Patent Publication Number: US-2005140587-A1

Title: Method and apparatus for decreasing an afterimage of a plasma display panel

Description:
This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 10-2003-0073312 filed in Korea on Sep. 21, 2003, the entire contents of which are hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a plasma display panel, and more particularly to a method and apparatus for decreasing an afterimage of a plasma display panel.  
      2. Description of the Background Art  
      A plasma display panel (hereinafter, referred to as “PDP”) displays an image by using a visible ray generated from a phosphor material when ultraviolet rays produced by a gas discharge excite the phosphor material. Such a PDP has many advantages over a cathode ray tube (CRT) which has been widely used so far in that it is thinner in thickness, higher in definition, and larger in a screen size.  
       FIG. 1  is a plan view schematically illustrating a conventional plasma display panel.  
      Referring to  FIG. 1 , a tri-electrode AC (Alternative Current) surface discharge type PDP includes scan electrodes Y 1  to Yn and sustain electrodes Z formed on an upper substrate  10 , and address electrodes X 1  to Xm formed on a lower substrate  18 . Discharge cells  1  of the PDP are formed at a part where the scan electrodes Y 1  to Yn, the sustain electrodes Z and the address electrodes X 1  to Xm are crossed with one another.  
      In the above-described PDP, a gray scale of an image is achieved on a time-division basis by dividing one frame into many subframes each having the different number of light emissions. Each subfield is further divided into a reset period for uniformly creating a discharge, an address period for selecting a discharge cell, and a sustain period for achieving a gray scale according to the number of light emissions. For example, if it is desired to display an image by 256 gray scales, a frame period (36.67 ms) corresponding to {fraction (1/60)} seconds is divided into 8 subfields. Each of the 8 subfields is further divided into a reset period, an address period and a sustain period. In this case, the reset period and the address period of each subfield are identical to each other, whereas the sustain period and the number of light emissions are proportional to the number of sustain pulses and increase at a rate of 2 n  (where n=0, 1, 2, 3, 4, 5, 6, and 7). Thus since the sustain period of each subfield becomes different, it is possible to obtain a desired gray scale of an image.  
       FIG. 2  is a block diagram illustrating a driving circuit of a conventional PDP.  
      Referring to  FIG. 2 , the PDP driving circuit includes a gain correcting part  22 , an error spreading part  23  and a subfield mapping part  24  connected between a first reverse gamma correcting part  21 A and a data arranging part  25 , and includes a average picture level (APL) calculating part  26  connected between a second reverse gamma correcting part  21 B and a waveform generating part  27 .  
      The first and second reverse gamma correcting parts  21 A and  21 B perform reverse gamma correction on red, green and blue (RGB) digital video data from an input line  20  to convert luminance corresponding to a gray scale of a video signal into a linear value.  
      The gain correcting part  22  compensates a color temperature by adjusting an effective gain according to respective data of RGB colors.  
      The error spreading part  23  minutely adjusts a luminance value by spreading to adjacent cells a quantization error of the RGB input digital video data received from the gain correcting part  22 .  
      The subfield mapping part  24  maps data received from the error spreading part  23  to a subfield pattern stored previously according to each bit and supplies the mapped data to the data arranging part  25 .  
      The data arranging part  25  supplies a data driving circuit of a PDP  28  with digital video data received from the subfield mapping part  24 . The data driving circuit connected to data electrodes of the PDP  28  latches data received from the data arranging part  25  on a horizontal line basis and supplies the latched data to the data electrodes of the PDP  28  in the unit of one horizontal period.  
      The APL calculating part  26  calculates an APL of RGB digital video data received from the second reverse gamma correcting part  21 B in the unit of one screen, and generates information about the number of sustain pulses corresponding to the calculated APL. The information about the number of sustain pulses corresponding to the APL calculated in the APL calculating part  26  is stored in a lookup table of a ROM (Read Only Memory) contained in the APL calculating part  26 . An APL-Nsus (the number of sustain pulses) curve shows an exponential function shape in which the number of sustain pulses decreases nonlinearly as the APL increases.  
