Patent Publication Number: US-7592977-B2

Title: Plasma display panel and method for processing pictures thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of Korean Patent Application 10-2005-0002818 filed in the Korean Intellectual Property Office on Jan. 12, 2005, the entire content of which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a plasma display device and an image processing method thereof. 
   BACKGROUND OF THE INVENTION 
   Recently, flat panel displays, such as liquid crystal displays (LCDs), field emission displays (FEDs) and plasma display panels (PDPs), have been actively developed. PDPs are advantageous over the other flat panel displays in regard to their high luminance, high luminous efficiency and wide viewing angle. Accordingly, PDPs are in the spotlight as a substitute for conventional cathode ray tubes (CRTs) for large-screen displays of more than 40 inches. 
   PDPs are flat panel displays that use plasma generated by gas discharge to display characters or images. PDPs include, according to their size, more than several hundreds of thousands to millions of pixels arranged in the form of a matrix. These PDPs are classified into a direct current (DC) type and an alternating current (AC) type according to patterns of waveforms of driving voltages applied thereto and discharge cell structures thereof. 
   A DC PDP has electrodes exposed to a discharge space, thereby causing current to directly flow through the discharge space during application of a voltage to the DC PDP. In this regard, the DC PDP has a disadvantage in that it requires a resistor for limiting the current. On the other hand, an AC PDP has electrodes covered with a dielectric layer that naturally forms a capacitance component to limit the current and protects the electrodes from the impact of ions during discharge. As a result, the AC PDP is superior to the DC PDP in regard to a long lifetime. 
   A plasma display device such as this divides an input video signal data of one frame into a plurality of subfields, and displays grayscales by time-dividing the subfields, as shown in  FIG. 1 . In general, the subfields can be expressed by temporal operation periods, i.e., a reset period, an address period and a sustain period. The reset period is a period to initialize the state of each cell such that an addressing operation of each cell is smoothly performed, and the address period is a period to select a cell to be turned on and a cell not to be turned on in the PDP. The sustain period is a period to apply sustain pulses to the addressed cell, thereby performing a discharge according to which a picture is actually displayed. 
     FIG. 1  illustrates a case where one frame is divided into 8 subfields in order to express 256 grayscale levels. Each subfield SF 1 -SF 8  includes a reset period (not shown), an address period A 1 -A 8  and a sustain period S 1 -S 8 . The sustain period S 1 -S 8  has light emitting periods 1 T, 2 T, 4 T, . . . , 128 T at ratios of 1:2:4:8:16:32:64:128. 
   For example, a grayscale level  3  is expressed by discharging a discharge cell during a subfield having a light emitting period of 1 T and a subfield having a light emitting period of 2 T so as to have a total light emitting period of 3 T In this way, a combination of different subfields having different light emitting periods produces pictures of 256 grayscale levels. 
   When an input video signal data of one frame is divided into a plurality of subfields and grayscales are displayed according to on/off states of the subfields as described above, a false contour may be generated due to human visual properties. That is, when a moving image is displayed, a false contour phenomenon may occur in which a grayscale, different from an actual grayscale, is perceived by human eyes because of visual properties of the human eyes that follows the movement of the image. 
   Further, when grayscales are displayed according to turning the subfields on and off, a certain grayscale may have a large gap between subfields that are turned on. For such a grayscale, a low discharge (meaning that a discharge is not effectively generated) may occur. 
   For example, in the subfield arrangement of  FIG. 1 , grayscale  4  is expressed when the first and second subfields SF 1  and SF 2  are off and the third subfield SF 3  is on. In this case, at the third subfield SF 3 , few priming particles may exist since the previous subfields SF 1  and SF 2  had been off. The third subfield may therefore fail to turn on. When this desired subfield is not turned on, expressing a corresponding grayscale becomes more problematic for low grayscales. 
   The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
   SUMMARY OF THE INVENTION 
   Various embodiments of the present invention may provide a plasma display device and an image processing method thereof having advantages of reducing a false contour and avoiding a low discharge of grayscales. 
   A plasma display device and an image processing method thereof according to an embodiment of the present invention expresses a grayscale by a combination of a plurality of subfields divided from a frame of an input video signal. 
   One embodiment of the image processing method includes detecting a moving image block and a still image block from input video signals. Output grayscales corresponding to original grayscales of the detected still image block are determined such that respective output grayscales corresponding to the original grayscales of the detected still image block satisfy a first condition that the number of consecutive non-lighting subfields is less than or equal to L among subfields driven previously to a last turn-on subfield of the corresponding grayscales. Output grayscales are further determined corresponding to original grayscales of the detected moving image block such that respective output grayscales corresponding to original grayscales of the detected moving image block satisfy a second condition that the number of consecutive non-lighting subfields is less than or equal to M and a third condition that a total number of non-lighting subfields is less than or equal to N among subfields driven previously to a last turn-on subfield of the corresponding grayscale. The determined output grayscales of the detected still image block and the moving image block are then displayed on the plasma display device. 
   The numbers L, M and N may be respectively given as 2, 1, and 2. 
   In a further embodiment, an image processing method of a plasma display device expresses a grayscale by a combination of a turn-on subfield in a first group of subfields and a turn-on subfield in a second group of subfields, the first and second groups of subfields are divided from a plurality of subfields having respective weights, 
