Patent GB-982997-A (GB)
Title: Signalling and control system for a receiver in a graphic communication system and graphic communication receiver therefor
IPC: G08C21/00
Filed: 1961-09-11
Abstract: 982,997. Writing telegraphs. VICTOR COMPTOMETER CORPORATION. Sept. 11, 1961 [Sept. 12, 1960], No. 32486/61. Heading G1N. The invention relates to a writing telegraph system wherein the transmitter 10 (Fig. 1) comprises two oscillators 26, 29 of different frequencies which are frequency-modulated by changes in capacitors or inductors 25, 28, controlled by the vertical and horizontal movements of a pen 14, the modulated signals being combined at 31 and sent to receivers 11A, 11B over a line 32. Each receiver comprises horizontal and vertical signal component filters 33, 34, respectively associated with demodulators 35, 36, amplifiers 37, 38 and pen linkage actuating motors 41, 40 to cause pen 20 to reproduce the motion of pen 14. The invention is characterized in that the vertical or horizontal signal at the transmitter may be suppressed by operation of push-button switches 42, 43 respectively, and a control circuit 44 at the receivers responds to the absence of this component to selectively control the operation of the receivers The circuitry of Fig. 2 shows the vertical frequency-modulated signal from filter 34 fed in push-pull through transformer 52 to transistors 56, 57 to be rectified by diodes 64, 65 and fed through LF fitters 67, 69 and 68, 69a to appear as D.C. voltages at terminals 81, 82 relative to a centre point 66 whose potential is determined by potentiometer 85. The frequency modulated signal is also fed through a circuit 80 tuned to the carrier frequency, amplified by transistor 74 and fed to transistors 56, 57 is parallel, thereby providing an unbalance between the D.C. voltages at terminals 81, 82 proportional to the frequency modulation. The D.C. voltage variation is amplified by pushpull amplifier 38 and energizes the control winding 40A of vertical motor 40. Winding 41A of horizontal motor 41 is similarly energized by the horizontal frequency-modulated signals. A control relay 141 is provided, whose energizing winding 142 is connected between the centre point 156 of winding 40A and earth. The potential at 156 follows that at 66, so that when a vertical signal is received the p.d. across winding 142 is sufficient to energize relay 141 and operate its contacts, but when no signal is received the relay falls and its contacts short-circuit the inputs to amplifiers 37, 38, preventing operation of motors 40, 41. Thus the receivers may be continuously ready, but the motors 40, 41 are effectively locked until a vertical carrier frequency signal is received. Two further control relays 161, 162 are provided which produce permanent short-circuit of the inputs to amplifiers 37, 38 when the output from the horizontal signal generator 29 is momentarily shortcircuited by operation of push-button switch 43 at the transmitter. A similar control circuit (Fig. 3 not shown) may be included in receiver 11B to operate when the vertical signal generator 26 is momentarily short-circuited. The receivers are restored to operating condition by closing both switches 42, 43, when relay 141 breaks the holding circuit for the other control relays.