set tuners may not cover it fully; secondly twisted pairs. Selection in the home was via a simple switch linking channel spacing may be closer and tuner selectivity may be inade- the required pair to the input of a special h.f. TV set. TV sound could be modulated with the video, but was often sent at baseband on its quate; thirdly the number of presettable tuning buttons may be inadequate to give convenient viewing. In countries with high own pair in company with other audio services. penetration SO called 'cable ready' TV sets are available to which a The system was straightforward, there being low loss at h.f., network can directly input. Otherwise set top units may be required capable of good quality, but of course limited in channel capacity such that it is now obsolete. to select out the required channel and offer it to the set at an appropriate frequency. In recent years further service types have come under consider- ation, the most prominent being pay per view. Whereas the tiers 57.3.3.2 Tree and branch coaxial received by a customer will be determined by the monthly subscrip- tion paid, pay per view gives the opportunity for access to a specific The obvious successor to multipair operation was coaxial cable premium programme at short notice. Technically this implies inter- carrying a multiplex of TV channels in the v.h.f. band (30MHz to action from the customer; though achievable by a telephone call, 300MHz). As this signal spreads out from the originating head end more automated techniques are preferable to deal with a potentially the structure resembles a tree trunk with branches arising as it is large volume response in a short time window. both amplified and split (Figure 57.19). Originally systems were The term video on demand can encompass both pay per view and truly confined to the v.h.f. region, but have in recent years spread the video library concept. Whereas pay per view remains a broad- into the u.h.f. region to accommodate greater channel numbers. By cast service, offered to all prepared to 