Patent GB-1496900-A (GB)
Title: Graphic symbol generation
IPC: G09G5/24
Filed: 1974-12-13
Abstract: 1496900 Alphanumeric symbol generators INFORMATION INTERNATIONAL Inc 13 Dec 1974 [13 Dec 1973] 53996/74 Heading H4T Apparatus for generating graphic symbols lineby-line on a C.R.T. display with the bases of the majority of the symbols co-inciding with a notional baseline, comprises a beam control system including a stroke-generator 26 for producing a number of spaced parallel strokes each normally starting from the baseline and a video control unit 23 for turning the beam on and off during each stroke to thereby generate linear elements of the symbols, and means 17 for generating starting positions for each stroke which positions may be offset from the baseline. The stroke-generator (Fig. 2, not shown) includes X and Y D/A converters controlling the magnitude and sign of the inputs to respective X and Y integrators the outputs X s , Y s  of which partly determine the size and slope of the display strokes whereby symbols may be italicized or rotated. To this end, "rate" signals are fed to the D/A converters from the CP system 10. The stroke deflection signals X s , Y s  are added in adders 12, 13 to starting-position signals X p , Y p  to generate raster signals for the C.R.T. 11. The start and clear signals for the integrators in stroke generator 26 are derived from video control unit 23 which contains a variable clock providing a second means for determining the size of the symbols. The CP system 10 supplies descriptors for each symbol to a symbol-generator control 22 and also supplies X, Y starting-position signals for a row or column to X, Y registers 24, 25. The X, Y registers 24, 25 are thereafter incremented by signals from stroke and symbol spacing registers 18, 19 a and b as generation of symbols and rows or columns of symbols proceeds. The registers 24, 25 are also incremented by X, Y offset signals stored in registers 18c, 19c fed from the symbol-generator control 22. Both X and Y offset may be necessary to enhance the appearance of an italicized symbol. The codes for offset form the initial part of the bitstream constituting the descriptor of each symbol. An italicized, offset symbol and the corresponding descriptive code are described in full in the Specification. Each vertical stroke may contain one or more segments during which the beam is turned on. Such display elements are specified by codes defining the end points (on and off) in terms of the change (increment or decrement deltas) from the previous stroke. The end points are re-defined whenever there is a change in the number of elements in a stroke or whenever the deltas are too large to be specified by the delta code. The video control circuit (Fig. 2) comprises a register 41 for the elements in a single stroke, on and off location memories 44, 55 with associated buffer registers 55, 56 and delta registers 42, 43 for updating the location memories at the beginning of each stroke. A counter 48 counts fast until advanced to all zeros and then, upon occurrence of the START signal, counts at a slower, programmed rate. As the on and off points of the first element are reached, comparators 51, 52 set and re-set a beam-control flip-flop 57 and advance the on and off memory counters 53, 54 to address the on and off points of the second element (if any) in the memories 44, 45. These points are then updated with deltas from memories 42, 43 and the process continues as for the first element. Updating of registers 55, 56 and advance of address counters 53, 54 follow after a delay provided by delay lines 58, 59. When comparator 60 detects that the turn-on point for an element exceeding the number of elements in the stroke has been reached, AND gate 67 is enabled to transmit an end-of-stroke signal. This causes the unit 22 to re-set the integrators of the stroke generator 26 with a CLEAR signal and to update the X, Y registers 18, 19 via the adders 20, 21.