GB1523753A - Two-way data transmission system for cable television network - Google Patents

Abstract

1523753 Selective signalling; educational apparatus RAI RADIOTELEVISIONE ITALIANA 29 Aug 1975 [16 Sept 1974] 35719/75 Headings G4H and G5G In a communication system, an answer format signal initiates counting to a characteristic count to define a time for sending a reply. In a cable TV system in which TV programmes are sent by cable from a central station to subscriber stations, the central station can also send repeated digital messages over the cable to the subscriber stations. A subscriber station demodulates a message and passes it down a series of shift registers connected by jumpers to define a first delay and then to a word decoder (involving shift registers and diode matrices). A "fast" clock (bit clock) derived from the message, drives all the shift registers. The word decoder feeds data and commands to external apparatus and a display on a keyboard unit, and provides sync, reset and answer format (i.e. one of two possibilities for answer length) signals. The "fast" clock is divided down and used, inter alia, to provide a "slow" clock to drive a subscriber counter which is reset by the reset signal, counting starting on appearance of either answer format signal. When the counter reaches a predetermined count, respective to the subscriber, an answer gate signal is produced, of duration dependent on the answer format (length). The subscriber counter thus defines a second delay. The answer gate signal is used to gate out multiplexed information (from the keyboard and the external apparatus, and including indications of the subscriber predetermined count, see above, and the TV channel in use and the commands from the word decoder, and parity bits, as appropriate) for transmission (after modulation) over the cable to the central station, which thus receives the information from the different subscriber stations in turn, due to differing first and second delays in the different stations. The central station can allow for the different answer lengths by adjusting the time at which it sends its answer format information (arrival of which starts the subscriber counter). Application to voting, teaching (question and answer), meter reading, purchasing, alarm systems and pay TV is mentioned.