   The image processing method of this embodiment includes detecting a moving image block or a still image block from the input video signals, determining output grayscales corresponding to original grayscales of the detected still image block such that respective output grayscales corresponding to the original grayscales of the detected still image block satisfy a first condition that the number of consecutive non-lighting subfields is less than or equal to L among subfields driven previously to a last turn-on subfield of the first and second group of subfields for the corresponding grayscale. The embodiment further includes determining output grayscales corresponding to original grayscales of the detected moving image block such that output grayscales corresponding to original grayscales of the detected moving image block satisfy a second condition that the respective number of consecutive non-lighting subfields is less than or equal to M and a third condition that the total of non-lighting subfields is less than or equal to N among subfields driven previously over the last turn-on subfield of the respective first and second groups of subfields for the corresponding grayscale. The determined output grayscales of the still image block and the moving image block are displayed on the plasma display device. 
   The numbers L, M and N may be respectively given as 2, 1, and 2. 
   In a yet further embodiment, a plasma display device includes a plasma display panel including a plurality of first and second electrodes, and a plurality of third electrodes crossing the first and the second electrodes. 
   The plasma display device includes a controller for controlling output grayscales corresponding to original grayscales by detecting a moving image block and still image block from input video signals. The controller further determines output grayscales corresponding to original grayscales of the detected still image block such that respective output grayscales corresponding to the original grayscales of the detected still image block satisfy a first condition that the number of consecutive non-lighting subfields is less than or equal to L among subfields driven previously to a last turn-on subfield of the corresponding grayscales. The controller further determines output grayscales corresponding to original grayscales of the detected moving image block such that respective output grayscales corresponding to original grayscales of the detected moving image block satisfy a second condition that the number of continuous non-lighting subfields is less than or equal to M and a third condition that a total number of non-lighting subfields is less than or equal to N among subfields driven previously over the last turn-on subfield of the corresponding grayscale. A plasma display panel driver is also included for driving the first electrodes, second electrodes, and third electrodes in response to control signals generated by the controller. 
   The input video signal may be a NTSC video signal. 
   In a yet further embodiment, a plasma display device includes a plasma display panel including a plurality of first and second electrodes, and a plurality of third electrodes crossing the first and the second electrodes. 
   The plasma display device further includes a controller for controlling output grayscales corresponding to original grayscales by dividing a plurality of subfields having respective weight values into a first group of subfields and a second group of subfields, determining output grayscales corresponding to original grayscales of the detected still image block such that respective output grayscales corresponding to the original grayscales of the detected still image block satisfy a first condition that the respective number of consecutive non-lighting subfields is less than or equal to L among subfields driven previously over the last turn-on subfield of the respective first and second group of subfields for the corresponding grayscales. The controller further determines output grayscales corresponding to original grayscales of the detected moving image block such that output grayscales corresponding to original grayscales of the detected moving image block satisfy a second condition that the respective number of continuous non-lighting subfields is less than or equal to M and a third condition that a total number of non-lighting subfields is less than or equal to N among subfields driven previously over the last turn-on subfield of the respective first and second group of subfields for the corresponding grayscale. A plasma display panel driver is further included for driving the first electrodes, second electrodes and the third electrodes in response to control signals generated by the controller. 
   The input video signal may be PAL video signal. 
   The controller may determine the output grayscale value such that the first coming subfield at the first group of subfields is turned on, and such that a sum of the weight values of turn-on subfields has a difference between the first group of subfields and the second group of subfields and the difference is less than a predetermined value. 
   In a yet further embodiment, a plasma display device includes a plasma display panel having a plurality of discharge cells for representing grayscales corresponding to the sum of weight values of the turn on subfields at a plurality of subfield having respective weight values. A controller is also included for detecting a moving image block and a still image block from input video signals. In case of inputting NTSC video signals, the controller controls the plurality of subfields driven successively and controls output grayscales corresponding to original grayscales by determining output grayscales corresponding to original grayscales of the detected still image block such that respective output grayscales corresponding to the original grayscales of the detected still image block satisfy a first condition that the number of consecutive non-lighting subfields is less than or equal to L among subfields driven previously to a last turn-on subfield of the corresponding grayscales. The controller further determines output grayscales corresponding to original grayscales of the detected moving image block such that respective output grayscales corresponding to original grayscales of the detected moving image block satisfy a second condition that the number of continuous non-lighting subfields is less than or equal to M and a third condition that a total number of non-lighting subfields is less than or equal to N among subfields driven previously to a last turn-on subfield of the corresponding grayscale. In the case of inputting a PAL video signal, the controller controls output grayscales corresponding to original grayscales