Patent GB-564511-A (GB)
Title: Improvements in or relating to combined television and sound systems
IPC: H03K7/00
Filed: 1943-01-07
Abstract: 564,511. Multiplex wireless signalling ; television. KHARBANDA, S. R. R., and PYE, Ltd. Jan. 7, 1943, No. 320. [Classes 40 (iii) and 40 (v)] In a combined sound and television system operating on a single carrier wave, sound is transmitted as a series of pulses modulated in width or phase, which are introduced during line-blanking periods when the vides signal is suppressed. As shown in Fig. 2, the pulses T are introduced during the blanking interval between the lines of the vides signal a, and are of opposite polarity to the synchronizing signals S which they immediately follow. A circuit, Fig. 3, for operating in this manner comprises a valve pair V1, V2 coupled by a common cathode resistance R6 and by a resistancecondenser combination R3, C2 between the anode of the valve V1 and the grid of V2. The synchronizing pulse is applied at the terminal 1 and is differentiated by the circuit C1, R1, so that the trailing edge of the pulse drives the grid of V1 positive, opening that valve and closing the valve V2. A positive pulse is thus produced on the anode of V2 and its duration is determined by the bias on the grid of V2 which is varied by the sound voltage applied through a transformer T1. The video signal is applied at 2 to the grid of a valve V4, the cathode of which receives the synchronizing signal from 1, the anodes of valves V2, V4 being connected to each other and to a terminal 3 from which the mixed signal is taken. At the receiver, Fig. 4, the mixed signal is applied to the grid of an inverting valve V5 which is coupled to a valve pair V6, V7. At the anode of valve V7 a pulse is generated which starts at the leading edge of the pulse S, Fig. 2, and finishes just before the vides signal a starts. This pulse opens a gate valve V8 to the control grid of which are applied the sound pulses T, the voltage across the condenser C in its anode circuit representing the sound signal. Pulses of constant width but variable in timing or phase by the sound signal may be used. For this purpose, the sound signal is applied in series with a saw-tooth wave to the input of a valve pair such as V1, V2, Fig. 3, the pulses being taken off from the anode of the valve V2. A diode is interposed between the saw-tooth source and the grid of the valve V1, Fig. 5 (not shown). The phase-modulated pulses are demodulated by applying them to the grid of a valve so as to discharge a condenser which had been charging to a variable degree during the preceding interval between pulses. The detected audio signal is taken off across the condenser through a low-pass filter, Fig. 6 (not shown). A similar circuit may be employed for demodulating width-modulated pulses, the condenser being then fully charged during intervals and discharged to a variable degree by the pulses, Fig. 7 (not shown). The synchronizing signals may be used as the modulated pulses, one edge being fixed and utilized to trigger the line scanning generator at the receiver, the other edge being variable to constitute width-modulated sound signals. The television system may employ negative video modulation, with the synchronizing and soundconveying pulses going in the positive direction. Specifications 434,882 and 434,890, and U.S.A. Specification 2,268,001 are referred to.