Patent GB-1173435-A (GB)
Title: Character Generator
Inventor: BOUCHARD RICHARD
IPC: G09G1/18
Filed: 1966-10-18
Abstract: 1,173,435. Cathode-ray tube displays. SANDERS ASSOCIATES Inc. 18 Oct., 1965, No. 46597/66. Heading H4T. A deflection waveform for the beam of a cathode-ray display tube is produced by charging a capacitor at rates determined by the amplitudes of each of a succession of control signals having different amplitudes and occurring in a preselected sequence, the voltage across the capacitor constituting the required waveform. The invention is described in connection with an Alpha-Numeric type of display in which the beam of the display tube is deflected, repetitively, to trace (if unblanked) a composite display matrix, selected elements or portions of elements of which are unblanked to form a desired character. Fig. 1 shows the general arrangement in which character identifying signals, derived from a computer 34 or from a keyboard encoder 32, are supplied to an editing system 36 which produces signals specifying the particular line and the position in that line in the trace produced on the screen 14 of the display tube 10 that the character is to occupy and supplies such signals to a character position circuit 40 whilst transferring the character identifying signals to a register 38. The output of the latter is then decoded in 52 and processed in a circuit 54 to produce a sequence of unblanking pulses to unblank the selected elements of the composite display matrix (e.g. see Fig. 2) to form desired characters (e.g. see Fig. 3). The matrix is produced by horizontal and vertical deflection (ramp) generators 46, 48 the outputs of which are combined with the position signals from circuit 40 in summing networks 44 and 42 respectively and fed to the beam deflection means 16. For the tracing of the matrix the horizontal and vertical deflection generators 46, 48 produce ramp waveforms which deflect the beam in the order shown in Fig. 2 through strokes 1 to 16, each stroke being traced in a one microsecond period and strokes 9, 11, and 14, which retrace strokes 3, 7 and 5 respectively, being automatically blanked. Fig. 6, which shows the general arrangement for generating the ramp waveforms, utilizes pulses T10 to T27 from a repeating sequence of 27 (T1 to T27) one-microsecond pulses and a timing pulse #3 generated together with other timing signals, under the control of a 4 Mc. clock (Figs. 4 and 5, not shown). These pulses are then processed in gating networks 92 and 270 to produce pulses controlling the charging of capacitors in the horizontal and vertical ramp generators 94, 276 which provide the beam deflection signals. Figs. 7A to 7D show the pulses for controlling the charging of the capacitor in the horizontal ramp generator and Fig. 7E shows the ramp voltage waveform produced for the horizontal component of deflection required to trace the strokes 1 to 16 during the indicated clock timing pulses T10-T27. Similarly, Figs. 9A and 9B show the pulses controlling the charging of the capacitor in the vertical ramp generator and Fig. 9C shows the ramp voltage waveform produced. Fig. 8 shows the horizontal ramp generator. This comprises a capacitor 114 which is charged from either a positive or from a negative constant current source 116 or 118 respectively, at instants and for durations determined by the pulses 7A to 7D which are supplied to the bases of respective switching transistors via terminals 104, 106, 108 and 110 respectively. Each transistor is switched on for the duration of the applied pulse and causes the constant current source transistor 116 or 118 with which it is connected to conduct current. which charges capacitor 114 at a rate determined by the adjustable resistors 136, 170, 198 and 218 associated with the switching transistors and the resistors 144, 152 and 176, 182 in the circuits of the transistors 116 and 118 respectively. The adjustable resistors form potential dividers with the resistors of the transistor circuit to which they are connected and by suitable adjustment of the former the rates of charging are controlled such that the different slopes of the waveform of 7E are produced. This waveform is then supplied to the output terminal 112 via a double emitterfollower circuit 120. At the end of the ramp generation period, i.e. during period T27 the latter pulse is utilized to operate a switch 122 comprising transistors 238, 264 which discharges capacitor 114 in preparation for the next cycle of ramp waveform generation. The vertical ramp waveform generator 276 (Fig. 6) is similar to the horizontal generator with the exception that the portion enclosed in the dashed-line " box " 284 is omitted. For generating the beam unblanking pulses the character identifying signals which are supplied to register 38 (Fig. 1) in serial form are supplied therefrom in parallel form to a decoder 52 (also Fig. 10, not shown) which produces a parallel set of " stroke " signals for each character &c. These sets are then converted in a "clocking gate network " (Figs. 11, 12 and 13, not shown) controlled by various timing signals from the circuit of Fig. 4 (not shown), into a single sequence of unblanking pulses appropriate to each character, symbol &c. to be displayed. Such unblanking pulses unblank the beam during the tracing of whole, threequarter, half or quarter elements (strokes) of the display matrix depending on the particular character, symbol &c. to be displayed and the Specification includes tables of Alpha-Numeric characters and other symbols together with the unblanking requirements appropriate to the display of such symbols. By injecting some of the vertical deflection waveform into the horizontal deflection circuit the vertical strokes in the display may be given a horizontal " slant " (Figs. 18 and 19, not shown). Instead of the form shown in Fig. 2 the composite display matrix may take the form shown in Fig. 20 or Fig. 21.