Patent GB-1488801-A (GB)
Title: Display systems
IPC: G09G3/288
Filed: 1976-01-19
Abstract: 1488801 Matrix displays WESTERN ELECTRIC CO Inc 19 Jan 1976 [21 Jan 1975 (3)] 2048/76 Heading G5C [Also in Division H4] The system includes a plurality of selectively energizable display members each of which corresponds to an element of the image to be displayed. It also includes means for accessing only those de-energized members whose corresponding image element intensity bears a first relationship to a threshold level associated with the respective member, and for accessing only those energized members whose corresponding element intensity bears a second relationship to an associated threshold level. For example, of those members that are "on" only those whose corresponding element intensity lies below a given threshold are turned "off", and similarly only the pertinent ones of those that are "off" are turned "on". This manner of operation allows members which can be accessed only slowly, e.g. plasma cells, to provide a satisfactory display of moving images since not all of the members have to be accessed during each frame period. The system may be modified to avoid scintillation of the image due to noise. In the embodiment illustrated, Fig. 1, a raster scanner 10 provides signals to an analog-digital converter 12 which quantizes the intensity level of image elements in 256 levels. The converter passes a sequence of 8-bit words to a comparator 21 where the intensity level represented by a word is compared with a threshold value which depends upon the position of the corresponding display member in a rectangular array of members. The members of the array are notionally subdivided into groups of 4 Î 4 members, and the members of a group have different threshold values, as indicated in Fig. 2, each group having the same arrangement of values. The threshold values are, in one mode of operation, obtained direct from a read-only memory 16 read out under the control of a cyclic counter 15. The comparator output passes to the display matrix 70 where it accesses the display member currently addressed by a cyclically operating address register 45 provided that the state of the member has to be changed. This is determined by comparing in excl-OR 41 the output of comparator 21 with the existing state of the member as recorded in a frame memory 50 read out by the address register 45. If the state is to be changed the member is accessed and the memory 50 updated. The display may include a buffer to accommodate rapid changes confined to a small area of the display. The display may provide a colour image by use of three colour members in place of each member referred to above. If an image element intensity lies close to the threshold value of the corresponding display member noise may cause the threshold to be crossed, resulting in undesirable scintillation in the displayed image. To avoid this effect the threshold values read from memory 16 are not passed directly to comparator 21 but instead are passed via a unit 31. If memory 50 indicates that the member currently addressed is energized a preset value from a register 32 is subtracted by unit 31 from the threshold value. On the other hand if the member is de-energized the preset value is added. Thus an energized member will be de-energized only if the intensity level falls below the first modified threshold value, and a de-energized member will be energized only if the level rises above the second modified threshold value, Fig. 5 (not shown). The preset value is chosen to be greater than the noise amplitude. The preset value may be increased if the number of changes in state occurring during a frame period becomes abnormally high. In alternative ways of avoiding scintillation the state of a display member is not changed unless change to the other state is required in, for example, two successive frames or unless the change would significantly alter the average intensity of the part of the display in the region of the member.