| 摘要 |
1414127 Wired broadcasting systems; automatic telephone systems COMMUNICATIONS PATENTS Ltd 21 Nov 1972 [3 Dec 1971] 56299/71 Headings H4R and H4K In a broadcasting system of the type disclosed in Specification 1,272,594 one of the two pairs of conductors connecting the subscriber station with the programme exchange can be used for telephony. In two embodiments (Figs. 1-4, not shown) the interstitial audio frequency conductors are used solely to connect a telephone at the subscriber station with a telephone exchange via the programme exchange, and the control signalling necessary for TV programme selection utilizes the high frequency vision conductors. Two different forms of signalling are used. In one embodiment (Figs. 1-2) programme selection D.C. pulse signals are used on one high frequency conductor, whilst a reset pulse is applied to the second conductor, a sheath of the cable being used as the return conductor in both cases. In the second embodiment, reverse polarity D.C. signalling is used on the two high frequency conductors only, i.e. positive pulses for programme selection with negative polarity for resetting. In a third embodiment (Figs. 5-6) both the telephone circuit and the control circuit utilize the audio frequency conductors, and a changeover switch S is used to select either the control or the telephone circuit. With the switch S in the circuit position shown the telephone subset is connected via the cable and relay contacts R 31 , R 32 in the programme exchange to a telephone exchange, and the audio output to the television set is muted. When switch S is put to the control circuit position, the high frequency conductors 1, 2 are looped to switch relay R 3 so as to connect the subscriber programme selector, in the form of a telephone type dial, to the programme selector switch at the programme exchange. |
