Abstract:
A method for driving a dark ring of a liquid-crystal-on-silicon (LCOS) display is provided to prevent the fringe effect (bright lines) due to the constant voltage difference between the dark ring and the adjoining pixels within the LCOS display. A dark ring is divided into a plurality of portions. The polarity of each portion is controlled in accordance with the polarity of the adjoining pixels within the LCOS display and the scan direction of gate drivers such that the polarity inversion for each portion will coincide with that for the adjoining pixels within the LCOS display so as to avoid the fringe effect (bright lines).

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a dark ring of a microdisplay and its driving method, in order to prevent the fringe effect occurred between the dark ring and the adjoining pixels within the microdisplay such as liquid-crystal-on-silicon (LCOS) display.  
         [0003]     2. Discription of the Prior Art  
         [0004]     LCOS display is one of the most popular microdisplays that are broadly adopted in large-sized TV. Because of mechanical and process limitations of the LCOS display, it is necessary to place a dark ring  110  in peripheral of the LCOS display  100  by using a top metal layer, as shown in  FIG. 1 .  
         [0005]     Referring to  FIG. 2 , in a normally-white mode, there are two voltages, V τ1  and V τ10 , at which liquid crystal contained within pixels will be biased to obtain a zero transmission rate. It suggests we can bias the liquid crystal at either V τ1  or V τ10  to show an all-black image. But the liquid crystal cannot be biased by DC voltages for a long time to avoid undesired deformation. Therefore, the dark ring is required to alternately bias at voltage V τ1  and V τ10  on a frame basis. Usually, it is called negative and positive polarity when biased at V τ1  and V τ10 .  
         [0006]     In the normally-white mode, when the LCOS display shows an all-black image with all pixels within the LCOS display changing from negative to positive polarity (that is, in case of frame inversion) and with gate drivers of the LCOS display scanning from top to bottom, the voltage of the dark ring will also change from V τ1  to V τ10  at time t 0  when activating the topmost scan line of the LCOS display. In this case, data voltages for the adjoining pixels on each scan line (from top to bottom) within the LCOS display will sequentially change from V τ1  to V τ10  at time t 0 , t 1 , and t n . It means there will be a constant voltage difference between the dark ring and the adjoining pixels on the lower scan lines within the LCOS display for almost a whole frame period, and the fringe effect (bright lines) between the dark ring and the adjoining pixels on the lower scan lines within the LCOS display will occur due to the constant voltage difference and decrease the quality of the image.  
         [0007]     This also applies when the LCOS display shows an all-black image with all pixels within the LCOS display changing from negative to positive polarity (that is, in case of frame inversion) and with gate drivers of the LCOS display scanning from bottom to top. Furthermore, this also applies when the LCOS display is performing line inversion or dot inversion.  
         [0008]     Therefore, there is a need to provide a new dark ring of the LCOS display and its driving method to prevent the fringe effect and to increase the quality of images.  
       SUMMARY OF THE INVENTION  
       [0009]     One object of the present invention is to provide a method for driving a dark ring of a microdisplay, more particularly of an LCOS display, in order to prevent the fringe effect (bright lines) due to a constant voltage difference between the dark ring and the adjoining pixels on the lower/upper scan lines of the microdisplay. The present invention divides the dark ring into a plurality of portions and respectively adjusts the polarity of each portion in accordance with the polarity of the adjoining pixels within the microdisplay and the scan direction of gate drivers in order to eliminate the voltage difference.  
         [0010]     Another object of the present invention is to increase the quality of images on a LCOS display without changing the manufacturing process. In accordance with the invention, we can adopt redundant pixels in side peripherals of the LCOS display area as the dark ring and eliminate the fringe effect (bright lines) by applying data voltages of alternate polarity to the redundant pixels in accordance with the polarity of the adjoining pixels of the LCOS display and the scan direction of gate drivers. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a conventional LCOS display with a dark ring.  
         [0012]      FIG. 2  illustrates the transmission rate for normally-white liquid crystal when biased at different voltages.  
         [0013]      FIG. 3  is a diagram of an embodiment of the present invention.  
         [0014]      FIG. 4  shows the polarity inversion for the LCOS display, in case of frame inversion, compared to that for each portion of the dark ring in  FIG. 3 .  
         [0015]      FIG. 5  is a diagram of another embodiment of the present invention.  
         [0016]      FIG. 6  shows a circuit implementation of  FIG. 5 .  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]     To eliminate the fringe effect (bright lines) due to the constant voltage difference between the dark ring and the adjoining pixels on the lower/upper scan lines within the LCOS display, we have to make the polarity inversion for each portion of the dark ring in peripheral of the LCOS display coincide with that for the adjoining pixels within the LCOS display. To achieve the above, the dark ring may be divided into several portions, with each portion electrically separated from the other. Therefore, we can control the polarity of each portion in accordance with the polarity of the adjoining pixels within the LCOS display and the scan direction of gate drivers, such that the polarity inversion for each portion would coincide with that for the adjoining pixels within the LCOS display.  
         [0018]     In a preferred embodiment, the dark ring can be divided into two portions, as shown in  FIG. 3 , that is, a dark ring up  310  and a dark ring down  320 . The dark ring up  310  includes the upper half of the dark ring, while the dark ring down  320  includes the lower half of the dark ring. In this case, both of the dark ring up  310  and dark ring down  320  may be implemented by a top metal layer. Further, there is no electrical connection between the dark ring up  310  and the dark ring down  320 .  