by dividing a plurality of subfields having respective weight values into a first and a second group of subfields, determining output grayscales corresponding to original grayscales of the detected still image block such that output grayscales corresponding to the original grayscales of the detected still image block satisfy a first condition that the respective number of consecutive non-lighting subfields is less than or equal to I among subfields driven previously to a last turn-on subfield of the respective first and second groups of subfields for the corresponding grayscales. The controller further determines output grayscales corresponding to original grayscales of the detected moving image block such that output grayscales corresponding to original grayscales of the detected moving image block satisfy a second condition that the respective number of continuous non-lighting subfields is less than or equal to J and a third condition that a total number of non-lighting subfields is less than or equal to K among subfields driven previously to a last turn-on subfield of the respective first and second groups of subfields for the corresponding grayscale. A plasma display panel driver is further included for driving the first electrodes, second electrodes, and third electrodes in response to control signals generated by the controller. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating a method for expressing a grayscale of a plasma display panel. 
       FIG. 2  is a schematic plan view of a PDP according to an exemplary embodiment of the present invention. 
       FIG. 3  is a schematic block diagram of a controller according to the embodiment of  FIG. 2 . 
       FIG. 4A  illustrates a part of an exemplary table used for grayscale conversion, when an input video signal is a still image. 
       FIG. 4B  illustrates a part of an exemplary table used for grayscale conversion, when an input video signal is a moving image. 
       FIG. 5A  illustrates an example of a 2×2 dithering matrix. 
       FIG. 5B  illustrates an example of an 8×8 dithering matrix. 
       FIG. 6  is a schematic block diagram of a controller according to another exemplary embodiment of the present invention. 
       FIG. 7A  illustrates a part of an exemplary table used for grayscale conversion, when an input video signal is a still image in a PAL format. 
       FIG. 7B  illustrates a part of an exemplary table used for grayscale conversion, when an input video signal is a moving image in a PAL format. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. 
   Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
   A plasma display device and an image processing method thereof according to an exemplary embodiment of the present invention will hereinafter be described in detail with reference to the accompanying drawings. 
   As shown in  FIG. 2 , a plasma display device includes a PDP  100 , a controller  200 , an address driver  300 , a scan electrode driver (hereinafter called a Y electrode driver)  400 , and a sustain electrode driver (hereinafter called an X electrode driver)  500 . 
   The PDP  100  includes a plurality of address electrodes A 1  to Am arranged as columns, and a plurality of scan electrodes Y 1  to Yn and a plurality of sustain electrodes X 1  to Xn alternately arranged as rows. The X electrodes X 1  to Xn are respectively formed corresponding to the Y electrodes Y 1  to Yn. The PDP includes a substrate (not shown) formed with the sustain and scan electrodes X 1 -Xn and Y 1 -Yn and another substrate (not shown) formed with the address electrodes A 1 -Am. The two substrates are facing each other such that the sustain and scan electrodes X 1 -Xn and Y 1 -Yn may perpendicularly cross the address electrodes A 1 -Am. Discharge cells are formed by discharge spaces formed at crossing regions where the address electrodes meet the scan and sustain electrodes. It is should be understood that such a structure of the PDP  100  is only an example, and the present invention is not limited thereto, since the spirit of the present invention may be applied to various other structures of a PDP. 
   The address driver  300  receives address driving control signals from the controller  200 , and applies display data signals for selecting desired discharge cells to the respective address electrodes A 1  to Am. The X electrode driver  400  receives X electrode driving control signals from the controller  200 , and applies driving voltages to the X electrodes X 1  to Xn. The Y electrode driver  500  receives Y electrode driving control signals from the controller  200 , and applies driving voltages to the Y electrodes Y 1  to Yn. 
   The controller  200  externally receives video signals, and outputs the address driving control signals, the X electrode driving control signals, and the Y electrode driving control signals. Also, the controller  200  drives the panel  100  by a plurality of subfields divided from a frame, wherein each subfield includes a reset period, an address period, and a sustain period in a temporal order. According to an exemplary embodiment of the present invention, the controller  200  converts grayscales of input video signals (i.e., R, G, B data) before outputting them, in order to solve low discharge and false contour problems. Also, the controller  200  applies a dithering algorithm for the converted grayscales so as to compensate the original grayscales. 
   A controller of a plasma display device according to an exemplary embodiment of the present invention will hereinafter be described in detail with reference to  FIG. 3 ,  FIG. 4A , and  FIG. 4B . In this embodiment, the controller is designed to solve a low discharge problem and a false contour problem at a subfield arrangement applied to an NTSC format. 
   As shown in  FIG. 3 , the controller  200  of a plasma display device includes a motion detector  220 , a still image grayscale converter  240 , a moving image grayscale converter  260 , and a dithering processor  280 . 
   The motion detector  220  divides whole pixels used for displaying one frame of a video signal into predetermined blocks, that is moving image blocks that display a moving image and still image blocks that do not. Because most of false contours are generated at moving images, the grayscale conversion for reducing a false contour may be performed at the moving image blocks, while the grayscale conversion for improving a low discharge at low grayscales is performed at the still image blocks. Whether a respective block displays a moving image can be determined by a sum of the difference of grayscales between the previous frame and current frame for respective pixels. The following equation 1 shows a method for calculating such a difference in grayscales. 
   Equation 1
 
