Patent GB-762402-A (GB)
Title: Improvements in television transmitting apparatus
Inventor: HUTTON JAMES ALAN
IPC: H04N5/06
Filed: 1954-04-30
Abstract: 762,402. Television. MURPHY RADIO, Ltd. May 2, 1955 [April 30, 1954], No. 12615/54. Class 40 (3). A number of signals of slightly different frame frequency and corresponding with frame periods which differ from one another by a whole number of line periods are derived from a master oscillator and a frequency comparator fed with the mains supply frequency and a standard frequency operates switching circuits by which the signal nearest the mains frequency is selected and connected for controlling frame synchronizing signals. The invention is applied to a television transmitter in which colour information is transmitted on a sub-carrier which differs by an odd harmonic of half the line frequency from the vision carrier, the two carriers being derived by oscillators independent of the master oscillator from which the frame and line synchronizing signals are derived. With such an arrangement it is not possible to lock the frame frequency to the mains frequency in the conventional manner by controlling the master oscillator frequency since this destroys the line frequency relationship between the two carriers. In Fig. 1 the output of master oscillator MO operating at twice line frequency is applied to three frequency dividers UF, MF and LF having division factors of 403, 405 and 407 and producing respectively frequencies of 50.25, 50 and 49.75 cycles per second corresponding to 201.5, 202.5 and 203.5 lines per frame in the standard British interlaced system. The normal signal of 50 c/s. is applied to a frequency comparator FC to which is also applied the mains supply M and produces an output voltage indicative of any difference in frequency. This voltage is then applied to a pair of gate stage G1, G2 and is effective to control a group of switches S1, S2, S3 which select that one of the divider outputs which at any instant is nearest in frequency to the mains supply. In an alternative arrangement, the mains frequency may be compared with that of a crystal-controlled oscillator to detect any variation. In order that the change in frame frequency may not disturb synchronism, it is arranged to occur when a pulse from the divider to be switched out synchronizes with a pulse from the divider to be switched in. To this end gate stages Gland G2, Fig. 2 (not shown), each receive inputs from two neighbouring dividers and are arranged to respond under the control of the frequency comparator FC to produce a gating pulse for the associated switch S1 or S2 only when the inputs coincide. Switches S1 and S2 which also receive the corresponding divider outputs then select from each pair that frequency which is nearer to the mains frequency and pass the selected frequency together with the gating pulse to switch S3. Switch S3 in turn makes a selection of the frequency nearer the mains frequency and applies the output to the wave forming circuits of the transmitter. Each switch, Fig. 3 (not shown), comprises a critically biased diode to which the gating pulses are applied, the diode output operating a trigger circuit controlling a relay whose contacts select the divider outputs.