RDS - Radio Data System
- an overview or tutorial about the basics of RDS, the radio data system used on FM transmissions for carrying data.
RDS or Radio Data System is standard on most car radios and hi-fi tuners today. RDS is used on VHF FM radio broadcast transmissions and provides a number of facilities that are of great use to all radio listeners, but particularly to those radio listeners in cars. RDS enables traffic reports to be received more easily, and provides many facilities including enabling the radio station name to be displayed on the radio display.
The system has gained a considerable amount of popularity and is widely used in Europe where it has been established for a number of years.
RDS, Radio Data System Development
The Radio Data System development took place mainly in Europe where it was first launched and deployed.
The first developments took place in Germany where system was developed to place traffic information onto FM broadcasts using a 57 kHz subcarrier.
This trial development was taken up by the European Broadcasting Union, EBU when in 1974 their technical committee proposed a development of the German project to carry traffic information as well as other data. It would also allow automatic re-tuning of a receiver when it went outside the range of one transmitter, and it would provide facilities such as programme information, etc.
Using experience from the original scheme as well as a modulation format from a Swedish paging system and baseband coding developed by the British Broadcasting Corporation, BBC and the Irish radio, IRT, the first RDS specification was launched in 1984.
The standard was subsequently enhanced in 1991 with features including alternative frequencies functionality and it was published under the auspices of the European Committee for Electrotechnical Standardization, CENELEC.
In North America the idea was taken up and the US National Radio Systems Committee issued their version known as the Radio Broadcast Data System, RBDS in 1992.
The CENELEC standard was updated in 1992 with the addition of Traffic Message Channel and in 1998 with Open Data Applications and, in 2000, RDS was published worldwide as IEC standard 62106.
The RDS system offers a wide range of very useful facilities. The most widely publicised one is that of being able to provide travel news. This is available on most local radio stations. All of these stations transmit the TP code to identify that travel messages are flagged by RDS. When the radio is set for travel news it will only tune to stations which carry the TP indication. As the station is about to broadcast a travel announcement the TA code is transmitted. If a CD or cassette is being played then most sets will actually pause the CD or tape and then allow the travel announcement to be heard. In addition to this the volume may also be set slightly higher to allow the announcement to be heard more easily.
RDS brings intelligence into the tuning of a radio. The autotuning facility comes into its own on long journeys when the car moves from the service area of one transmitter to the next. Without RDS the radio has to be manually tuned to the next station. This is not always easy because it is difficult to reliably detect which is the strongest station.
An RDS set will look for the Programme Identification or PI code. A national network will be broadcast from a large number of different transmitters around the country. The station or network eg Radio 4 will have its own PI code. When the radio moves out of the range of one transmitter the radio will seek the strongest signal which has the same PI code, allowing the radio to remain tuned to the same programme.
When radios fitted with RDS store a station frequency, they also store the PI code along side it. This has the advantage that when the radio is turned on in a place outside the coverage area for the transmitter frequency which is stored then the radio will seek the strongest signal which has the correct PI code.
Local radio stations also have a PI code. In view of the local nature of these stations the PI code works slightly differently.
If the station has two or more transmitters then the PI code will operate in the normal way when it is range of these transmitters. However when the radio moves outside this coverage area it will retune to the strongest signal of the same type of station.
The PI code consists of four characters. The first indicates the country of origin and for the UK this is C. The next one indicates the type of coverage. The figure "2" indicates a national station, and the final two characters are the programme reference. For example Radio 3 has the PI code C203 and BBC GLR has C311.
RDS instant tuning
It takes a number of seconds for the radio to search for the strongest signal with the correct PI code. During this time the radio would mute itself and the listener would have an annoying gap in listening. To enable the set to tune itself very quickly from one transmission to the next each transmitter broadcasts a short list of frequencies of adjacent transmitters. This vastly reduces the amount of seeking which the radio set has to perform. In addition to this a second front end is often employed to constantly detect the strength of the alternative frequency transmissions. This results in much faster changes in setting - to the extent that the listener should not be able to detect when the radio changes from one transmitter to another.
Another facility associated with tuning is called the Programme Service Name (PS). This enables the set to display the station name. This normally takes a second or two to come up on the display after the station has been tuned in. However it is a most useful facility with the ever-increasing number of stations on the air
Additional RDS facilities
A new feature which has been added to RDS is called Enhanced Other Networks (EON). This allows the set to listen to one station like a national network, but still be interrupted by travel news from a local radio station. This feature even allows announcements to be heard whilst travelling in silence or listening to a tape.
EON requires a large amount of co-ordination between the different stations. To achieve this, the BBC have a central computer specifically for this purpose. When a local radio station is about to transmit a traffic message the fact is flagged to the computer. In turn this directs the relevant national radio transmitters to indicate this fact, thereby enabling the radios to change frequency to the local radio station to receive the message. Once the message is complete the radio will return to its original station.
EON is relatively new and the first sets to have it included only appeared in 1991. Although it is being introduced on more sets, the majority still does not have it. However with manufacturers constantly bringing new sets onto the market EON should be included on far more sets in a year or two.
By Ian Poole
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