GB1359674A - Display system with circle drawing - Google Patents

Abstract

1359674 Cathode-ray tube displays ELLIOTT BROS (LONDON) Ltd 9 June 1972 [11 June 1971] 27430/71 Heading HAT In a cathode-ray tube display curved lines are approximated by a succession of adjoining straight lines. Two embodiments are described. In the first (Fig. 1) the beam of the display tube 15 is deflected in incremental steps in the X and Y directions to trace each of the straight lines (e.g. the sides of a polygon when the curved line is a circle) under the control of incremental quantities contained in a Read Only Memory, ROM and in the second embodiment (Fig. 2, not shown) the tube beam is deflected raster wise and the incremental quantities from the ROM are employed to control the instant of beam bright-up to produce a succession of bright points the lengths of which may be varied to form elements of the straight lines. As shown in Fig. 1 (which is concerned with the tracing of a 100 sided polygon to approximate to a circle) the five most significant bits in an eight bit radius register R1-R8 define the length of a side of the polygon this number being compared in comparator 20 with the output of a (side) length counter L1-L5 driven by timing signals at input 10 and the output of the comparator on equality of inputs supplying a signal via gate 22 which resets counter L1-L5 and "clocks on" a side counter S1-S5 by one count. The count in the latter is decoded by circuitry 24 and causes the ROM to supply the incremental deflection quantities DX1, DY1 for the next side to gates 11, 12 through which they are supplied on the occurrence of the timing signals to the beam deflection circuitry. Since the polygon is symmetrical wirh respect to the 45 degrees axis DX, DY incremental quantities need be stored for only 13 sides in ROM, a quadrant counter Q1-Q2 controlled by the overflow of the side counter S1-S5 controlling the memory output circuit 27 to set the sign of the incremental quantities in accordance with the particular octant in which the display is being produced. The lengths of all sides or of selected sides of the polygon and hence the size of the polygon may be varied under the control of the three least significant bits R6 to R8 in the radius register by comparing the latter in circuit 25 with the count in the side and quadrant counters and employing the output to block gate 22 and thus prevent line length counter from being reset until the desired length has been traced.