1512376 Programme exchange systems COMMUNICATIONS PATENTS Ltd 30 July 1975 [6 Aug 1974] 34512/74 Heading H4R In a programme exchange system, a two stage selection process is performed by a subscriber to gain access to some of the programmes. The system is essentially a television distribution system including an exchange, figure 2, to which each subscriber is connected by a four wire line including a programme pair 4, 5 and a control pair 2, 3. In a first embodiment the exchange includes three multiple programme sources 19-21 and each subscriber is provided with a three-position changeover switch 9 for selecting a respective one of the sources. In the position shown the changeover switch does not connect a potential source to either programme wire 4, 5 and both transistor circuits 26, 28 at the exchange remain inoperative to connect the subscribers selected to the programme source 19. The subscriber can then use a known dialling arrangement to control the position of his selector at the exchange to couple him with one of the programme wires 22-25 of source 19. Moving the changeover switch to a second position applies a positive potential to programme wire 5 to turn on transistor 45 and operate relay 46 to connect the subscribers selector to the programme wires of source 20. The third programme source is likewise selected by moving the changeover switch from its second to its third position transistor circuit 28 then being operated over wire 4. In an alternative embodiment (Figure 4, not shown) the changeover switch selection of the exchange programme source is replaced by an arrangement using single frequency tones in the range (30-500) KHz, the subscriber being provided with a keyboard for selecting the programme source 19-21. One of the programme sources may be a digi-TV machine in which a multi track magnetic drum or disc carries on each track a recording of one television picture frame. Such a machine may be associated with a computer store from which a piece of information selected by the subscriber may be transferred onto one of the tracks. For information to be used by one or a small group of subscribers, a 'dedicated' programme output can be provided as disclosed in Specification No 1,420,628.
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