         [0019]     According to the scan direction of gate drivers and the polarity, negative or positive, of the adjoining pixels within the LCOS display, the voltages of the dark ring up  310  and the dark ring down  320  are respectively determined by controlling the polarity inversion for each of the dark ring up  310  and dark ring down  320  to coincide with that of the adjoining pixels within the LCOS display to eliminate the fringe effect (bright lines).  
         [0020]     Referring to  FIG. 4 , the polarity inversion for the dark ring up  310  and that for the dark ring down  320  are reversed to halve the duration for the constant voltage difference occurred between the black ring down  320  and the adjoining pixels on the bottom scan lines can within the LCOS display. In case of frame inversion, when the scan direction of gate drivers is from top to bottom and the polarity of the adjoining pixels within the LCOS display change from positive to negative polarity, the polarity for the dark ring up  310  also changes from positive to negative polarity to coincide with the polarity inversion for the topmost scan line, while the polarity of the dark ring down  320  simultaneously changes from negative to positive. From this, the duration for the constant voltage difference can be halved and the fringe effect (bright lines) due to the constant voltage difference can be reduced accordingly.  
         [0021]     This also applies to situations when the scan direction of gate drivers is from top to bottom and the polarity of the adjoining pixels within the LCOS display changes from negative to positive polarity, when the scan direction of gate drivers is from bottom to top and the polarity of the adjoining pixels within the LCOS display changes from positive to negative polarity, and when the scan direction of gate drivers is from bottom to top and the polarity of the adjoining pixels within the LCOS display changes from negative to positive polarity.  
         [0022]     In another preferred embodiment, the dark ring includes a dark ring up  510 , a dark ring down  520  and two dark ring sides  530 , as shown in  FIG. 5 . The dark ring up  510  is disposed in a top peripheral of the LCOS display area  500 . The dark ring down  520  is disposed in a bottom peripheral of the LCOS display area  500 . The dark ring sides  530  are disposed in side peripherals of the LCOS display area  500 . The dark ring up  510  and dark ring down  520  are implemented by a top metal layer, while the dark ring sides  530  are implemented by redundant pixels in side peripherals of the LCOS display area  500 , as shown in  FIG. 6 .  
         [0023]     Referring to  FIG. 6 , a logic circuit including a logic up  560 , a logic down  550  and a logic side  540  is provided to control the dark ring up  510 , the dark ring down  520  and the dark ring sides  530  respectively by controlling the polarity of the dark ring up  510 , the dark ring down  520  and the dark ring sides  530  in accordance with the polarity of the adjoining pixels within the LCOS display area  500  and the scan direction of gate drivers.  
         [0024]     Each of the logic up  560 , the logic down  550  and the logic side  540  respectively includes a plurality of inputs. The inputs include V τ1 , V τ10 , POL and UD, wherein V τ1  refers to a voltage of negative polarity, V τ10  refers to a voltage of positive polarity, POL refers to the polarity of the adjoining pixels within the LCOS display area  500 , that is, negative or positive polarity, and UD refers to the scan direction of gate drivers, that is, from top to bottom or from bottom to top.  
         [0025]     As shown in  FIG. 6 , the polarity inversion for the adjoining pixels within the LCOS display area  500  compared to that for each portion of the dark ring is illustrated below. Also, as mentioned above, the dark ring up  510  and the dark ring down  520  are implemented by a top metal layer, and the dark ring sides  530  are implemented by redundant pixels in side peripherals of the LCOS display area  500 . In this case, the polarity inversion for the dark ring up  510  will coincide with that for the adjoining pixels on the topmost scan line, the polarity inversion for the dark ring down  52  will coincide with that for the adjoining pixels on the bottommost scan line, and the polarity inversion for the redundant pixels on each scan line of the dark ring sides  530  will coincide with that for the adjoining pixels on the same scan line within the LCOS display area  500 . From this, the duration for the constant voltage difference can be further reduced to within a scan line period, and the fringe effect (bright lines) due to the constant voltage difference can be further reduced.  
         [0026]     For example, referring to  FIG. 4 , when the scan direction of gate drivers is from top to bottom and the polarity of all pixels change from positive to negative polarity (in case of frame inversion), the polarity of the dark ring up  510  also changes from positive to negative polarity in synchronous with the topmost scan line within the LCOS display area  500  to coincide with the polarity inversion for the adjoining pixels on the topmost scan line within the LCOS display area  500 . Further, the polarity of the dark ring down  520  will change from positive to negative polarity in synchronous with the bottommost scan line within the LCOS display area  500  to coincide with the polarity inversion for the adjoining pixels on the bottommost scan line within the LCOS display area  500 . Further, the polarity of the redundant pixels (adjoining pixels) on each scan line of the dark ring sides  530  will change from positive to negative polarity in synchronous with the same scan line within the LCOS display area  500  to coincide with the polarity inversion for the adjoining pixels on the same scan line within the LCOS display area  500 . From this, the polarity of the dark ring will immediately follow that of the adjoining pixels within the LCOS display area  500 , and the fringe effect (bright lines) due to the constant voltage difference will be greatly eliminated.  
         [0027]     This also applies to situations when the scan direction of gate drivers is from top to bottom and the polarity of the adjoining pixels within the LCOS display area  500  changes from negative to positive polarity, when the scan direction of gate drivers is from bottom to top and the polarity of the adjoining pixels within the LCOS display area  500  changes from positive to negative polarity, and when the scan direction of the gate drivers is from bottom to top and the polarity of the adjoining pixels within the LCOS display area  500  changes from negative to positive polarity.  
         [0028]     The foregoing is offered primarily for purpose of illustration. It will be readily apparent to those skilled in the art that the operating conditions, materials, procedural steps and other parameters of the system described herein may be further modified or substituted in various ways without departing from the spirit and scope of the invention.