Channel allocation
In order to cover a large area, a number of transmitting stations are used. Each transmitting station is assigned a number of 8-MHz channels. However, for analogue television broadcasting, to avoid inter-channel interference, adjacent channels on either side of the TV channel are not used resulting in a number of channels remaining empty, some of which have now been allocated for digital television. The UK is divided into several areas each served by a station transmitting UHF frequencies in bands IV and V. For example, Crystal Palace serves the Greater London area and uses the fol- lowing analogue channels:
The frequency spectrum for an NTSC channel as used in the USA is shown in Figure 1.10. The total channel bandwidth is 6 MHz with a sound carrier situated at 4.5 MHz away from the vision carrier. The vestigial sideband is ± 0.75 MHz and a maximum video frequency of 4.5 MHz which is ade- quate for SD television.