Abstract:
The invention is related to a new kind of dithering method for plasma display panels. In a former Europena Patent Application a dithering method for the plasma display technology has been presented that utilizes a 3-dimensional dither pattern for the repeated use in a video sequence. A first dimension corresponds to a number of video frames, a second dimension corresponds to a number of video lines, and a third dimension corresponds to a number of cells or pixels in a video line. It has been found that this dither pattern has for some video levels the drawback of a generating a noticeable pattern in homogenous surfaces on the screen that reduces the picture quality. In order to overcome this drawback the invention proposes a new degree of freedom for the dither pattern. Different dither patterns are provided for different entries in a number of least significant bits of the data word representing the input video level. The invention makes it possible to suppress the disturbing patterns occurring on the plasma screen when using the conventional dither patterns.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method and apparatus for processing video picture data for display on a display device. More specifically the invention is closely related to a kind of video processing for improving the picture quality of pictures which are displayed on matrix displays like plasma display panels (PDP) or other display devices where the pixel values control the generation of a corresponding number of small lighting pulses on the display. 
     The Plasma technology now makes it possible to achieve flat colour panel of large size (out of the CRT limitations) and with very limited depth without any viewing angle constraints. 
     Referring to the last generation of European TV, a lot of work has been made to improve its picture quality. Consequently, a new technology like the Plasma one has to provide a picture quality as good or better than standard TV technology. On one hand, the Plasma technology gives the possibility of “unlimited” screen size, of attractive thickness . . . but on the other hand, it generates new kinds of artefacts which could degrade the picture quality. 
     Most of these artefacts are different as for CRT TV pictures and that makes them more visible since people are used to see the old TV artefacts unconsciously. 
     A Plasma Display Panel (PDP) utilizes a matrix array of discharge cells which could only be “ON” or “OFF”. Also unlike a CRT or LCD in which grey levels are expressed by analogue control of the light emission, a PDP controls the grey level by modulating the number of light pulses per frame (sustain pulses). This time-modulation will be integrated by the eye over a period corresponding to the eye time response. 
     Since the video amplitude determines the number of light pulses, occurring at a given frequency, more amplitude means more light pulses and thus more “ON” time. For this reason, this kind of modulation is also known as PWM, pulse width modulation. 
     This PWM is responsible for one of the PDP image quality problems: the poor grey scale portrayal quality, especially in the darker regions of the picture. This is due to the fact, that the displayed luminance is linear to the number of pulses, but the eye response and its sensitivity to noise is not linear. In darker areas the eye is more sensitive than in brighter areas. This means that even though modern PDPs can display e.g. 255 discrete video levels for each colour component R,G,B, the quantisation error will be quite noticeable in the darker areas. Further on, the required degamma operation in PDP displays, increases quantisation noise in video dark areas, resulting in a perceptible lack of resolution. 
     There are known some solutions which use a dithering method for reducing the perceptibility of quantisation noise in PDP&#39;S. 
     From a former European Patent Application of the applicant having the number 00250099.9 it is known to use a 3-dimensional static dither patterns for the cells of a plasma display panel to improve the grey scale rendition. The three dimensions corresponding to a number of frames, a number of lines and a number of columns on the PDP. With this pattern for some video levels some checked patterns could be seen in homogenous surfaces that reduce the picture quality. 
     From EP-A-0 994 457 it is known to store a set of dither patterns in memory and to select the right dither pattern in dependence on time data (field data) and positional data indicating the locations of the pixels on the display panel. In another embodiment different dither patterns are used independence on what video level an input data word represents. The different video level ranges are e.g. defined to be 0 . . . 15, 16 . . . 31, 32 . . . 47, and 48 . . . 63. 
     From U.S. Pat. No. 6,069,609 it is known to store in memory a number of different dither patterns called ‘dither types’. These dither patterns are selected with the 5 higher bits of an 8 bit input data word and the three lower bits determine which of the eight dither values from the dither type needs to be taken for dithering. 
     SUMMARY OF THE INVENTION 
     To overcome the drawback of reduced picture quality when using a static 3-dimensional dither pattern, the present invention, reports a dithering technique that makes use of different dither patterns for different entries in a number of least significant bits of the data word representing the input video level. In case that the dither patterns itself are also 3-dimensional, the invention utilizes quasi a 4-dimensional dither pattern with the fourth dimension being the entries in a number of least significant bits of an input video level data word. The invention makes it possible to suppress the disturbing patterns occurring on the plasma screen when using the 3-dimensional dither pattern. 
     Further advantageous embodiments are apparent from the dependent claims. 
     The input data words need to be transformed in a form where they have more bits than necessary for the final bit resolution that is required in the subsequent sub-field coding process. The bit resolution needs to be increased corresponding to the bit resolution for the dithering process. 
