lay the station name, a facility which is very useful with the large variety of stations on the air today. A further facility which was not available at the launch of RDS is called enhanced other networks (EON). This allows the set to listen to a national network, but to be interrupted by travel news on a local station. This enables the set to be sensitive only to relatively local traffic problems even though the set is tuned to a national network. To achieve this facility requires a large amount of co-ordination between the different stations. To implement this feature the BBC have a central computer specifically for this. When a local station is about to transmit a traffic message this fact is communicated to this computer. It directs the relevant transmitters in the national network to indicate this fact, enabling the receiver to retune to the relevant local radio station. The data transmitted includes an alternative frequencies list that may be used to pick up the required local station, and on the best frequency for the particular location of the receiver. To be able to receive RDS data three circuit blocks are required. The first is a 57 kHz filter which is used to isolate the signals from the baseband audio and the stereo difference signals. Next this must be fed into a demodulator. This removes the data from the 57 kHz subcarrier. Once the data is available it needs to be processed for use by the receiver. As most car radio sets these days incorporate a processor, it can also be used to perform this function. Broadcasting 215 Table 7.2 RDS terminology Alternative frequencies - A list of the frequencies used by a station in adjacent coverage areas. Clock time and date - Data containing time and date information. This enables the clock to display the correct time and adjust between changes in winter and summer time without the need for manual setting. Decoder information - This signal allows for miscellaneous functions to be controlled in the radio. Enhanced other networks - Information which is transmitted