sembles or even to AM or FM broadcasts. The DAB transmissions are very efficient in their use of spectrum. In view of their resilience to interference it means that adjacent transmitters do not need to use different frequencies as is the case for normal AM and FM services. Instead the same frequency is reused as required to give continuous coverage in what is termed a single frequency network (SFN). In the UK, a single national network for FM occupies 2.2 MHz. A DAB transmission occupying only 1.5 MHz can carry several audio services as well as having an additional data capability. To enable the receiver to be able to decode the data being transmitted it must have a defined format. This is shown in Figure 7.10. Each information channel Synchronization Main service channel Figure 7.10 DAB data frame structure 220 Newnes Guide to Radio and Communications Technology Orthogonal frequency Audio Channel Tuner division decoder decoder multiplex and amplifier decoder services information Packet channel demultiplexer Control bus Controller Receiver controls and display Figure 7.11 Block diagram of a DAB receiver transmission frame starts with a null signal to give approximate synchronization. This is followed by a phase reference for the demodula- tion process. After these references the next symbols of data are reserved for the fast information channel (FIC) used for data. The remaining frames are then used for the main service channel (MSC). Each programme or service within the MSC section of the frame is allocated its own time for decoding purposes. The total length of the frame for terrestrial transmissions which are being made at the moment is 96 ms, although additional standards exist for 48 or 24 ms. These are intended for use on other frequencies and for satellite broadcasts which are planned for the future. Receivers for DAB naturally require totally different decoding circuitry to that used for more typical AM or FM receivers. They make heavy use of digital signal processing techniques to reconstruct and process