     This transformation can advantageously be done in a degamma calculation step in which the input video levels are amplified in order to compensate for the gamma correction in the video source. 
     In cases where the set of admissible input video levels is restricted, for the purpose of optimising the sub-field coding process by taking only those video levels to which sub-field code words are assigned that are less sensitive to the dynamic false contouring, the transformation can also be done in a resealing step where the data words for the reduced set of video levels are translated into data words having less bits. 
     For an apparatus for processing video pictures according to the invention it is advantageous when the dither unit comprises a table of different dither patterns for different entries in a number of least significant bits of the data word representing the input video level and a selector that assigns to a given video level one of the dither patterns stored in the table. This is a simple implementation of the invention. 
     Further advantageous is if for each colour component R, G, B of the input video signal a dedicated table of dither patterns is provided in the dither unit. This allows to use a single clock signal, e.g. pixel clock for dither number retrieval in the dithering process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. 
     FIG. 1 shows an illustration for the plasma cell activation with small pulses in sub-fields; 
     FIG. 2 shows an illustration for pixel-based and cell-based dithering; 
     FIG. 3 shows a 3-dimensional cell-based static dither pattern; 
     FIG. 4 illustrates the effect that patterns occur on a screen when each colour component has a fixed low video level and the dithering technique with use of a static dither pattern is used; 
     FIG. 5 shows the result of temporal integration of the patterns shown in FIG. 4; 
     FIG. 6 shows the result of temporal integration of the patterns shown in FIG. 3 for different input video levels for one colour component; 
     FIG. 7 shows the different 3-dimensional cell-based dither patterns according to the invention for different input video levels; 
     FIG. 8 shows the 3-dimensional cell-based dither patterns according to the invention in modified form for different input video levels but only for one frame; 
     FIG. 9 illustrates the resulting patterns that occur on a screen when the dithering technique with use of the dither patterns as shown in FIG. 7 or  8  is used; 
     FIG. 10 shows a block diagram of a circuit implementation of the invention in a PDP. 
     FIG. 11 shows a block diagram of a dithering unit, and 
     FIG. 12 shows a block diagram of a dithering evaluation unit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the general concept of light generation in plasma display panels is illustrated. As mentioned before, a plasma cell can only be switched on or off. Therefore, the light generation is being done in small pulses where a plasma cell is switched on. The different colours are produced by modulating the number of small pulses per frame period. To do this a frame period is subdivided in so called sub-fields SF. Each sub-field SF has assigned a specific weight which determines how many light pulse are produced in this sub-field SF. Light generation is controlled by sub-field code words. A sub-field code word is a binary number which controls sub-field activation and inactivation. Each bit being set to 1 activates the corresponding sub-field SF. Each bit being set to 0 inactivates the corresponding sub-field SF. In an activated sub-field SF the assigned number of light pulses will be generated. In an inactivated sub-field there will be no light generation. A typical sub-field organisation with 12 sub-fields SF is shown in FIG.  1 . The sub-field weights are listed at the top of the figure. 
     The frame period is illustrated slightly longer than all the sub-field periods together. This has the reason that for non-standard video sources the video line may be subject of jittering and to make sure that all sub-fields SF fit into the jittering video line, the total amount of time for all sub-fields SF is slightly shorter than a standard video line. 
     For clarification, a definition of the term sub-field is given here: A sub-field is a period of time in which successively the following is being done with a cell: 
     1. There is a writing/addressing period in which the cell is either brought to an excited state with a high voltage or with lower voltage to a neutral state. 
     2. There is a sustain period in which a gas discharge is made with short voltage pulses which lead to corresponding short lighting pulses. Of course only the cells previously excited will produce lighting pulses. There will not be a gas discharge in the cells in neutral state. 
     3. There is an erasing period in which the charge of the cells is quenched. 
     In some specific plasma driving schemes (NFC coding) the addressing or erasing periods are not present in each sub-field. Instead, a selective addressing/erasing is performed ahead or after a group of sub-fields. 
     As mentioned before, plasma uses PWM (pulse width modulation) to generate the different shades of grey. Contrarily to CRTs where luminance is approximately quadratic to the applied cathode voltage, luminance is linear to the number of discharge pulses in PDPs. Therefore, an approximately quadratic gamma function has to be applied to the input video signal components R,G,B before the PWM. 
     The effect of this gamma function on the input video data is shown in the following table, where a quadratic gamma function is applied (calculated with 16-bit resolution). After applying the quadratic degamma function to the input video data, in the next column the effect of this degamma function is depicted. The numbers in this column were achieved after dividing the quadratic numbers in the previous column by 256 and truncation. By doing this it is assured that the output video range and the input video range is identical. 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 11 Bit 
                   
