GB1147604A - Improvements in or relating to television transmission systems - Google Patents

Abstract

1,147,604. Television; multiplex radio signalling. MULLARD Ltd. 13 April, 1966 [15 July, 19651, No. 30074/65. Headings H4F and H4L. A method of television transmission employs at the transmitter time-sharing between a plurality of programme sources 1, 2, 3, 4 by transmitting cyclically frames appertaining to each programme in turn so that each cycle of frames contain frames from all the programmes together with added programme identification signals, and employs at each receiver recognition of said identification signals and selection and storage of the frames of a desired programme. The invention is described primarily as applied to the transmission of a large number e.g. 100, educational television programmes in which the programme content primarily comprises still pictures, but reference is also made to transmitting monitoring or warning information. Each programme may be associated with an identification signal which differs from the identification signals of all other programmes. The recognition and selection means at each receiver are adapted to recognize any one of said identification signals. Alternatively, the identification signals may be constituted by a reference signal occurring at one point in each cycle of programme frames. In this case the recognition and selection means at each receiver are adapted to detect each reference signal and thereby initiate a count of a number of frames such as to terminate at the frame appertaining to the desired programme. Combinations of the above two modes of operation can be adopted, namely, a plurality of reference signals may be transmitted in each cycle, each signal being spaced from the next one by a whole number or series of frames. Where a separate identification signal is utilized for each programme this may conveniently be inserted in the field blanking interval. The signal may comprise a group of pulses arranged to identify the programme according to a binary code. The signal is repeated in each of six successive lines in the field blanking interval to ensure positive operation of the identification circuits in a receiver. Where the identification signal is inserted at a point in the cycle and a frame is determined by initiating a count, the identification signal may occupy the whole of a frame period. As illustrated in the drawing, the identification signal generator controls the sequencing switch whereby the programme sources 1, 2, 3, 4 are selected and adds the identification signal to the final composite signal. In order to permit the identification signal to be distinguished from interference the pulses which comprise the signal may be comprised of bursts " of tone, Fig. 4b (not shown). At the receiver, Fig. 3 (not shown), the identification signal is separated from the video signal by means of a gate stage (G) which is controlled during the appropriate portion of the field blanking interval by means of pulses from a field gate pulse generator. Where the pulses of the identification signal comprise " bursts " of tone a filter (F) and detector (#) are included in the path to gate G. The individual pulses of the identification signal are directed into the input channels of a register (R) by means of a sequence switch (X1) controlled by a sequence pulse generator (Q). The register is cleared before each identification signal so that it takes up in turn each of the identification signals as they are received. A desired programme is selected by means of a switch (X2) which sets up a binary code combination corresponding to that of the desired programme. A coincidence recognition gate (C) compares the code combination for the desired programme with the code combinations of the incoming programmes as setup in register (R) and when coincidence occurs produces the pulse which operates a mode pulse generator (M). This then opens a gate (V) to allow the corresponding frame of the video input to pass into a video store (S). The video store (S), which comprises for example a cathode-ray tube store or a magnetic tape apparatus, stores the programme frame and thereafter permits it to be reproduced repeatedly to provide signals to a cathode-ray tube display (D). Mode pulse and generator (M) produces the required signals to cause store (S) to be erased and primed at the appropriate time before the arrival of the next frame of the selected programme. Reference is made to systems for linear and branching educational programmes in which the required frames of information are selected by operating switch (X2) to change from programme to programme in the cycle of programmes which are received.