Patent GB-1051581-A (GB)
IPC: H04N9/455
Abstract: 1,051,581. Television; multiplex radio signalling. ELECTRIC & MUSICAL INDUSTRIES Ltd. July 9, 1963 [April 10, 1962; June 6, 1962], Nos. 13674/62 and 21884/62. Headings H4F and H4L. A television signal conveys a main video signal and an additional video or facsimile signal by modulating a signal which occurs in the interval between lines of the main signal, the effective scanning rate of the additional signal being such that one line of the scan of the additional signal occupies the same period of time as one field of the scan of the main video signal. In a first example the additional signal has an effective line frequency of 50 per second so that a 600-line picture requires a transmission time of 12 seconds. The main signal is a conventional 625-line signal, information about one picture element only of the additional signal being transmitted during each inter - line interval. A resolution of 300 elements per line is obtained, the signal bandwidth being 7À5 kc/s. In another example the additional signal comprises 50 lines per second and one field per second, the picture being given an aspect ratio of 6:1. Transmission may be effected by modulating a pulse, which may be the line synchronizing pulse or an additional pulse, in amplitude or width, or by modulating in frequency, phase or amplitude a subcarrier " burst " where the main signal is colour signal. The additional signal may be a stereoscopic signal involving two signal components, in which case transmission is effected by modulation of two subcarriers in phase quadrature. Reference is also made to an additional signal which is a colour signal, transmission being effected by an additional pulse amplitude modulated by brightness information and a subcarrier modulated in phase quadrature for the colour information. An additional sound signal may be transmitted by modulating the trailing edge of the line synchronizing pulse for the main signal. In an arrangement based on the first example referred to above the additional signal is obtained from a normal 625-line video signal using a sampling technique as illustrated in Fig. 1. Sampling pulses are derived from the normal line synchronizing pulses and gradually advanced in time over an interval of one line, the period of advance being 12 seconds. The operation is effected by the circuit shown in Fig. 2 where the additional signal at terminal A is applied through a video amplifier B to a sampling gate C which is opened by pulses derived from a comparitor K which responds to a normal line frequency sawtooth wave applied at J and a 12-second period sawtooth obtained from a generator I triggered by a flip-flop H triggered once in each twelve seconds by a divider G driven from 50 c/s. field drive pulses F. Pulses from the comparitor, which gradually advance in time over a 12-second period, are differentiated and applied to the base of a symmetrical transistor comprising the gate. The resulting pulses are stored in a capacitor and then resampled by an output gate D controlled by pulses from a generator M which occur in the interline interval. The final amplitude modulated pulses are combined in mixer E with a main video signal. At a receiver, Fig. 3, line synchronizing pulses are separated at 3 and applied through a delay A to control a gate 2 to pass the pulses modulated by the additional signal. The pulses are then applied via an amplifier 8 to control a display device 9. Field synchronizing pulses from separator 3 control a 50 c/s. line scan generator 5 directly and control field scan generator 7 via a divider 6, whereby the generator produces one cycle of scan in 12 seconds. As an alternative, reference is made to the inclusion of separate synchronizing signal in field retrace interval for the main signal. The receiver, Fig. 3, may be incorporated in the receiver for the main signal, separator 3 and generator 5 being common to the two receivers. Where colour signals are involved for both the main and additional signal common subcarrier detectors may be used for both signals. Display device 9 may be a storage C.R. tube or a picture recorder using a thermoplastic, photographic, electrographic or magnetographic recording medium. Reference is also made to recording the signals on magnetic tape. Reference is made to the use of the invention in testing transmission lines by comparing received main and additional pictures.