diff_criterion( x,y )=| i   n ( x,y )− i   n-1 ( x,y )|
 
   In Equation 1, i n (x,y) designates a grayscale at the (x,y) position of the present frame image data, and i n-1 (x,y) designates a grayscale at the (x,y) position of the previous frame. In this case, the “block-wise” difference of grayscales is acquired by adding up the difference of grayscales calculated in the equation 1 for respective pixels in a block. When the block-wise difference of grayscales is greater than or equal to a predetermined value, the corresponding block is determined as a moving image block. When the block-wise difference of grayscales is less than the predetermined value, the corresponding block is determined as a still image block. The predetermined value may be obtained to be an appropriate value based on empirical data. The method for obtaining an appropriate value of the predetermined value will be readily understood to a person of ordinary skill in the art, and is not described in further detail. 
   The motion detector  220  includes a frame memory (not shown) for storing data from a previous frame, and is used to detect moving image signals through methods such as Equation 1. The blocks divided from the whole pixels for representing data of the one frame may be preset to a predetermined size, for example, to a size corresponding to one pixel or a whole screen. 
   In this manner, the motion detector  220  detects whether blocks display moving images or still images, and sends the detected information to the still image grayscale converter  240  and the moving image grayscale converter  260 . 
   In order to improve a low discharge at low grayscales of still image blocks, one embodiment of the still image grayscale converter  240  converts still image grayscales using the table shown in  FIG. 4A  and outputs the converted grayscales. As shown in  FIG. 4A , the image grayscale converter  240  outputs, for grayscales (e.g., grayscales  2 ,  4 ,  6  . . . ) that may suffer from a low discharge, output grayscale candidates. The output grayscale candidates are adjacent grayscales. These adjacent grayscales are output to avoid a low discharge at a low grayscale. For example, grayscale  1  and  3  are output instead of grayscale  2 , grayscale  3  and  5  instead of grayscale  4 , etc. For grayscales which are not expected to suffer from a low grayscale (e.g., grayscales  1 ,  3 ,  5  . . . ), the grayscale converter  240  directly outputs input grayscales. A method for acquiring such a table as  FIG. 4A  will be hereinafter described in detail. 
     FIG. 4A  is a predetermined table satisfying conditions for improving a low discharge at low grayscales in the case where weights of respective subfields are arranged as followed: {1(sf 1 ), 2(sf 2 ), 4(sf 3 ), 8(sf 4 ), 16(sf 5 ), 32(sf 6 ), 42(sf 7 ), 44(sf 8 ), 52(sf 9 ), 54(sf 10 )}. The low discharge at low grayscales is generated at predetermined subfields in which light is not emitted because sufficient priming particles are not present. 
   Accordingly, the predetermined grayscales satisfying the following conditions are used: a condition (hereinafter, referred to as ‘condition  1 ’) that a first-coming subfield sf 1  is a turn-on subfield; and a condition (hereinafter, referred to as ‘condition  2 ’) that the number of the consecutive non-lighting subfields is less than or equal to L (in this case L=2). A “non-lighting subfield” is defined as a turn-off subfield driven previously to the last turn-on subfield to express the corresponding grayscale among a group of successively driving subfields (i.e., among the whole subfield in the case of NTSC video signal input, and among a later described group of sub-frames in the case of PAL video signal input). When at least one of condition  1  and condition  2  is not satisfied, adjacent higher and lower grayscales satisfying both condition  1  and condition  2  are selected as output grayscale candidates. When the first-coming subfield sf 1  (hereinafter simply called a first subfield) is turned on, reset and sustain discharges caused thereby generate a large amount of priming particles. Thus, the priming particles may remain even when a non-lighting subfield follows.  FIG. 4A  is a predetermined table satisfying these conditions  1  and  2 . 
   For example, when an input grayscale is a grayscale  2 , the first subfield sf 1  is not turned on and the condition  1  is not satisfied. Accordingly, grayscale  1  and grayscale  3  satisfying the conditions  1  and  2  are selected as output grayscale candidates, wherein the grayscale  1  and grayscale  3  are adjacent lower and higher grayscales of the grayscale  2 . Also, when an input grayscale is a grayscale  8 , the first subfield sf 1  through the third subfield sf 3  are not turned on and the condition  2  is not satisfied. Accordingly, grayscale  7  and grayscale  9  satisfying the conditions  1  and  2  are selected as the output grayscale candidates, wherein the grayscale  7  and grayscale  9  are the adjacent lower and higher grayscales of the grayscale  8 . When an input grayscale is a grayscale  3 , both of the conditions  1  and  2  are satisfied. Accordingly, a grayscale  3  is adopted as an output grayscale candidate. 
   The subfield weight arrangement {1(sf 1 ), 2(sf 2 ), 4(sf 3 ), 8(sf 4 ), 16(sf 5 ), 32(sf 6 ), 42(sf 7 ), 44(sf 8 ), 52(sf 9 ), 54(sf 10 )} shown in  FIG. 4A  is one example. If both of the conditions  1  and  2  are satisfied, the arrangement can be obviously varied by a person skilled in the art. 