                 16 Bit De- 
                 8 Bit Out- 
                 11 Bit De- 
               
               
                 8 Bit Input 
                 16 Bit De- 
                 8 Bit Output 
                 Degamma 
                 8 Bit Input 
                 gamma 
                 put Video 
                 gamma 
               
               
                 Video Data 
                 gamma Data 
                 Video Data 
                 Data 
                 Video Data 
                 Data 
                 Data 
                 Data 
               
               
                 (X) 
                 (X**2) 
                 (X**2/255) 
                 (X**2)/32 
                 (X) 
                 (X**2) 
                 (X**2/255) 
                 (X**2)/32 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 0 
                 0 
                 0 
                 128 
                 16384 
                 64 
                 512 
               
               
                 1 
                 1 
                 0 
                 0 
                 129 
                 16641 
                 65 
                 520 
               
               
                 2 
                 4 
                 0 
                 0 
                 130 
                 16900 
                 66 
                 528 
               
               
                 3 
                 9 
                 0 
                 0 
                 131 
                 17161 
                 67 
                 536 
               
               
                 4 
                 16 
                 0 
                 0 
                 132 
                 17424 
                 68 
                 544 
               
               
                 5 
                 25 
                 0 
                 0 
                 133 
                 17689 
                 69 
                 552 
               
               
                 6 
                 36 
                 0 
                 1 
                 134 
                 17956 
                 70 
                 561 
               
               
                 7 
                 49 
                 0 
                 1 
                 135 
                 18225 
                 71 
                 569 
               
               
                 8 
                 64 
                 0 
                 2 
                 136 
                 18496 
                 72 
                 578 
               
               
                 9 
                 81 
                 0 
                 2 
                 137 
                 18769 
                 73 
                 586 
               
               
                 10 
                 100 
                 0 
                 3 
                 138 
                 19044 
                 74 
                 595 
               
               
                 11 
                 121 
                 0 
                 3 
                 139 
                 19321 
                 75 
                 603 
               
               
                 12 
                 144 
                 0 
                 4 
                 140 
                 19600 
                 76 
                 612 
               
               
                 13 
                 169 
                 0 
                 5 
                 141 
                 19881 
                 77 
                 621 
               
               
                 14 
                 196 
                 0 
                 6 
                 142 
                 20164 
                 79 
                 630 
               
               
                 15 
                 225 
                 0 
                 7 
                 143 
                 20449 
                 80 
                 639 
               
               
                 16 
                 256 
                 1 
                 8 
                 144 
                 20736 
                 81 
                 648 
               
               
                 17 
                 289 
                 1 
                 9 
                 145 
                 21025 
                 82 
                 657 
               
               
                 18 
                 324 
                 1 
                 10 
                 146 
                 21316 
                 83 
                 666 
               
               
                 19 
                 361 
                 1 
                 11 
                 147 
                 21609 
                 84 
                 675 
               
               
                 20 
                 400 
                 1 
                 12 
                 148 
                 21904 
                 85 
                 684 
               
               
                 21 
                 441 
                 1 
                 13 
                 149 
                 22201 
                 87 
                 693 
               
               
                 22 
                 484 
                 1 
                 15 
                 150 
                 22500 
                 88 
                 703 
               
               
                 23 
                 529 
                 2 
                 16 
                 151 
                 22801 
                 89 
                 712 
               
               
                 24 
                 576 
                 2 
                 18 
                 152 
                 23104 
                 90 
                 722 
               
               
                 25 
                 625 
                 2 
                 19 
                 153 
                 23409 
                 91 
                 731 
               
               
                 26 
                 676 
                 2 
                 21 
                 154 
                 23716 
                 93 
                 741 
               
               
                 27 
                 729 
                 2 
                 22 
                 155 
                 24025 
                 94 
                 750 
               
               
                 28 
                 768 
                 3 
                 24 
                 156 
                 24336 
                 95 
                 760 
               
               
                 29 
                 841 
                 3 
                 26 
                 157 
                 24649 
                 96 
                 770 
               
               
                 30 
                 900 
                 3 
                 28 
                 158 
                 24964 
                 97 
                 780 
               
               
                 31 
                 961 
                 3 
                 30 
                 159 
                 25281 
                 99 
                 790 
               
               
                 32 
                 1024 
                 4 
                 32 
                 160 
                 25600 
                 100 
                 800 
               
               
                 33 
                 1089 
                 4 
                 34 
                 161 
                 25921 
                 101 
                 810 
               
               
                 34 
                 1156 
                 4 
                 36 
                 162 
                 26244 
                 102 
                 820 
               
               
                 35 
                 1225 
                 4 
                 38 
                 163 
                 26569 
                 104 
                 830 
               
               
                 36 
                 1296 
                 5 
                 40 
                 164 
                 26896 
                 105 
                 840 
               
               
                 37 
                 1369 
                 5 
                 42 
                 165 
                 27225 
                 106 
                 850 
               
