mber of facilities being expected, new data capabilities are also required. All of these have been incorporated into DAB together with an improved ease of tuning. The problem of reflections is at the core of determining how the new system may work. Although digital audio is widely used in television broadcasts, these have the advantage that direction antennas are used which will greatly limit the effect of any reflections. For a car system that uses an omnidirectional antenna, any system must be resilient to reflections, without suffering any degradation in performance. The system is also designed to make tuning easier. On the medium wave or VHF FM bands, there are many stations that can be heard, often making it difficult to locate the wanted station. DAB aims to simplify this to a very large degree. The main problem with a digital system is to ensure that reflections do not corrupt the data being sent as shown in Figure 7.7. For satisfactory operation the system must be able to cope with significant delays. This reduces the data rate well below that required for a system using normal linear encoding like that used for a CD. For these discs the data rate is about Mbits per second. To enable the data to be successfully transmitted, two data rate reduction systems are employed. The first is an audio processing system, and the second is a totally different form of modulation called coded orthogonal frequency division multiplex (COFDM). The audio processing system relies upon the fact that the ear can only detect certain sounds. Below a given level it does not hear them. This Delayed signal from reflection causing Wanted signal uncertainty at the receiver Figure 7.7 Data corruption caused by reflections Broadcasting 217 Sound level Threshold of hearing (a) No sound present Frequency Threshold modified sound by level sound Sound Threshold level of hearing Weaker sound not heard (b) With sound present Frequency Figure 7.8 Threshold of hearing of the ear threshold of hearing varies with frequency as shown in Figure 7.8