pay the premium at the time, the end of the 1980s 550MHz systems were being installed with video library introduces narrowcasting, whereby a customer (or a 860MHz ones in the pipeline. With a 550MHz system channel limited group of them) accesses a particular item from a centralised capacity can be from between 40 and 80 channels, the exact number source. This concept has had only limited trials to date, but con- depending on factors such as channel bandwidth and spacing, for- tinues to excite interest. bidden frequency bands, and compatibility with TV receivers and Radio relay has been a long term service provided by networks, over the air broadcasts, all of which vary between and perhaps with it being relatively easy to transmit the FM band at good quality within countries. on a coaxial cable. Recently there is interest in providing additional The network is of a broadcast nature with all channels reaching all hi-fi audio services, including the possibility of an audio library. connected customers, unless special measures are taken. Condi- Cable TV networks are starting to look beyond their traditional tional access is achieved in two main ways. A frequency trap services into the following types of area: external to the premises can remove blocks of channels. It is cheap x(MHz) x-1.25 Channel gaps may have to be left to avoid Vestigial sideband form of each channel forbidden frequencies with adjacent spacing or likely interference Return band Radio Figure 57.21 Typical spectrum on a tree and branch network. (8MHz spacing for PAL-1 shown; 6MHz and 7MHz spacing also used) 57/20 Video transmission but very inflexible; changes in tiers wanted (spin) means a physical sic noise and non linearity of optical sources; technology ad- change of trap, and more general re-arrangement of channels into vances have dramatically changed this recently. different tiers is awkward for the cable operator. 2. Frequency modulation. The FM improvement factor gives A more flexible technique is to scramble all but the basic tier of great