Patent GB-1187521-A (GB)
Title: Character & Symbol Generator
IPC: G09G1/12
Filed: 1968-01-01
Abstract: 1,187,521. Cathode-ray displays; sawtooth circuits. TASKER INDUSTRIES. 1 Jan., 1968 [30 Dec., 1966], No. 36/68. Headings H3P, H3T and H4T. In a character-symbol generator a plurality of ramp voltage waveforms having the same slope but different zero-voltage crossing times is derived from a single ramp voltage waveform having the same, predetermined, slope, selected ones of the plurality of ramp waveforms being supplied to circuit means individual to different characters which produce output ramp waveforms of different slopes and time durations which are then summed to provide waveforms for deflecting the beam of a cathode-ray tube to trace the character. Fig. 1 shows the general arrangement in which a generator 14 supplies a ramp voltage waveform to a separation network producing the plurality of time shifted ramp voltage waveforms. These are then supplied over individual lines 22a ... 22f to circuit means 24 which includes a separate circuit selected by computer controlled means 34, 36, for each character, the means 24 operating to produce ramp voltage waveforms which are summed in amplifiers 28 to produce X and Y deflection waveforms and beam intensity (on-off) control signals for the cathode-ray display tube 30. Fig. 2a shows part of one of the character generating units in block 24. The circuit operates as follows. A positive reference voltage from circuit 34 of Fig. 1 is supplied at E s  and one of the time shifted ramp voltages from network 20 increasing in the positive direction is supplied at V in . When the ramp voltage crosses the zero axis diode 44 commences to conduct and with the decreasing conduction of diode 42 as the input ramp voltage increases in the positive direction a decreasing voltage drop occurs across resistors 46 and 48. This continues (see Fig. 2b, which shows the output V out  via an inverting amplifier 50) until the ramp reaches a voltage V determined by resistors 46 and 48 at which value the output levels off and remains constant for values of V in  more positive than V. Thus an output ramp voltage V  out  is produced, the slope and amplitude (at the levelling-off point) of which are determined by the value of resistors 46, 48. When it is required to develop an output V  out  whilst V in  is still negative the latter may be supplied to diode 42 via a resistor (Figs. 3a and 3b, not shown) and when, additionally, it is required for V out to level-off whilst V  in  is still negative an additional input V  in  - K (where K is a constant) may be supplied to diode 42 via a further resistor (Figs. 4a and 4b, not shown). A plurality of circuits of the kind shown in Fig. 2a may be combined as shown in Fig. 5a so that in response to V  in , output waveforms I  out  86, 88, 90 (Fig. 5b) are developed across the (summing) resistors 70, 72, 74 which are combined in summing amplifier 71 to produce a composite output waveform V  out  as shown at 92 (Fig. 5b). The times at which waveforms 88 and 90 commence are determined by bias sources 76, 78 and the slopes of all waveforms are determined by the relative values of the two resistors associated with each diode. Fig. 13 shows an extension of the arrangement of Fig. 5 in which the various diode circuits are supplied with ramp waveforms having different zero crossing times and the summed outputs utilized to control the X and Y deflection of the cathode-ray tube to produce the required display. This circuit also utilizes the basic ramp waveform from generator 14 (Fig. 1) in inverted form and provides ON-OFF signals via amplifier 224 for controlling the beam of the display tube. Additionally a feedback signal to generator 14 (Fig. 1) is provided to stabilize the operation. A separate circuit of the type shown in Fig. 13, is provided for each character and selected by circuit 34 (Fig. 1). Figs. 6 and 7 show one form which the ramp separation networks 20 of Fig. 1 may take the resistor chains providing the various time staggered output ramp voltages from the various tappings when the input V in  is applied as shown. Frequency compensation may be provided by shunting each resistor by a capacitor (Fig. 8, not shown) and the disadvantage of the attenuating effect of the resistors may be avoided by driving the resistor chain by an amplifier from each end (Fig. 9, not shown) or from one end with the other end supplied from a constant-current source. With the latter driving arrangement the resistors may be replaced by Zener diodes or by ordinary diodes (Figs. 11 and 12, not shown).