      The waveform generating part  27  generates a timing control signal in response to the information about the number of sustain pulses from the APL calculating part  26  and supplies the timing control signal to a scan driving circuit and a sustain driving circuit which are not shown. The scan driving circuit and the sustain driving circuit supply sustain pulses to the scan electrodes and sustain electrodes of the PDP  28  during a sustain period in response to the timing control signal received from the waveform generating part  27 .  
      Although the PDP can reduce consumed power by controlling the APL, since the APL-Nsus curve is nonlinear, an afterimage becomes severer. For instance, as shown in  FIG. 4 , the PDP driving circuit partially displays a bright part on the PDP and then displays the bright image on the whole screen with the same brightness. However, even though the bright image is displayed on the whole screen with the same brightness, the bright part which has been discharged becomes brighter and an afterimage remains. This is because the number of sustain pulses decreases abruptly when the partially bright image (that is, an image with a low APL) changes to the entirely bright image (that is, an image with a high APL). Therefore, priming charge particles exist within a cell at a previously discharged part, and due to the priming charge particles, the previously discharged part appears to be brighter than a part which has been not discharged. In other words, the afterimage phenomenon is caused by an abrupt increase or decrease ΔNsus in the number of sustain pulses according to the ALP by the APL-Nsus curve of an exponential function shape.  
     SUMMARY OF THE INVENTION  
      Accordingly, an object of the present invention is to solve at least the problems and disadvantages of the background art.  
      An object of the present invention is to provide a method and an apparatus for decreasing an afterimage of a PDP by using a prescribed function period on an APL-Nsus curve.  
      According to an embodiment of the present invention, a method for decreasing an afterimage of a plasma display panel by using an average picture level, comprises the steps of: generating an average picture level-to-sustain pulse number curve having a trigonometric function period or a linear function period; and driving the plasma display panel by calculating an average picture level of an input image, and deriving the number of sustain pulses corresponding to the calculated average picture level from the average picture level-to-the number of sustain pulses curve.  
      According to an embodiment of the present invention, an apparatus for decreasing an afterimage of a plasma display panel by using an average picture level, comprises: an average picture level calculating part for calculating an average picture level of an input image, and deriving the number of sustain pulses corresponding to the calculated average picture level from an average picture level-to-sustain pulse number curve having a trigonometric function period or a linear function period; and a driving part for a driving the plasma display panel by using the number of sustain pulses derived by the average picture level calculating part.  
      The method and appratus for decreasing an afterimage of a PDP by using an APL according to the present invention can minimize the afterimage by using an APL-Nsus curve having a trigonometric function period or a linear function period and preventing an abrupt change in the number of sustain pulses when the APL varies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will be described in detail with reference to the following drawings in which like numerals refer to like elements.  
       FIG. 1  is a plan view schematically illustrating a conventional plasma display panel (PDP).  
       FIG. 2  is a block diagram illustrating a driving circuit of a conventional PDP.  
       FIG. 3  is a graph illustrating a conventional APL (average picture level)-Nsus (the number of sustain pulses) curve.  
       FIG. 4  is a diagram illustrating an example of generating an afterimage when a PDP is driven using the APL-Nsus curve shown in  FIG. 3 .  
       FIG. 5  is a block diagram illustrated to describe a method and apparatus for decreasing an afterimage of a PDP according to the present invention.  
       FIG. 6  is a graph illustrating an APL-Nsus curve according to a first embodiment of the present invention.  
       FIG. 7  is a graph illustrating an APL-Nsus curve according to a second embodiment of the present invention.  
       FIG. 8  is a graph illustrating an APL-Nsus curve according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      Preferred embodiments of the present invention will be described in a more detailed manner with reference to the drawings.  
      According to an embodiment of the present invention, a method for decreasing an afterimage of a plasma display panel by using an average picture level, comprises the steps of: generating an average picture level-to-sustain pulse number curve having a trigonometric function period or a linear function period; and driving the plasma display panel by calculating an average picture level of an input image, and deriving the number of sustain pulses corresponding to the calculated average picture level from the average picture level-to-the number of sustain pulses curve.  
