By R. P. Gabriel, B. Sc., FlEE, MIEEE, Chairman, Rediffusion International Limited
In the hub type network of Dial-a-Program the requirement for two-way transmission of data, voice or vision is met in a very simple and straightforward manner. Indeed, Dial-a-Program is potentially a complete communication network carrying not only television programs in the ordinary sense but telephone and full bandwidth picturephone as well. Figure 1 shows the general arrangement for the distribution of television programs. Subscribers select their desired program from those available at the program exchange by remote control. All vision signals are carried in the channel 3-9 MHz on identical phase locked carriers. Each subscriber is connected to the exchange by means of two balanced pairs in a multipair cable of special construction known as Qwist. A cross section of these two pairs is shown in Figure 2 where a comparison is also given of the three pairs of telephone cable which are required for the narrow band picturephone service. The larger pair in the Qwist unit carries the television program and the smaller pair, which has a usable bandwidth up to about 3 MHz, may be used in various ways. In the simplest case it is used only for the control signals which operate the selector switch in the exchange.