   In order to improve a false contour problem of moving image blocks, one embodiment of the moving image grayscale converter  260  converts the grayscales of the corresponding blocks using such a table as shown in  FIG. 4B , and outputs the converted grayscales. As shown in  FIG. 4B , the moving image grayscale converter  260  outputs, for predetermined grayscale values (i.e., grayscale  2 ,  4 ,  6  . . . ) that may suffer from a false contour, output grayscale candidate values. Output grayscale candidates are adjacent grayscales to avoid a false contour, i.e., grayscale  1  and  3  instead of grayscale  2 , grayscale  3  and  5  instead of grayscale  4 . For predetermined grayscale values (i.e., grayscale  1 ,  3 ,  5  . . . ) that are not expected to suffer from a false contour, the grayscale converter  240  directly outputs input grayscales. A method for acquiring such a table as  FIG. 4B  will hereinafter be described in detail. 
     FIG. 4B  is a predetermined table satisfying conditions for improving false contour in the case that weights of respective subfields are as follows: {1 (sf 1 ), 2(sf 2 ), 4(sf 3 ), 8(sf 4 ), 16(sf 5 ), 32(sf 6 ), 42(sf 7 ), 44(sf 8 ), 52(sf 9 ), 54(sf 10 )}. A false contour is generated at moving images and dissimilar subfield lighting patterns. Accordingly, to avoid the false contour, the subfield lighting pattern should be set to be similar between adjacent grayscales. Accordingly, the predetermined grayscales satisfying the following conditions are used: a condition (hereinafter, referred to as ‘condition  3 ’) that a first-coming subfield sf 1  is a turn-on subfield, a condition (hereinafter, referred to as ‘condition  4 ’) that the number of consecutive non-lighting subfields is less than or equal to M (in this case M=1); condition (hereinafter, referred to as ‘condition  5 ’) that the total number of the non-lighting subfields is less than or equal to N (in this case N=2). When at least one of condition  3  through condition  5  is not satisfied, adjacent higher and lower grayscales, satisfying condition  3  through condition  5  are selected as output grayscale candidates.  FIG. 4B  is a predetermined table satisfying these conditions  3  through  5 . 
   For example, when an input grayscale is a grayscale  2 , the first subfield sf 1  is not turned on and the condition  3  is not satisfied. Accordingly, grayscale  1  and grayscale  3  satisfying the conditions  3  through  5  are selected as output grayscale candidates, wherein the grayscale  1  and grayscale  3  are respectively adjacent higher and lower grayscales of the grayscale  2 . When an input grayscale is a grayscale  4 , the conditions  3  and  4  are not satisfied. Accordingly, adjacent higher and lower grayscales  3  and  5  are selected as output grayscale candidates. When an input grayscale is a grayscale  9 , second subfield sf 2  and third subfield sf 3  are not turned on so that there are consecutive two non-lighting subfields; as a result, the condition  4  is not satisfied. Accordingly, adjacent higher and lower grayscales  7  and  11  are selected as output grayscale candidates, 
   The subfield weight arrangement {1(sf 1 ), 2(sf 2 ), 4(sf 3 ), 8(sf 4 ), 16(sf 5 ), 32(sf 6 ), 42(sf 7 ), 44(sf 8 ), 52(sf 9 ), 54(sf 10 )} shown in  FIG. 4B  is one example. If the conditions  3  through  5  are satisfied, the arrangement can be varied as desired by a person skilled in the art. 
   The data processed in this manner by the still image grayscale converter  240  and the moving image grayscale converter  260  are sent to the dithering processor  280 . 
   When two output grayscale candidates are produced according to the table shown in  FIG. 4A  or  FIG. 4B , the two grayscale candidates have grayscale differences from an actual input grayscale. The grayscale differences may be used to display the desired input grayscale in an averaged manner by spatially mixing the two determined output grayscale candidates in a predetermined ratio. Operation of the dithering processor  280  for expressing the input grayscale in such an averaged manner will be described hereafter. 
   For the grayscales having the two output grayscale candidates for one input grayscale, the dithering processor  280  applies a dithering process in order to compensate the grayscale difference. In other words, the dithering processor  280  is used to select an appropriate candidate from among the determined output candidates and represent a grayscale close to the desired grayscale within a predetermined area. 
   When the output grayscale candidates are 3 and 5 corresponding to the input grayscale  4 , for example, the two grayscales  3  and  5  in a 2×2 display area are respectively determined to be output, the mean value in the 2×2 area becomes 4 and it is hence possible to represent the input grayscale  4 . In this instance, an output value of each pixel in the 2×2 area is determined from among the output grayscale candidates according to a threshold value of the pixel. That is, when the grayscale  4  is smaller than a pixel&#39;s threshold value, the grayscale  3  is output and when the grayscale  4  is larger than a pixel&#39;s threshold value, the grayscale  5  is output. The following equation 2 shows a method for expressing such dithering calculation. 
   Equation 2 
   IF(i(x,y)&lt;Threshold(x,y)) 
   {
         result(x,y)=level min ;       