               
                 38 
                 1444 
                 5 
                 45 
                 166 
                 27556 
                 108 
                 861 
               
               
                 39 
                 1521 
                 5 
                 47 
                 167 
                 27889 
                 109 
                 871 
               
               
                 40 
                 1600 
                 6 
                 50 
                 168 
                 28224 
                 110 
                 882 
               
               
                 41 
                 1681 
                 6 
                 52 
                 169 
                 28561 
                 112 
                 892 
               
               
                 42 
                 1764 
                 6 
                 55 
                 170 
                 28900 
                 113 
                 903 
               
               
                 43 
                 1849 
                 7 
                 57 
                 171 
                 29241 
                 114 
                 913 
               
               
                 44 
                 1936 
                 7 
                 60 
                 172 
                 29584 
                 116 
                 924 
               
               
                 45 
                 2025 
                 7 
                 63 
                 173 
                 29929 
                 117 
                 935 
               
               
                 46 
                 2116 
                 8 
                 66 
                 174 
                 30276 
                 118 
                 946 
               
               
                 47 
                 2209 
                 8 
                 69 
                 175 
                 30625 
                 120 
                 957 
               
               
                 48 
                 2304 
                 9 
                 72 
                 176 
                 30976 
                 121 
                 968 
               
               
                 49 
                 2401 
                 9 
                 75 
                 177 
                 31329 
                 122 
                 979 
               
               
                 50 
                 2500 
                 9 
                 78 
                 178 
                 31684 
                 124 
                 990 
               
               
                 51 
                 2601 
                 10 
                 81 
                 179 
                 32041 
                 125 
                 1001 
               
               
                 52 
                 2704 
                 10 
                 84 
                 180 
                 32400 
                 127 
                 1012 
               
               
                 53 
                 2809 
                 11 
                 87 
                 181 
                 32761 
                 128 
                 1023 
               
               
                 54 
                 2916 
                 11 
                 91 
                 182 
                 33124 
                 129 
                 1035 
               
               
                 55 
                 3025 
                 11 
                 94 
                 183 
                 33489 
                 131 
                 1046 
               
               
                 56 
                 3136 
                 12 
                 98 
                 184 
                 33856 
                 132 
                 1058 
               
               
                 57 
                 3249 
                 12 
                 101 
                 185 
                 34225 
                 134 
                 1069 
               
               
                 58 
                 3364 
                 13 
                 105 
                 186 
                 34596 
                 135 
                 1081 
               
               
                 59 
                 3481 
                 13 
                 108 
                 187 
                 34969 
                 137 
                 1092 
               
               
                 60 
                 3600 
                 14 
                 112 
                 188 
                 35344 
                 138 
                 1104 
               
               
                 61 
                 3721 
                 14 
                 116 
                 189 
                 35721 
                 140 
                 1116 
               
               
                 62 
                 3844 
                 15 
                 120 
                 190 
                 36100 
                 141 
                 1128 
               
               
                 63 
                 3969 
                 15 
                 124 
                 191 
                 36481 
                 143 
                 1140 
               
               
                 64 
                 4096 
                 16 
                 128 
                 192 
                 36864 
                 144 
                 1152 
               
               
                 65 
                 4225 
                 16 
                 132 
                 193 
                 37249 
                 146 
                 1164 
               
               
                 66 
                 4356 
                 17 
                 136 
                 194 
                 37636 
                 147 
                 1176 
               
               
                 67 
                 4489 
                 17 
                 140 
                 195 
                 38025 
                 149 
                 1188 
               
               
                 68 
                 4624 
                 18 
                 144 
                 196 
                 38416 
                 150 
                 1200 
               
               
                 69 
                 4761 
                 18 
                 148 
                 197 
                 38809 
                 152 
                 1212 
               
               
                 70 
                 4900 
                 19 
                 153 
                 198 
                 39204 
                 153 
                 1225 
               
               
                 71 
                 5041 
                 19 
                 157 
                 199 
                 39601 
                 155 
                 1237 
               
               
                 72 
                 5184 
                 20 
                 162 
                 200 
                 40000 
                 156 
                 1250 
               
               
                 73 
                 5329 
                 20 
                 166 
                 201 
                 40401 
                 158 
                 1262 
               
               
                 74 
                 5476 
                 21 
                 171 
                 202 
                 40804 
                 160 
                 1275 
               
               
                 75 
                 5625 
                 22 
                 175 
                 203 
                 41209 
                 161 
                 1287 
               
               
                 76 
                 5776 
                 22 
                 180 
                 204 
                 41616 
                 163 
                 1300 
               
               
                 77 
                 5929 
                 23 
                 185 
                 205 
                 42025 
                 164 
                 1313 
               
               
                 78 
                 6084 
                 23 
                 190 
                 206 
                 42436 
                 166 
                 1326 
               
