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摘要

1470409 Closed circuit television systems RCA CORPORATION 31 July 1974 [13 Aug 1973] 33693/74 Heading H4R In a closed circuit television distribution system in which return signals, e.g. in respect of burglar alarms, fire alarms, polling, pay to, meter reading &c., are sent from the subscribers to the central station over the same lines as television signals are sent from the central station to the subscribers, one, or groups of, the subscribers may be isolated, so far as return signals are concerned, from the central station either when the return signal from that subscriber, or group of subscribers, is below a predetermined level, or in the absence of a particular interrogation code transmitted from the central station in respect of that subscriber or group of subscribers. As described television signals are transmitted from a central station 10, Fig. 1, in the band of 50 to 270 MHz via a trunk line 22, with repeaters 40, 41 &c., bridger amplifiers 44, feeder lines 24, 26, 28, and line repeaters 50, 51, 52 &c., to subscribers, e.g. 100, branched off from tapping points 30, 32 &c. Return signals from subscribers, in the band 5 to 30 MHz, are by-passed round the repeater amplifiers by frequency diplexers 75 and return signal amplifiers such as 60, 61, 70, 71, &c. At each return signal amplifier, e.g. 60, which is adjacent a bridging amplifier, e.g. 44, and at each return amplifier, e.g. 70, adjacent the head end, a muting circuit, 82, Fig. 2A, is provided which open circuits the return amplifier 80 output if the signal thereat is below a predetermined level. The switch may be preceded by a delay circuit 84, Fig. 2B, and/or an a.g.c. circuit 86, Fig. 2C, to enable switch operation before the signal to be switched arrives and/or to restore the signal level to a predetermined level. Alternatively the switch can be closed by an interrogation signal, in respect of the subscribers connected to that trunk, and feeder, transmitted from the central station and detected adjacent the switched return amplifier. The arrangement reduces noise on the return signals received at the head end since only noise from subscribers lines actually engaged in returning signals reaches the central station.