Patent GB-723235-A (GB)
Title: Improvements relating to television relay apparatus
Inventor: GARGINI ERIC JOHN
IPC: H04N7/10
Filed: 1952-01-01
Abstract: 723,235. Distributing music &c. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Dec. 15, 1952 [Jan. 1, 1952; April 25, 1952], Nos. 1/52 and 10417/52. Class 40 (4). In a television relay system a source of periodic substantially rectangular voltage pulses of substantially uniform amplitude with intervals between the pulses of short duration compared with the pulses during which intervals the voltage has a reduced amplitude is applied to the deflecting circuit of a cathode-ray tube at a subscriber's station over a cable channel, the deflecting circuit deriving operating energy from the pulses and having the timing of its deflecting cycles determined by the intervals. The general layout is as shown in Fig. 1, the apparatus in rectangle 101 being a distributing unit, and that in rectangle 102 a receiver. The receiver channel 103 either picks up broadcast television waveforms by means of an aerial, or receives signals of the waveform described in Specification 704,141; and in each case it delivers a carrier wave modulated with video signals via 104, and synchronizing signals via 105. Separator 106 separates the line and frame components, feeding the latter to 107 which delivers a sawtooth waveform to conductors 104. The line components are applied to flip-flop 108 which produces pulses of line frequency each of a duration approximately 85 per cent of a line period. Paraphase versions of these pulses are applied via levelling circuits 110, 111 to gates 112, 113 which are gated alternately at half line frequency by the output of frequency divider 114 driven by the line frequency output of separator 106. The outputs of gates 112, 113 are combined and fed to conductors 115. At the receiver, the filter 117 separates the modulated carrier from the frame waveforms, feeding the former to video channel 119 and the latter to the frame circuit 120, from which heater current for the receiver may be derived. The conductors 115 are connected to the line scanning circuit 118 which may be as shown in Fig. 2. A voltage of waveform as shown in Fig. 4 is thus applied to the primary of transformer 1. The anode 13 of gas tube 11 is grounded through the tapped winding 14 and line scanning coils 15 are connected between the tapping point and ground. In operation the amplitude of the positive and negative excursions of potential applied to the cathodes 9 and 10 may be 110 volts with respect to ground and the control grid 12 is biased - 100 V. to ground by means of components 17, 18, 19. During a negative excursion applied to a cathode it emits electrons to the anode which may be brought to - 100 V. and the forward stroke of a sawtooth scanning current is generated in the line scan coils 15. At the termination of the excursion (i.e. of the pulse) the tube 11 is cut off and the current in the scanning coils performs a free half-cycle of oscillation to generate the return stroke of the sawtooth current. During this stroke the anode potential may rise to more than 1000 V. positive which potential is applied via inductance 21 and condenser 22 to the E.H.T. rectifier 20 in such magnified form that E.H.T. of the order of 5 KV. is available. In the Fig. 3 modification the coil 14 is connected to the cathodes of diodes or metal rectifiers 30, 31 and condenser 32 may supply H.T. to the receiver valves. In all the modifications E.H.T. may be produced as in Fig. 2. In the operation of the circuit shown in Fig. 5 the rectifiers 35, 36 conduct alternately to produce full-wave rectification of the Fig. 4 waveform and maintain condenser 41 charged to provide H.T. for valve 40 and other valves if desired. Rectifiers 37, 38 pass current which produces a negative sawtooth waveform voltage across condenser 43 so that valve 40 is switched off by the leading edge of each pulse and switched on again when condenser 43 has charged via resistance 44. When valve 40 is conducting the anode current generates a part of the long flank of the sawtooth waveform oscillation of line frequency which is applied via transformer 39 to the coils 15. The return stroke is generated when valve 40 is switched off, rectifiers 35, 36 being non- conductive due to the high positive anode potential developed. When, at the end of the return stroke, the anode potential tends to swing negative, rectifiers 35, 36 conduct to damp the oscillation, the current taken by them providing the first part of the next forward stroke. In a further modification (Fig. 6, not shown) a separate rectifier is provided for damping the return stroke oscillation, and the scan coils are in the anode circuit of the valve 40. In another embodiment (Figs. 7, 9, 10, not shown), the pulses are all positive and the linescanning circuits are modified to produce the sawtooth scanning waveform from these unidirectional pulses. The second Provisional Specification refers also to Specifications 723,122 and 4319/52.