               
                 79 
                 6241 
                 24 
                 195 
                 207 
                 42849 
                 168 
                 1339 
               
               
                 80 
                 6400 
                 25 
                 200 
                 208 
                 43264 
                 169 
                 1352 
               
               
                 81 
                 6561 
                 25 
                 205 
                 209 
                 43681 
                 171 
                 1365 
               
               
                 82 
                 6724 
                 26 
                 210 
                 210 
                 44100 
                 172 
                 1378 
               
               
                 83 
                 6889 
                 27 
                 215 
                 211 
                 44512 
                 174 
                 1391 
               
               
                 84 
                 7056 
                 27 
                 220 
                 212 
                 44944 
                 176 
                 1404 
               
               
                 85 
                 7225 
                 28 
                 225 
                 213 
                 45369 
                 177 
                 1417 
               
               
                 86 
                 7396 
                 29 
                 231 
                 214 
                 45796 
                 179 
                 1431 
               
               
                 87 
                 7569 
                 29 
                 236 
                 215 
                 46225 
                 181 
                 1444 
               
               
                 88 
                 7744 
                 30 
                 242 
                 216 
                 46656 
                 182 
                 1458 
               
               
                 89 
                 7921 
                 31 
                 247 
                 217 
                 47089 
                 184 
                 1471 
               
               
                 90 
                 8100 
                 31 
                 253 
                 218 
                 47524 
                 186 
                 1485 
               
               
                 91 
                 8281 
                 32 
                 258 
                 219 
                 47961 
                 188 
                 1498 
               
               
                 92 
                 8464 
                 33 
                 264 
                 220 
                 48400 
                 189 
                 1512 
               
               
                 93 
                 8649 
                 33 
                 270 
                 221 
                 48841 
                 191 
                 1526 
               
               
                 94 
                 8836 
                 34 
                 276 
                 222 
                 49284 
                 193 
                 1540 
               
               
                 95 
                 9025 
                 35 
                 282 
                 223 
                 49729 
                 195 
                 1554 
               
               
                 96 
                 9216 
                 36 
                 288 
                 224 
                 50176 
                 196 
                 1568 
               
               
                 97 
                 9409 
                 36 
                 294 
                 225 
                 50625 
                 198 
                 1582 
               
               
                 98 
                 9604 
                 37 
                 300 
                 226 
                 51076 
                 200 
                 1596 
               
               
                 99 
                 9801 
                 38 
                 306 
                 227 
                 51529 
                 202 
                 1610 
               
               
                 100 
                 10000 
                 39 
                 312 
                 228 
                 51984 
                 203 
                 1624 
               
               
                 101 
                 10201 
                 40 
                 318 
                 229 
                 52441 
                 205 
                 1638 
               
               
                 102 
                 10404 
                 40 
                 325 
                 230 
                 52900 
                 207 
                 1653 
               
               
                 103 
                 10609 
                 41 
                 331 
                 231 
                 53361 
                 209 
                 1667 
               
               
                 104 
                 10816 
                 42 
                 338 
                 232 
                 53824 
                 211 
                 1682 
               
               
                 105 
                 11025 
                 43 
                 344 
                 233 
                 54289 
                 212 
                 1696 
               
               
                 106 
                 11236 
                 44 
                 351 
                 234 
                 54756 
                 214 
                 1711 
               
               
                 107 
                 11449 
                 44 
                 357 
                 235 
                 55225 
                 216 
                 1725 
               
               
                 108 
                 11664 
                 45 
                 364 
                 236 
                 55696 
                 218 
                 1740 
               
               
                 109 
                 11881 
                 46 
                 371 
                 237 
                 56169 
                 220 
                 1755 
               
               
                 110 
                 12100 
                 47 
                 378 
                 238 
                 56644 
                 222 
                 1770 
               
               
                 111 
                 12321 
                 48 
                 385 
                 239 
                 57121 
                 224 
                 1785 
               
               
                 112 
                 12544 
                 49 
                 392 
                 240 
                 57600 
                 225 
                 1800 
               
               
                 113 
                 12769 
                 50 
                 399 
                 241 
                 58081 
                 227 
                 1815 
               
               
                 114 
                 12996 
                 50 
                 406 
                 242 
                 58564 
                 229 
                 1830 
               
               
                 115 
                 13225 
                 51 
                 413 
                 243 
                 59049 
                 231 
                 1845 
               
               
                 116 
                 13456 
                 52 
                 420 
                 244 
                 59536 
                 233 
                 1860 
               
               
                 117 
                 13689 
                 53 
                 427 
                 245 
                 60025 
                 235 
                 1875 
               
               
                 118 
                 13924 
                 54 
                 435 
                 246 
                 60516 
                 237 
                 1891 
               
               
                 119 
                 14161 
                 55 
                 442 
                 247 
                 61009 
                 239 
                 1906 
               
