Radio Data Services in UK Independent Local Radio

Most professional FM radio transmitters are capable of accepting baseband signals from 40 Hz to 100 kHz. Stereo programme transmissions require a base bandwidth up to 53 kHz, which leaves a potential capacity for other baseband signals in the range 53 to 100 kHz. In the United States this capacity has been exploited for some years, and various systems are in operation under the generic title of Subsidiary Communications Authorisation (SCA) issued by the FCC. In the USA, sub-carriers up to 92 kHz have been used and are possible due to the FM channel spacing used in US frequency assignment planning. In the UK, CCIR Recommendation 450 has been followed due to the need for closer channel spacing limits in the European Frequency Plan. This effectively limits sub-carrier frequencies to a maximum of 76 kHz. The two Independent Local Radio stations for the London area both carry the EBU Radio Data System (RDS) signal on a sub-carrier of 57 kHz, and both stations also carry financial data information services on sub-carriers of 69 kHz. This latter service is known as Radio Teletext