Patent GB-1224224-A (GB)
Title: Character display system
IPC: G09G1/10
Filed: 1968-08-30
Abstract: 1,224,224. Cathode-ray tube displays. SANDERS ASSOCIATES Inc. 30 Aug., 1968 [1 Sept., 1967], No. 41633/68. Heading H4T. In an arrangement for tracing characters on the screen of a cathode-ray tube each character is composed of a string of strokes chosen from a repertoire of strokes extending in various directions. Fourteen strokes a, a<SP>1</SP>, b to f, g, g<SP>1</SP>, h to k and m (Fig. 2) having twelve different directions are provided the tracing of which requires component deflections of + X,- X, + 2X, - 2X, + 2Y and - 2Y. Each character is traced in a space 26 (see also Figs. 1 and 3A) having X and Y dimensions corresponding respectively to eight units of deflections and the lower left-hand corner of which constitutes the origin at which the tracing commences. The different characters are identified by signals contained in a memory 32 (Fig. 1) from which, on request, they are supplied via a register 34 to a decoder 46 which provides output signals on individual lines indicative of the different characters. The latter signals are then supplied, together with signals from a clock 50, to a gate matrix 48 which generates a succession of timed signals selected one of which are supplied to the horizontal and vertical ramp generators 52; 57 respectively and to an unblanking amplifier 62. The outputs of the ramp generators are then combined in summing amplifiers 42, 40 with signals from a positioning circuit 36 (controlling the gross position of the character) and fed to the X and Y deflection control means 22. Fig. 3A, shows the sequence of steps in tracing the character A. Thus, commencing at the lower left hand corner, four of the c strokes (of the repertoire shown in Fig. 2) are produced during four clock pulses followed by four k strokes during the next four clock pulses after which two e strokes (superimposed on the last two k strokes) and two g strokes complete the character. All the deflection signals required for tracing this pattern are produced by a portion of matrix generator 48 which is concerned solely with character A and which additionally produces unblanking signals so that the beam remains blanked whilst being directed to the origin and during the two strokes e (which retrace the last two k strokes in the opposite direction) in order to maintain uniformity of illumination. The character B (Fig. 3B) is traced in a similar manner. Fig. 5, shows the gate matrix 48 of Fig. 1 including the portion 48-A concerned with the tracing of the character A. Such portion comprises a number, five, of AND gates 70, such number being determined by the changes required in stroke direction and in beam unblanking when tracing the character. All the of gates receive as a first input a constant signal on output line 46-A of decoder 46 (identifying the character A) and have as second inputs various sequencies of the timing pulses from clock 50 (Fig. 1) commencing with pulse T10 (pulses T1 to T9 allowing time for the positioning circuit 36 to effect the required shift). During the interval T10 AND gate 70-1 produces an output pulse which is fed to OR gates 74a, 74e and 74g. Likewise during the intervals T14, T18, T25 and T22 the other AND gates 70 produce output pulses which are supplied to the various OR gates 74a to 74g as shown. Of the latter gates, 74a to 74f inclusive are concerned respectively with controlling deflections of +X, -X, +2X, - 2X, +2Y and -2Y units and gate 74g is concerned with unblanking requirements. The outputs of all these gates are fed to similar respective bi-stable circuits 80a ... 80g comprising conventional bi-stable flip-flops with gated inputs the outputs from the bi-stable circuits being utilized to control the appropriate deflection and blanking circuits. The arrangement operates as follows. Considering circuit 80a (the operation of the others being identical), the output pulse from OR gate 74a during period T10 (curve 78, Fig. 6) together with the negative transition of a pulse train T s  (from clock 50) causes the flip-flop FF to change its state and produce an output 83 on line 81d which is maintained until the next output pulse from gate 74a during period T18. The output on line 81d controlling the +X deflection is supplied to input 51a of the horizontal ramp generator 52 which then generates a deflection signal having a slope 67 one unit (see 85a, Fig. 7) and which corresponds to the horizontal component of the c and k strokes 1-8 (Fig. 3A). Simultaneously, i.e. during period T10, the output from AND gate 70-1 is applied to OR gate 74e the output from which is applied to bi-stable circuit 80e which controls the vertical ramp generator 57 to produce a deflection (104a, Fig. 7) signal having a slope of two units (+2Y) corresponding to the vertical component of the c strokes 1-4 (Fig. 3A). This deflection signal is maintained until period T14 when the next input to OR gate 74e terminates the production of this deflection signal. The latter input (from AND gate 70-5) is aslo supplied to OR gate 74f which then controls bi-stable circuit 80f to produce a deflection signal (104b, Fig. 7) having a negative scope of two units (-2Y) corresponding to the vertical component of the k strokes 5-8 (Fig. 3A) and which is maintained until the next input to OR gate 74f during period T18 from AND gate 70-9. During period T10 the output from AND gate 70-1 is also supplied to OR gate 74g which supplies an output to bi-stable circuit 80g to unblank the beam so that the c and k strokes are visible on the tube screen. At period T18, however, when the output of AND gate 70-9 is employed to control the tracing of the e strokes 9 and 10 (Fig. 3A) by utilizing inputs to OR gates 74a and 74b to terminate the + X and - 2Y deflection signals and inputs to OR gates 74b and 74e to produce the - X and + 2Y deflection signals required for the e strokes, an input is also supplied to OR gate 74g which terminates the unblanking output of bi-stable circuit 80g so that the e strokes, which retrace k strokes 8 and 7 are blanked out. The g strokes 11 and 12 are then traced during period T20 the beam being again unblanked and during period T22 the beam is again blanked which it is being returned to the origin on i stroke 13. Other characters are traced in similar manner by supplying the appropriate decoder output 46-A to 46-N to the corresponding section 48A to 48N of the gate matrix. At the end of the tracing of a character the cessation of an output signal from an OR gate 150 having inputs from each of the bi-stable circuits 8a ... 80b operates via an inverter 151 to reset the clock 50 so that the next timing signal is a T41 signal (the last of the cyclic output pulses from the clock) which prepares the system to display the next character. The matrix sections 48-A to 48-N comprise individual plug-in modules so that the character set may be readily changed, the clock cycle being enlarged if the new character set involves larger or more complex characters. A reduction in the number of components utilized in the matrix sections of Fig. 5 may be effected by taking into account the fact that many characters of a set require the same combination of stroke components and unblanking during the same stroke interval (Fig. 8, not shown). A reduction in the number of such components may also be effected by modifying the clock and distributer 50 (Fig. 1) so that it produces timing signals defining 2, 3, 4, or more stroke intervals (Fig. 9, not shown) and modifying the matrix sections to utilize such timing signals (Fig. 11 not shown).