               
                 120 
                 14400 
                 56 
                 450 
                 248 
                 61504 
                 241 
                 1922 
               
               
                 121 
                 14641 
                 57 
                 457 
                 249 
                 62001 
                 243 
                 1937 
               
               
                 122 
                 14884 
                 58 
                 465 
                 250 
                 62500 
                 245 
                 1953 
               
               
                 123 
                 15129 
                 59 
                 472 
                 251 
                 63001 
                 247 
                 1968 
               
               
                 124 
                 15376 
                 60 
                 480 
                 252 
                 63504 
                 249 
                 1984 
               
               
                 125 
                 15625 
                 61 
                 488 
                 253 
                 64009 
                 251 
                 2000 
               
               
                 126 
                 15876 
                 62 
                 496 
                 254 
                 64516 
                 253 
                 2016 
               
               
                 127 
                 16129 
                 63 
                 504 
                 255 
                 65025 
                 255 
                 2032 
               
               
                   
               
             
          
         
       
     
     As it can be seen from the values in the columns headed 8 bit output video data, for smaller input values, many input levels are mapped to the same output level. This is due to division by 255 and truncation. In other words, for darker areas, the quantisation step is higher than for the higher areas which corresponds to non-linear quantisation. In particular the values smaller than 16 are all mapped to 0 (this corresponds to four bit video data resolution which is unacceptable for video signal processing). 
     Dithering is a known technique for avoiding to lose amplitude resolution bits due to truncation. This technique only works if the required resolution is available before the truncation step. But this is the case in the present application, because the video data after degamma operation has 16 bit resolution and in the corresponding columns there are no two identical values. Dithering can in principle bring back as many bits as those lost by truncation. However, the dithering noise frequency decreases, and therefore becomes more noticeable, with the number of dither bits. 
     1 bit-dithering corresponds to multiply the number of available output levels by 2, 2 bit-dithering corresponds to multiply the number of available output levels by 4 and 3 bit-dithering corresponds to multiply the number of available output levels by 8. 
     Looking at the table above, in particular to the input values less than 16 reveals that at minimum 3 bit-dithering is required to reproduce the 256 video levels more correctly with the required grey scale portrayal of a ‘CRT’ display device. 
     In the table above the columns headed 11 Bit Degamma Data contain the output data from the degamma unit. These values are derived from the values in the columns headed 16 Bit Degamma data by dividing them by 32 or better by truncation of 5 bits. How these values are used in the dithering process will be explained later on. 
     Next, the cell-based dithering will be explained in detail. 
     With cell-based dithering as illustrated in FIG. 2 a dither number is added to every panel cell in contrast to every panel pixel as usually done. A panel pixel is composed of three cells: red, green and blue cell aligned in a video line for each pixel. The cell-based dithering has the advantage of rendering the dithering noise finer and thus making it less noticeable to the human viewer. 
     Because the dither pattern is defined cell-wise, it is not possible to use techniques like error-diffusion, in order to avoid colouring of the picture when one cell would diffuse in the contiguous cell of a different colour. This is not a big disadvantage, because it has been observed sometimes an undesirable low frequency moving interference, between the diffusion of the truncation error and a moving pattern belonging to the video signal. Error diffusion works best in case of static pictures. 
     Instead of using error diffusion, a new degree of freedom is added to the dither patterns. Starting from a 3 dimensional dither pattern, this leads to a 4-dimensional dither pattern. Before explaining the 4-dimensional dither patterns, it is advantageous to first understand the concept of a 3-dimensional dither pattern which has already been disclosed in a previous European Patent Application of the Applicant having the number 00250099.9. It is expressively referred to this document also for the disclosure of the new invention. 
     FIG. 3 shows one example for a 3-dimensional dither pattern. 3-bit-dithering is used in this example. This means that the dither numbers have values from 0 to 7. The static 3-dimensional dither pattern is defined for a cube of 4*4*4 cells (4-lines with 4 cells each, repeatedly taken from 4 frames). 
     The use of a 3 bit-dithering requires that the degamma operation is performed with 3 bits more than final resolution. The final resolution is supposed to be 8 bit resolution. The sub-field coding range is therefore from 0 to 255. Then the output range of the degamma operation should be from 0 to 2040. It is noted that the maximum dither number with 3 bit dithering is 7. If this number is added to 2040, the result is 2047 which is the highest possible 11 bit binary number %11111111111. A slightly lower value than 2040. e.g. 2032 could also be used. This has the advantage that the corresponding values can simply be derived from the 16 bit degamma data by truncating the 5 least significant bits. 
