| 摘要 |
1480749 Wired broadcasting systems COMMUNICATIONS PATENTS Ltd 7 March 1975 [16 March 1974] 11791/74 Heading H4R In a wired broadcasting system a wide band coaxial cable 3 carries multichannel programme signals from a programme source 1 to a number of programme exchanges 4 to each of which a number of subscriber units 7 are connected. The channels in the wide band cable 3 are stacked one upon another in frequency, and each exchange has, for each subscriber a respective frequency translator 10 which is controlled from the respective subscriber unit 7 to convert a selected one of the stacked frequency signals to a predetermined other frequency which the subscriber is able to receive. As described the central station 1 transmits the programme signals on low deviation fm carriers, although standard UHF or VHF modulation can be used, and the cable 3 has a bandwidth of 600 MHz. Also applied to the cable are one or more pilot tones from generator 5. Each exchange can be connected to its subscribers by respective "Qwist" cables 8 of the type described in Specification 1,272,594 fed from the frequency translators 10 with a unit 11 to bypass the pilot signal around the translators 10 and apply it direct to the line. At the subscribers unit the pilot signal is utilized via frequency dividers or multipliers to generate selectively control signals which, fed back to the exchange, determine the parameters of the frequency translation so that the selected programme is returned to the subscriber over the respective one of lines 8 at the frequency which the subscriber can receive. In order to restrict reception of particular programmes the control unit 11 at the exchange may restrict the pilot tones applied to the subscribers line so that not all the programmes available can be selected by the subscriber to prevent subscribers receiving pay programmes &c. If desired the control unit 11 may be under the control of a central pay tv computer. In an alternative embodiment, Fig. 2 (not shown) the exchange 4 is linked to the subscribers units 7 via a single wide band cable (15) and individual subsidiary cables (16) and the various subscribers are allotted respective transmission frequencies, those near the exchange having frequencies higher than those more distant to increase the service area of the exchange. The subscriber will be provided with a frequency translator to change the frequency of the programme received from his line to that appropriate his receiver, the frequency translator utilizing the pilot tones received from the exchange and being located on or outside the subscribers premises.
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