GB675062A - Improvements in television receivers - Google Patents

Abstract

675,062. Television systems. ZENITH RADIO CORPORATION. May 8, 1950, No. 11454/50. Class 40 (iii). A television receiver which co-operates with a transmitter as described in Specification 675,061, comprises means for receiving and storing the key-signal pattern, which is transmitted from the transmitter to the receiver via telephone lines, the stored pattern being scanned continuously and repetitively to restore the key signal which is utilized to effect the decoding of the received television picture signals. The television picture signals are coded at the transmitter by periodically changing the timing of the line scanning waveform applied to the camera unit with respect to the line-synchronizing signals in accordance with a repetitive key signal which is generated by the continuous scanning of a magnetic-tape upon which a key-signal pattern is stored. The key signal is also transmitted over telephone wires continuously to a central station which is in telephone communication with each person using a co-operating receiver. Upon request by the person using the receiver, a short length of key signal comprising the complete key-signal pattern is sent from the central station over lines 18 to the receiver where it is recorded by head 19 upon continuously-rotating magnetic tape 15. Any previous signal stored on the tape is removed before this operation by means of an erasing signal applied from unit 22. The received picture signals are detected at 2, amplified at 3 and applied to cathode-ray tube 4. Synchronizing signals from separator 5 are applied to the vertical-sweep generator 6 and horizontal-sweep generator 8 to control the scanning in cathode-ray tube 4, the linesynchronizing pulses being applied to generator 8 through a decoding circuit 7. This circuit comprises a delay line which is switched into and out of circuit in synchronism with a corresponding operation at the transmitter, so that the changes in timing introduced between the synchronizing pulses and scanning waveform at the transmitter are compensated for at the receiver. The switching of circuit 7 is controlled by a unit 11 to which are applied frame - blanking pulses from vertical - sweep generator 6 and key-signals which are picked off tape 15 by the pick-up head 14, the coincidence of both signals effecting the switching operation, which thus occurs only during the interval between successive frames. The invention may be modified to include a plurality of magnetic tapes which are coded according to signals stored on respective tapes at the transmitter, the tape used being selected according to a predetermined sequence. Synchronizing key signal at receiver with that at transmitter. The motor 25 which drives the tape 15 is driven synchronously by a generator 28 which is synchronized with the frame synchronizing signals. Frame-synchronizing signals from separator 5 are applied through a switch circuit 30 to a multi-vibrator 27 which controls the generator 28. Since the corresponding motor at the transmitter is also synchronized by the frame-synchronizing signals, the tapes at the transmitter and receiver rotate at the same speed. To obtain correspondence of phase between the two tapes, an additional synchronizing signal is recorded on the tapes both at the transmitter and receiver and a further signal is radiated from the transmitter with the picture signal in correspondence therewith. This latter signal is applied by separator 5 and unit 33 to switch circuit 30 to which the synchronizing signal recorded on tape 15 is also applied via conductors 32. Coincidence between these signals maintains switch circuit 30 conductive so that the tapes at the transmitter and receiver continue to be driven in synchronism. Should, however, the synchronizing signal not be applied simultaneously to the circuit 30, pressing a press-button upon circuit 30 causes it to become non-conductive, whereupon the multi-vibrator 27 becomes freerunning until coincidence occurs once more between the two signals and the circuit is restored to its conductive condition.