The Radio Data System, RDS, is intended for application to VHF/FM sound broadcasts in the range 87.5 MHZ to 108.0 MHZ which may carry either stereophonic (pilot-tone system) or monophonic programs. The main objectives of RDS are to enable improved functionality for FM receivers and to make them more user-friendly by using features such as Program Identification, Program Service name display and where applicable, automatic tuning for portable and car radios, in particular. The relevant basic tuning and switching information shall therefore be implemented by the type 0 group, and it is not optional unlike many of the other possible features in RDS.
Modulation characteristics of the data channel (physical layer)
The Radio Data System is intended for application to VHF/FM sound broadcasting transmitters in the range 87.5 to 108.0 MHZ, which carry stereophonic (pilot-tone system) or monophonic sound broadcasts.
It is important that radio-data receivers are not affected by signals in the multiplex spectrum outside the data channel.
The system can be used simultaneously with the ARI system, even when both systems are broadcast from the same transmitter. However, certain constraints on the phase and injection levels of the radio-data and ARI signals must be observed in this case.
The data signals are carried on a subcarrier which is added to the stereo multiplex signal (or monophonic signal as appropriate) at the input to the VHF/FM transmitter. Block diagrams of the data source equipment at the transmitter is shown in figures 1.
Figure 1. RDS encoder schematic
Subcarrier frequency
During stereo broadcasts the subcarrier frequency will be locked to the third harmonic of the 19-kHz pilottone.
Since the tolerance on the frequency of the 19-kHz pilot-tone is ± 2 Hz, the tolerance on the frequency of the subcarrier during stereo broadcasts is ± 6 Hz.
During monophonic broadcasts the frequency of the subcarrier will be 57 kHz ± 6 Hz.