      Further, the trigonometric function period or the linear function period exists between 5 and 95 percent of a peak average picture level on the average picture level-to-the sustain pulse number curve.  
      According to an embodiment of the present invention, an apparatus for decreasing an afterimage of a plasma display panel by using an average picture level, comprises: an average picture level calculating part for calculating an average picture level of an input image, and deriving the number of sustain pulses corresponding to the calculated average picture level from an average picture level-to-sustain pulse number curve having a trigonometric function period or a linear function period; and a driving part for a driving the plasma display panel by using the number of sustain pulses derived by the average picture level calculating part.  
      Further, the trigonometric function period or the linear function period exists between 5 and 95 percent of a peak average picture level on the average picture level-to-sustain pulse number curve.  
      Hereinafter, Preferred embodiments of the present invention will be described in a more detail with the drawings.  
       FIG. 5  is a block diagram illustrated to describe a method and apparatus for decreasing an afterimage of a PDP according to the present invention. Referring to  FIG. 5 , a PDP driving circuit according to the present invention includes a gain correcting part  32 , an error spreading part  33  and a subfield mapping part  34  connected between a first reverse gamma correcting part  31 A and a data arranging part  35 , and includes a trigonometric function/linear function APL calculating part  36  connected between a second reverse gamma correcting part  31 B and a waveform generating part  37 .  
      The first and second reverse gamma correcting parts  31 A and  31 B perform reverse gamma correction on RGB digital video data from an input line  30  to convert luminance corresponding to a gray scale of a video signal into a linear value.  
      The gain correcting part  32  compensates a color temperature by adjusting an effective gain according to respective data of RGB colors.  
      The error spreading part  33  minutely adjusts a luminance value by spreading to adjacent cells a quantization error of the RGB input digital video data received from the gain correcting part  32 .  
      The subfield mapping part  34  maps data received from the error spreading part  33  to a subfield pattern stored previously according to each bit and supplies the mapped data to the data arranging part  35 .  
      The data arranging part  35  supplies a data driving circuit of a PDP  38  with digital video data received from the subfield mapping part  34 . The data driving circuit connected to data electrodes of the PDP  38  latches data received from the data arranging part  35  on a horizontal line basis and supplies the latched data to the data electrodes of the PDP  38  in the unit of one horizontal period.  
      The trigonometric function/linear function APL calculating part  36  calculates an APL of data received from the second reverse gamma correcting part  31 B, and derives the number of sustain pulses, Nsus, corresponding to the calculated APL from an APL-Nsus curve having a trigonometric function period. The NPL-Nsus curve having the trigonometric function period includes a sine (Sin) or cosine (Cos) function as shown in  FIG. 6  or  7 . Further, the trigonometric function/linear function APL calculating part  36  calculates an APL of data received from the second reverse gamma correcting part  31 B, and derives the Nsus corresponding to the calculated APL from an APL-Nsus curve having a linear function period.  
      An example of the NPL-Nsus curve having the linear function period is shown in  FIG. 8 . The trigonometric function period or the linear function period exists between 5 and 95 percent of a peak APL, that is, a maximum average brightness on the APL-Nsus curve. The APL-Nsus curve is stored in a ROM (not shown) as lookup table data.  
      The waveform generating part  37  generates a timing control signal in response to information about the Nsus from the trigonometric function/linear function APL calculating part  36  and supplies the timing control signal to a scan driving circuit and a sustain driving circuit which are not shown. The scan driving circuit and the sustain driving circuit supply sustain pulses to the scan electrodes and sustain electrodes of the PDP  38  during a sustain period in response to the timing control signal received from the waveform generating part  37 .  
      As described above, the method and apparatus for decreasing an afterimage of a PDL according to the present invention drives the PDP by using the APL-Nsus curve having the trigonometric function period or the linear function period. Therefore, the method and apparatus can prevent the number of sustain pulses from being abruptly changed when the APL varies. As a result, even though a bright part is partially displayed on the PDP and then it is displayed on the whole screen with the same brightness, an afterimage remains hardly because the number of sustain pulses does not change abruptly.  
      The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.