   } 
   ELSE 
   {
         result(x,y)=level max ;       

   } 
   In Equation 2, i(x,y) is a current grayscale, Threshold(x,y) is a threshold value, and result(x,y) is a grayscale finally output by the plasma display device, and, level min  and level max  respectively represent a lower grayscale and a higher grayscale from among the found output candidates. The lower grayscale level min  is output as output grayscale result(x,y) when the current input grayscale i(x,y) is smaller than threshold(x,y), and the higher grayscale level max  is output as output grayscale result(x,y) when the current input grayscale i(x,y) is larger than threshold(x,y). 
   In this case, a threshold(x,y) value of each pixel is determined depending on the given dithering matrix and two output candidates.  FIG. 5A  shows an example of a 2×2 dithering matrix and  FIG. 5B  shows an example of 8×8 dithering matrix. For example, in the case of considering a 2×2 area, the value between the two output candidates is divided with gaps of the same size and the gaps are filled with threshold values in the four positions of the 2×2 area. The process for determining the threshold values may be expressed as the following Equation 3. 
   
     
       
         
           
             
               
                 
                   Threshold 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     
                       x 
                       , 
                       y 
                     
                     ) 
                   
                 
                 = 
                 
                   
                     level 
                     min 
                   
                   + 
                   
                     
                       
                         
                           level 
                           max 
                         
                         - 
                         
                           level 
                           min 
                         
                       
                       
                         Dither_Size 
                         + 
                         1 
                       
                     
                     × 
                     
                       
                         Dither 
                         ⁡ 
                         
                           [ 
                           
                             y 
                             ⁢ 
                             % 
                             ⁢ 
                             D_h 
                           
                           ] 
                         
                       
                       ⁡ 
                       
                         [ 
                         
                           x 
                           ⁢ 
                           % 
                           ⁢ 
                           D_w 
                         
                         ] 
                       
                     
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 3 
               
             
           
         
       
     
   
