GB1503362A - Digital raster display system - Google Patents

Abstract

1503362 Cathode-ray tube displays INTERNATIONAL BUSINESS MACHINES CORP 15 May 1975 [11 June 1974] 20485/75 Heading H4T The invention is directed to a computer driven digital television type (i.e. raster scanned) display system and is particularly concerned with providing a system which allows selective modification of data and the erasing of data without erasing overlaying material, e.g. data common to intersecting vectors, whilst reducing the storage requirements which such manner of operation requires in prior art systems. This is brought about by employing a technique described as 'on-the-fly' or 'implicit refresh' in which all displayable data retains its identity in computer coded form up to the final stages of video signal generation. As described, a refresh buffer 28 (Fig. 1) is loaded (e.g. from a computer) with lists of coded data each list representing displayed elements having an endpoint on the same raster line the raster lines being different for each list and the lists being transferred in operation to an intermediate buffer store 38 from which they are transferred, in synchronism with the display of raster lines, to a symbol generator 40 and a vector generator 42. The latter then operate to generate from each list the sequence of binary digits (video ON-OFF signals) appropriate to the raster line represented by the list and returns to the intermediate store 38 only that coded data which relates to display elements which extend to the next raster line to that represented by the list. The intermediate buffer 38 is organized into a pre-load area and an active area with a total capacity of 256 32-bit words data words being transferred from buffer 28 to buffer 38 as room becomes available in the pre-load area and from the latter area to the active area as required for display. All vector and symbol video data are assembled in "one line" of a partial raster assembly store (PRAS) 44, which has a capacity of "two lines", during the line time preceding it display and then read out at video rate whilst the next line is being assembled. The read out video is then mixed with synch and timing signals in output driver 46 and supplied to the display tube. Three PRAS's 44, one for each primary colour of a colour display tube may be driven by the generators 40, 42. The invention is described in the context of its use in a system in which each of a plurality of colour display consoles may interact with a "host" computer via a trackball (for cursor control), function keyboard, alphanumeric keyboard and a light pen.