xRDS, a new technology for increase data rate of the old FM radio
The slow penetration of terrestrial digital radio, the delayed switchovers suggest that the old analogue FM radio still has 10-15 years to stay here. FM broadcasting is mature, widespread and most of the listeners are happy with its sound. The only problem we had to face with is the slow data transmission rate provided by the Radio Data System (RDS) belonging inseparable to FM radio.
In the era of multimedia and wireless communication broadcasters are looking for multimedia solution which can be easily implemented using digital media but not with the analogue ones.
Radio Data System deployed more than a quarter century ago provides a very modest data transmission rate. The bigger part of the net 421.8 bit/s RDS data stream is used to make tuning and program selection easier while about 20-25% of the capacity is allocated for Other Data Applications (ODA) like transmission of encoded road traffic information for satellite navigation devices.
RDS data is added to the radio program by injecting a single sub-carrier into the baseband multiplex signal containing the mono or stereo audio. The system was devised so that it should not disturb the analogue radio transmission. This is why it has a moderate data rate.
In the past decades there were some attempts for the implementation of solutions like DARC and DirectBand for faster data transmission using the FM broadcast channel, but due to mainly their incompatibility with the RDS they could not proliferate and has been used only in some countries.
The Extended Radio Date System shortly xRDS is a solution with which broadcasters can multiply the speed of data transmission in the FM channel in a simple but compatible way. By using further normal RDS subcarriers, shifted into the higher frequencies in the FM multiplex.
Coding and modulation of the xRDS sub-carriers can happen by the use of standard RDS encoders what makes the implementation cost effective, or by new encoders what makes the compressed mode possible, this increase the capacity 25-50% for each stream. The individual subcarriers are mixed up to their frequencies defined by the standard being prepared at the time of writing. Finally they are added to the baseband mpx signal modulating the FM exciter.
On the receiving side the xRDS subcarriers should be mixed down to 57 kHz so that they can be demodulated and decoded by standard RDS decoders. After this the outputs of the individual RDS decoders are uncompressed if necessery and multiplexed into one data stream having a data rate of multiples of the standard RDS data rate. Modern DSP-based receivers can be easily programmed for xRDS decoding. According to this the price of the xRDS ready receivers will not rise considerable.
While the RDS recommendation defines the amount of the Other Data Application data to be transmitted, for xRDS there is no such restriction. Therefore the net data rate usable for other applications is not direct proportional to the number of xRDS sub-carrier injected, but it rises even faster. Beyond this xRDS has some reserves. The standard RDS data stream contains frequently repeated information not necessary to be transmitted in the xRDS streams. In order to leave out these data certain modification of the RDS encoders needed, but it will increase the data rate of the system further.
The standard will make it possible to use more than two xRDS sub-carriers but in practice not more than three of them are probable to be used.