   In Equation 3, Dither_Size represents a maximum size of the dithering matrix, and Dither_Size has a value of 4 in such a dithering matrix as shown in  FIG. 5A  and a value of 64 in such a dithering matrix as shown in  FIG. 5B . Dither□□ is a dithering matrix which is used for determining arrangement positions of the determined threshold values. D_w and D_h are dimensions of a width and a height of the dithering matrix respectively, and % is an operator for calculating a remainder and is used to apply a predetermined dimension of the dithering matrix to the whole image corresponding to one frame without superposition. Therefore, the threshold values of the respective pixels are calculated throughout the whole frame of image according to Equation 3. 
   According to one embodiment of the present invention, the controller  200  generates a final grayscale signal using the dithering processor  280 , and sends it to the PDP drivers that is the address driver  300  and the scan and sustain drivers  400  and  500 . 
   A controller of plasma display device according to another embodiment of the present invention will hereinafter be described in detail with reference to  FIG. 6 ,  FIG. 7A , and  FIG. 7B . In this embodiment, the controller is designed to solve a low discharge problem and a false contour problem at a subfield arrangement applied to a PAL format. The PAL format divides subfield weight values used at one frame in two sub-frames (a group of subfields) to reduce flicker. 
   As shown in  FIG. 6 , a controller  200 ′ includes a motion detector  220 ′, a still image grayscale converter  240 ′, a moving image grayscale converter  260 ′ and a dithering processor  280 ′. The still image grayscale converter  240 ′ and the moving image grayscale converter  260 ′ are operated according to different grayscale conversion conditions than the previously described embodiments. The motion detector  220 ′ and dithering processor  280 ′ are operated as described above. 
   The input grayscale is reset to satisfy a condition (hereinafter, referred to as ‘condition  6 ’) to turn on the first subfield sf 1  and a condition (hereinafter, referred to as ‘condition  7 ’) that the number of the adjacent non-lighting subfields is less than or equal to I (in this case I=2) for the respective sub-frames. When both of condition  6  and condition  7  are satisfied, the input grayscale is directly output by the still image grayscale converter  240 ′, and when at least one of condition  6  and condition  7  are not satisfied, adjacent higher and lower grayscales, satisfying both of condition  6  and condition  7  are output. As above noted, because subfield weight values used at one frame are divided in two sub-frames in the PAL format, for the respective sub-frames, the condition  7  is requested, i.e., the number of the adjacent non-lighting subfields is less than 2. In the PAL format, in order to reduce flicker occurring when a sum of weights of the light emitting subfields is much different between the two sub-frames, a condition (hereinafter, referred to as ‘condition  8 ’) that the difference of these sums is less than the predetermined value (i.e., 20) may be added. For example, as shown in  FIG. 7A , when an input grayscale is a grayscale  11 , at the 1 sub-frame, the sum of the weight values of turn-on subfields is given as 7. At the 2 sub-frame, the sum of the weight values of turn-on subfields is given as 4 so that the difference of these sums is given as 3. The predetermined values can be acquired experimentally and the value  20  can be varied as desired by a person skilled in the art. 
     FIG. 7A  is a predetermined table satisfying conditions  6  and  7  for improving a low discharge at low grayscales in the case where weights of respective subfields are arranged as follows: 1 sub-frame={1(sf 1 ), 2(sf 2 ), 4(sf 3 ), 8(sf 4 ), 16(sf 5 ), 32(sf 6 ), 68(sf 7 ), 116(sf 8 )}, 2 sub-frame={4(sf 1 ′), 12(sf 2 ′), 24(sf 3 ′), 40(sf 4 ′), 68(sf 5 ′), 116(sf 6 ′)}. Referring to  FIG. 7A , when an input grayscale is given as  12 , a first subfield sf 1  is turned off in the 1 sub-frame and three non-lighting subfields are arranged consecutively. Therefore the grayscale  12  does not satisfy conditions  6  and  7 . Accordingly, adjacent higher and lower grayscales  11  and  13  satisfying the conditions  6  through  8  are selected as output grayscale candidates. 
