gnals on these channels have the correct programme identity (PI) code. The AF codes carry the VHF Band II channels on which the station may be heard. These channels start at 87.5 MHz with channel 0, then 87.6 is channel 1, and SO forth. Up to 25 alternative channels may be catered for under the basic system, although this can be expanded to cater for more when required. Normally the list is kept as short as possible, but large lists may be needed when there are small relay stations that do not decode the data, and only retransmit the 214 Newnes Guide to Radio and Communications Technology full complete signal. When radios fitted with RDS store the frequency of a station, they normally store the programme identification (PI) code along side it. In this way if the receiver is located outside the coverage area for the given transmitter it can still locate the correct network. While the system operates in this manner for national networks some changes have to be made for local stations. Here the radio will tune to the strongest station of the same type when moving out of the service area of the first station. The PI code is structured, consisting of four characters. The first indicates the country of origin, and it is 'C' for the UK. The second indicates the type of coverage. The figure '2' indicates a national station, and the final two letters are used for the programme reference. As an example C203 is the code for the BBC Radio 3 network and C311 is used for the BBC local radio station GLR. It can be imagined that when moving from the service area of one transmitter to the next there may be a small gap as the receiver checks for the strongest signal, and that it has the correct PI. This can be improved by employing a second front end section in the set to check the alternative frequencies. Some sets incorporate this and the change from one transmitter to the next is virtually unnoticeable. Another facility which is becoming increasingly useful is the pro- gramme service (PS) name. This enables the receiver to disp