     Some other examples derived from the table above: if sub-field coding range would be from 0 to 175, output range of degamma operation should be from 0 to 1400; and finally if coding range is from 0 to 127, output range should be from 0 to 1016. For every panel cell and for every frame, the corresponding dither pattern value is added to the output of the degamma function, and consequently truncated to the final number of bits. 
     The final bit resolution does not need to be necessarily 8 bit resolution. In another European Patent Application of the applicant having the application number 01250158.1 an example is explained where the final bit resolution is 6 bits only due to a decimation of the set of input video levels in order to avoid dynamic false contouring. In this case 9 bit data words need to be provided by the degamma and rescaling unit, thus corresponding to truncation of 7 bits from the 16 bit degamma data words. If the video range is from 0 to 36, then the output range of the degamma operation in 9 bit resolution should be from 0 to 10. 
     The 3-bit dither pattern shown in FIG. 3 is static. This means that it is repeatedly used for the whole panel. From FIG. 3 it can be seen that the dither pattern is repeated in horizontal direction of the panel. However, it also repeats in vertical direction and in time direction accordingly. 
     It is noted that the proposed pattern, when integrated over time, always gives the same value for all panel cells. If this were not the case, under some circumstances, some cells could acquire an amplitude offset compared to other cells which would correspond to an undesirable fixed spurious static pattern. 
     A problem of the 3-dimensional dither patterns is that mainly in static pictures having homogenous surfaces for some video levels some noticeable patterns can occur. This problematic can be seen for example for a video level of ⅛ after degamma operation, to be displayed with 3 bits of dithering. In this case, when the previous dithering is applied, the pattern shown in FIG. 4 will appear on the PDP. The temporal integration of this pattern over the 4 frames gives for each color a checked pattern with 2×2 squares. This is illustrated in FIG.  5 . This is a static pattern that also occurs in the following 4 frame periods as the pixels which have ‘0’ in the FIG. 5 are always ‘off’, while the others are ‘on’ once in four frames. This pattern is quite noticeable in big monochrome areas and also in an area having a dominant colour component. 
     In FIG. 6 the temporal integration of the dither patterns are shown for the video levels 0, ⅛, ¼, . . . ⅞. The displayed patterns are patterns for one colour component, namely R. It is pointed out that the disturbing patterns will not only occur for these 8 lowest video levels but also for the video levels 1, 1⅛, 1¼, 1⅜, and so on. Of course, as the human eye is more sensitive to relative luminance/chrominance differences, the most disturbing patterns are in the low video level range (dark scenes). 
     This noisy static pattern could be reduced to a checked pattern twice smaller by the use of a modified 3-dimensional dither pattern, but in this case, the previous pattern will appear for another video level. 
     Another idea to suppress this pattern could be to use a dithering based on 8 frames, but in this case the temporal frequency of the dithering will be too low, and so the problem of flickering will appear. 
     In fact, it is not possible with only one 3-dimensional dither pattern to suppress all the noisy static patterns for all the dither values. It is therefore an idea of the invention to use more 3-dimensional dither patterns. This is equivalent with the concept of a 4-dimensional dither pattern which will be explained next. 
     FIG. 7 shows an example of eight different dither patterns for the different video levels after degamma X, X⅛, X¼, X⅜, X½, X⅝, X¾, X⅞ where X stands for any number 0 . . . 255. Only the trivial dither pattern for the level X where all entries are Zero is not shown. The depicted pattern is valid for the colour component R. The dither patterns shown in FIG. 7 could be written in the form of FIG. 3, too. From the aspect of hardware or software expenditure for the implementation it is better to memorize the dither patterns in the form of FIG. 7, because here, for each cell only one single bit needs to be stored in memory. The evaluation of the entries in a dither pattern is performed in such a manner that in each case where a “1” entry is given the binary value %111 will be added to the input value and in each case where a “0” entry is given either no value is added or a Zero value is added. It need not be added a value less than seven in this case because for these values after truncation of the three least significant bits there will not remain an effect on the higher bits. In the illustration of FIG. 7 the truncation effect is already considered. 
     The 4-dimensional dither pattern is defined in case of 3-bit dithering for a cube of 8*4*4*4 cells (8-level ranges, 4-plasma cells, 4-lines, 4-frames). It should be noted that other alternatives of 4-dimensional dither patterns can be found. Other cubes can be defined and the patterns could also be differently defined. 
     A problem is to find easily the right entries for a pixel because in each frame the order is R,G,B,R. This problem can be solved by making simply a wrap around each time the four entries for a line have been read out. Then the next entry follows in the first entry of the same line and so on. However, as the processing for the colour components is done quasi in parallel, there is the problem of rightly incrementing the index counter for the table. The counter needs to be incremented three times for each colour component of a pixel and the dithering units for each colour components have to access the same memory having stored the dither pattern. The memory needs to be very fast because there cannot be much delay between the accesses of the dithering units to the memory in order to assure quasi parallelism. 