   The subfield weight arrangement shown in  FIG. 7A  is one example. If the conditions  6  and  7  are satisfied, the arrangement can be varied as desired by a person skilled in the art. 
   Next, in order to reduce a false contour of a moving image block, the moving image grayscale converter  260 ′ converts the output grayscales to satisfy several conditions. An input grayscale is reset to satisfy the following conditions: a condition (hereinafter, referred to as ‘condition  9 ’) to turn on a first subfield sf 1 , a condition (hereinafter, referred to as ‘condition  10 ’) that the number of the consecutive non-lighting subfields is less than or equal to J (in this case J=1) for the respective sub-frames and a condition (hereinafter, referred to as ‘condition  11 ’) that the total number of the non-lighting subfields is less than or equal to K (in this case K=2) for the respective sub-frame. For example, when all of the conditions  9  through  11  are satisfied, the input grayscale is directly output. When at least one of conditions  9  through  11  is not satisfied, adjacent higher and lower grayscales, satisfying condition  9  through condition  11 , are output. Because the PAL format divides subfield weights used at one frame into two sub-frames, the number of the non-lighting subfields or the like are determined for the respective sub-frames as conditions  10  and  11 . In order to reduce flicker occurring when the sum of weight values of the light emitting subfields is much different between the two sub-frames, a condition (hereinafter, referred to as ‘condition  12 ’) that the difference of these sums is less than the predetermined value (i.e., 20) may be added. The predetermined values may be obtained to be an appropriate value based on empirical data and the value  20  can be varied as desired by a person skilled in the art. 
     FIG. 7B  is a predetermined table satisfying conditions  9  through  11  for reducing a false contour in the case that weights of respective subfields are arranged as followed: 1 sub-frame={1(sf 1 ), 2(sf 2 ), 4(sf 3 ), 8(sf 4 ), 16(sf 5 ), 32(sf 6 ), 68(sf 7 ), 116(sf 8 )}, 2 sub-frame={4(sf 1 ′), 12(sf 2 ′), 24(sf 3 ′), 40(sf 4 ′), 68(sf 5 ′), 116(sf 6 ′)}. Referring to  FIG. 7B , when an input grayscale is given as  12 , the first subfield (sf 1 ) is turned off at the 1 sub-frame; three non-lighting subfields are arranged consecutively, and the grayscale  12  does not satisfy conditions  9  through  11 . Accordingly adjacent lower and higher grayscales  11  and  13  satisfying the conditions  9  through  11  are selected as output grayscale candidates. 
   The subfield weight arrangement shown in  FIG. 7B  is one example. If the conditions  9  to  11  are satisfied, the arrangement can be varied as desired by a person skilled in the art. 
   The data processed in this manner by the still image grayscale converter  240 ′ and the moving image grayscale converter  260 ′ are sent to the dithering processor  280 ′ and are applied to the dithering algorithm in the same manner as described above. 
   When the common conditions  1 ,  3 ,  6  and  9 , relating to the first subfield sf 1  being turned on, are not used to improve a low discharge of grayscales and a false contour, these conditions may be omitted. The number of consecutive non-lighting subfields and the total number of the non-lighting subfields noted in conditions  2 ,  4 ,  5 ,  7 ,  8 ,  10 ,  11  and  12  are examples of conditions, and these numbers or the like can be varied experimentally as desired to improve a low discharge problem of grayscales and a false contour problem by a person skilled in the art. 
   While this invention has been described in connection with various embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims and their equivalents. 
   As above described, the input video signal is determined to be a moving image or a still image. When a still image is detected, the detected still image is converted into grayscales for avoiding a low discharge of grayscale. When a moving image is detected, the moving image is converted into grayscales for reducing a false contour, thereby reducing both false contour and avoiding low scale discharge.