Patent GB-530776-A (GB)
Title: Improvements in or relating to the secret transmission of television and like signals
IPC: H04N7/169
Filed: 1939-03-06
Abstract: 530,776. Television ; cathode-ray tubes ; valve generating circuits. SCOPHONY, Ltd., and ROSENTHAL, A. H. March 6, 1939, Nos. 7230 and 8522. [Classes 39 (i), 40 (iii) and 40 (v)] In a secret television system, the frame synchronizing pulses, or the frame deflecting potentials, are used at the transmitter and receiver to generate line synchronizing pulses at irregular intervals by controlling the scanning of a pattern. In the transmitter shown in Fig. 1, the frame pulses generated at 2 control a time base 3 producing saw-tooth waves which control the transmitter 1 and also cause the beam of an auxiliary cathode-ray tube 4 to scan a screen 5 along a straight path, the moving light spot thus produced scanning a transparent screen 6 having opaque lines 11, Fig. 2, at irregular intervals. The pulses thus set up in the cell 7 control a line timebase circuit 8 which in turn controls the transmitter 1. At the receiver, the frame pulses or deflecting potentials, separated from the picture signals at 12, Fig. 3, control the time base 14 producing saw-tooth waves which control the receiver 13 and also cause the beam of an auxiliary cathode-ray tube 15 to produce irregular line synchronizing impulses as in Fig. 1. Instead of traversing a linear pattern, as in Fig. 2, the light beam may traverse an annular pattern or patterns, Fig. 6 (not shown), or the electron beam may itself move over a metal electrode 27, Fig. 7 having high secondary electron emission, with narrow lines of carbon inscribed thereon at irregular intervals, a pulse being produced in the circuit including the anode 29 and electrode 27 each time the electron beam strikes a carbon line. Instead of or in addition to transmitting synchronizing pulses, the complete time base oscillations may have their wave-form varied, e.g. by using a pattern as in Fig. 2 but with graduated shading in each section of the pattern. Owing to the irregular timing of the line pulses, the length and spacing of the scanning lines will differ. To avoid difficulties due to this, (1) the picture area may be reduced, (2) the amplitude of the saw-tooth line-deflecting current may be limited by a saturated valve 19, Fig. 1, or (3) the speed of frame scanning may be modified (a) by additional deflecting plates fed from the line-scanning circuit Fig. 4 (not shown), (b) by superposing currents derived from the linescanning circuit on the frame-scanning circuit, or (c) by utilizing the line-scanning voltages to charge a condenser which is discharged by the framesynchronizing pulses to effect the frame flyback, Fig. 5 (not shown). The first Provisional Specification describes an application of the invention to mechanical scanners in which the picture signals are stored in the form of a visible record of a picture line on the fluorescent screen of a cathode ray tube, or in the form of a supersonic wave train in a liquid, the record being scanned by electronic means responsive to the irregular synchronizing pulses. The second Provisional Specification describes a modified system in which the picture signals are sent by radio and the frame synchronizing signals and sound over a cable. Specification 516,247 is referred to.