. 2  shows a block diagram of an apparatus for improving the image-sticking effect of a liquid crystal display in accordance with the preferred embodiment of the present invention. As illustrated in  FIG. 2 , at least an image datum is transmitted by an image input device  210  continuously to a comparator  220  and is then stored in a storage, like an image memory  240 . Further, the image datum represents a series of image signals transmitted therein in every frame time in the preferred embodiment. When receiving the image datum, the comparator  220  compares the image datum with a previous image datum stored in the image memory  240  first by analyzing image signals thereof between each other. As a proportion of image signals varies, for example, up to 80% of image signals of the image datum differ from those of the previous image datum, the comparator  220  regards the image datum as a moving image. When a moving image is detected, the comparator  220  transmits the image datum directly to an image output device  290 . The image output device  290  may be a panel of the liquid crystal display. Otherwise, if a certain percentage of image signals of an image datum is monotonous, the image datum is considered as a still image.  
      On the other hand, when a still image is detected, a value of a frame counter  260  is increased by, for example, “1”. Meanwhile, an inset device  280  produces an inset image transmitted to the image output device  290  if the value of the frame counter  260  accumulates successively to a threshold value, for example, “n”, which depends on the characteristics of the liquid crystal display. Thereby the still image appearing on a panel of the liquid crystal display for a critical period is changed imperatively by displaying the inset image. The residual DC charges therein are thus offset owing to alteration of displayed pictures and hence the image-sticking effect is reduced. Further, the inset image comprises a black inset image, a white inset image, or an intermediate gray inset image.  
      Referring to  FIG. 3 , which illustrates a flow chart of a method for improving the image-sticking effect of a liquid crystal display in accordance with the preferred embodiment of the present invention. After receiving an image datum in step  315 , a comparator compares the image datum with a previous image datum and determines whether the image datum is a still image or not in step  325 . If an image datum is not a still image, the image datum is transmitted immediately to an image output device  390  that comprises a panel of the liquid crystal display. A counter is then reset in step  395 , and returns to step  315  thereafter. Or, if a still image is considered, a value of the counter is then increased by, for example, “1”, in step  345 . Next, in step  365 , the value of the counter is checked if the same equals a critical value, for instance, “n”, which relies on the characteristics of the liquid crystal display. At the meantime, if the value of the counter has not reached the critical value yet, then an image datum is continuously received in step  315 . On the contrary, if the value of the counter is equal to the critical value, an inset image is produced by an inset device and is transmitted to the image output device  390  in step  385 . Afterward the value of the counter is reset, that is, set to “0”, in step  395 , the process returns to step  315 . In addition, the inset image may be a black inset image, a white inset image, or an intermediate gray inset image. The still image displayed on the panel for a critical period is forced to vary by inserting the inset image. Accordingly, the residual image disappears.  
      According to the aforementioned preferred embodiment of the present invention, an inset image is transmitted to a panel of a liquid crystal display when a still image displayed on the panel for a certain period is detected. The image-sticking effect resulting from residual DC charges is thus improved by altering the displaying images. Therefore the residual image is diminished when the same picture has been displayed for a long time.  
      While the invention has been particularly shown and described with reference to the preferred embodiments thereof, these are, of course, merely examples to help clarify the invention and are not intended to limit the invention. It will be understood by those skilled in the art that various changes, modifications, and alterations in form and details may be made therein without departing from the spirit and scope of the invention, as set forth in the following claims.