ices dates back to the 1960s. In the USA many FM stations use a subcarrier to carry additional low 212 Newnes Guide to Radio and Communications Technology bandwidth subsidiary audio for use as background music in areas such as shops or stores. In view of this it is called 'storecasting'. This is covered by the Subsidiary Communications Authorization of the FCC, and as a result the system is also referred to as SCA. In Europe this idea was not taken up as it was felt that the level of crosstalk from the SCA channel to the main programme was too high. Instead it was felt that a low data rate channel would be able to operate without giving rise to unacceptable levels of crosstalk. Initial development of various ideas started in the early 1970s but it was not until 1976 that work was co-ordinated across Europe and in 1983 the final specification was agreed. The system operates by adding a data signal to the baseband modulation. It is carried above the stereo difference signal on a 57kHz subcarrier. This is three times the stereo pilot carrier as shown in Figure 7.5. This was chosen because the data is based around a carrier at a Amplitude Pilot RDS signal Frequency (kHz) Figure 7.5 Baseband modulation for an FM signal carrying RDS harmonic of the pilot tone that is also used for the stereo signal and as a result it creates fewer audible intermodulation products. The data is modulated onto this subcarrier using a form of modulation called quadrature phase shift keying (QPSK). This gives good immunity to data errors while still allowing sufficient data to be carried. Being a phase shift keying system it also gives good immunity to interference to the audio signal being carried. The data message is divided into small separate entities that can be received, decoded and processed independently to overcome the effects of signal variation and multipath distortion. Data is transmitted at a rate of 1187.5 bits per second which is equal to the RDS subcarrier frequency divided by 48. By adopting this approach the decoding circu