     An alternative implementation is to memorize separately a table for each colour component per frame and per video level. In this case with a simple modulo-4 counter the right entry will be easily found for each pixel. The counters can be incremented in parallel and that is easy to implement with a clock signal. For the dither patterns of each colour component a separate memory can be used. This avoids problems with multiple access to a single memory. FIG. 8 shows the dither patterns for the different video levels and for the colour components R, G, B but only for the first frame. The dither patterns for the remaining frames are not shown, but they can be easily derived from FIG.  7 . Both alternatives of dither patterns can be regarded as equivalent because they generate the same patterns on the screen. 
     The dither patterns of FIGS. 7 and 8 have been chosen in order to reduce the size of the noisy static patterns, line flicker, and also the noise introduced by asymmetries between the different dither patterns. 
     FIG. 9 shows the temporal integration of these dither patterns like in FIG.  6 . It is evident from this figure, that the size of noisy static patterns is really reduced. Instead of a checked pattern with 2*2 pixel blocks a checked pattern with 1*1 pixel blocks is generated. Of course, the checked pattern with 1*1 pixel blocks is less noticeable than the checked pattern with 2*2 pixel blocks and so with dither patterns according to the invention, the picture quality is really improved. 
     In FIG. 10 a circuit implementation of the invention is illustrated. Input R,G,B video data is forwarded to degamma unit  10 . The degamma unit  10  performs the 11-bit degamma function and delivers 11 bit video data RGB at the output. The dither evaluation unit  12  computes the dither numbers: DR for red, DG for green and DB for blue based on the degamma data coming from degamma unit  10 . To do that it requires the sync signals H and V to determine which pixel is currently processed and which line and frame number is valid. These information is used for addressing a lookup table in which the dither pattern is stored. The R, G and B components are used in this unit for evaluating the video level range of each component. In calculation unit  11  the resulting dither numbers and the degamma output values are added and the 3 least significant bits of the resulting data words are truncated so that the final output values R, G and B are achieved. These values are forwarded to a sub-field coding unit  13  which performs sub-field coding under control of control unit  16 . The sub-field code words are stored in memory unit  14 . Reading and writing from and to this memory unit is also controlled by the external control unit  16 . For plasma display panel addressing, the sub-field code words are read out of the memory device and all the code words for one line are collected in order to create a single very long code word which can be used for the line-wise PDP addressing. This is carried out in the serial to parallel conversion unit  15 . The control unit  16  generates all scan and sustain pulses for PDP control. It receives horizontal and vertical synchronising signals for reference timing. 
     In FIG. 11 the components of the calculation unit  11  are shown. Reference number  110  denotes the adder that adds the dither value DR, DG, DB to the the 11 bit degamma data R, G, B. Reference number  111  denotes the truncation stage in which the three least significant bits of the resulting 11 bit data words are truncated. Finally the resulting 8 bit data words are output and they will be used for sub-field coding in sub-field coding unit  13 . 
     FIG. 12 shows in more detail the structure of the dither evaluation unit  12 . With reference number  120  the tables with the dither patterns are denoted. In order to be able to store the complete 4-dimensional dither pattern the memory range is subdivided in 8 sectors having each the dither patterns for one of the different eight possible input values corresponding to the entries in the three least significant bits of the input value. For the addressing of the memory the following components are provided. First, the input value is fed to a selector  121 . The three least significant bits determine which value is required in pointer  122 . This pointer points to the beginning of a memory sector. Furthermore, a set of three modulo-4-counters  123  to  125  is provided. One is a frame counter  123 , one is a line counter  124  and one is a pixel counter  125 . The frame conuter  123  determines which of the 4*4 tables for the 4 successive frames needs to be taken. The line counter  124  determines the line within the 4*4 table and the pixel counter  125  determines the position within the selected line. All the three modulo-4-counters are clocked by the same clock signal Φ that corresponds to the pixel clock in the PDP signal processing. The right address for memory addressing is determined by multiplying the entry in the frame counter by 16, multiplying the entry in the line counter by 4, and adding the resulting values plus the entry in the position counter to the pointer value of pointer  122 . The entry in the resulting address is read out and determines which dither value DR is output by the dithering evaluation unit  12 . For this purpose a second selector  126  is provided. The dither value 7 is output if the read out value is equal to “1” and the value 0 is output if the read out value is equal to “0”. The components depicted in FIG. 12 while shown only for the colour component R are also required for the other color components G and B. 
     The invention can be used in particular in PDPs. Plasma displays are currently used in consumer electronics, e.g. for TV sets, and also as a monitor for computers. However, use of the invention is also appropriate for matrix displays where the light emission/generation is also controlled with small pulses in sub-fields, i.e. where the PWM principle is used for controlling brightness. 
     It is noted that the disclosed embodiment is an example and that the number of dither bits as well as the size and type of dither pattern can be subject of modification in other embodiments of the invention.