Radio data systems (RDS) transmitter equipment changed in-car audio forever

25 years ago this month: 'RDS: The way ahead for radio'

In April 1989, the idea that a driver could ‘listen to Radio 2 all the way from Newcastle to Newquay’ without having to retune their car radio several times was still something of a novelty. Writing in E&T’s predecessor magazine IEE Review, B Marks gave a BBC perspective on the emerging technology of radio data systems.

In the UK, broadcasters have been working on radio data systems (RDS) transmitter equipment installations for the last 3 or 4 years, and something like 80 per cent of the population are within reach of an FM transmitter which is already transmitting some RDS features.

Well over 150 BBC transmitters are now radiating RDS and work is progressing to complete the necessary installations throughout the UK. Fortunately, a fairly high proportion of the remaining transmitters are relays of others, so they will start RDS as soon as their mother station is on air with RDS.

Across Europe, all major European Broadcasting Union members have now said they will provide RDS over the next few years, and several countries, like the UK, are well ahead with installing the necessary RDS encoders, notably Austria, France, Eire, Sweden and the Federal Republic of Germany. All broadcasters are conforming to the EBU specification [1, 2] which has been carefully nurtured by the broadcasters to create a large market, making it worthwhile for the radio manufacturing industry to develop the technology to take advantage of RDS.

There are some 16 RDS features already standardised, and it is interesting to note the differing features that the broadcasters have chosen to provide at the initial stages. Sweden, for example, has a network and local radio structure comparable with those of the UK, and broadcasters have chosen similar features, apart from the addition of radio paging (RP), which is already served in the UK. This is not entirely surprising, since RDS has its origins in their earlier 57 kHz paging system. France has started RDS with RP being a major consideration, because it can provide revenue to enable the long-term expansion of other RDS features.

Comprehensive details of RDS have already been described in other articles [3] and I shall not go into them in depth here. Instead, I shall attempt to describe the background and give a limited insight into the technology.

RDS has been designed to be upwardly mobile, allowing the broadcaster and radio manufacturer to add features when the demand develops. It will also be possible compatibly to add new features which have not yet even been dreamed of. BBC Radio felt that RDS was of major importance in helping the listener to find the already high-quality programmes — both editorial and technical — that it offered. So the prime objective was to find features which would meet this criterion, yet cost relatively little to provide, because it was not desirable or possible to arrange for RDS to be charged to the listener by an increased licence fee. Also, there would initially be relatively few listeners able to make use of RDS, because new radios would be required to take advantage of the system.

At this stage, it is worth describing the features and introducing the concept of static and dynamic RDS. All BBC FM transmitters with RDS (both national network and local radio) provide the following static RDS features:

• Programme service name (PS) (e.g. BBC Radio 3)

• Programme identification code (PI) (e.g. C203)

• Alternative frequencies list (AF)

• Other networks information (ON)

• Clock time (CT)

Briefly, these features provide automatic tuning capability, station naming, near-instant retuning to other services, and that ubiquitous feature of modern electronic consumer equipment: time.

The advantage that the PS will give the listener is obvious. All those confusing frequency numbers are redundant — all that matters is the station's identity (e.g. BBC Radio 3, which can be found on some 20 different frequencies around the UK).

The PI code C203 is never seen by the listener but it is vitally important for the radio to ensure that all data received with that reference are relevant to BBC Radio 3 and do not become confused with any other data received. Alternatively, you may like local radio and already have your BBC local station pre-programmed on a memory button on the receiver. If so, then by a special method being pioneered by the BBC, known as generically linked PI codes, second-generation radios will build up knowledge about other BBC local services and load them into the memory behind your local-radio button, so that once you are out of range of your first choice you can press the button again and the radio will retune to an adjacent BBC local-radio service.

The AF list comprises all the frequencies geographically surrounding the presently received transmission, so that in whatever direction the radio should be taken it will always have a selection of possible new frequencies to choose from to maintain reception of that service.

Supposing you want to retune to another BBC service after driving 150 miles or so — the likelihood of knowing the appropriate frequency for BBC Radio 4 (say) is quite slim. RDS can come to your aid with the ON feature, which has been telling your receiver, as a background routine, all about the other BBC services, so the radio will already know what frequency BBC Radio 4 is available on in that area. Additionally, ON allows the radio to react to status changes on other services and thus provides an 'intelligent' response to various useful radio advances.

Clock time (CT), of course, cannot be totally static. In fact, it is transmitted in a special group on the minute edge, but the RDS encoder has this built into the software and no external command is required to put this group into the data stream. In the BBC system, CT is synchronised by off-air reception of MSF 60 kHz from Rugby, to give the accuracy considered essential these days.

Apart from CT, the other features are all derived from PROM-held data in the RDS encoder, which will continue cycling around to provide the necessary groups in a predetermined sequence. Hence the description 'static RDS'.

It is important to note that, at present, local radio transmitters can provide only static RDS because they only have a programme circuit from the local studio to the transmitter, and no data connection with the central RDS computer in London. However, network transmitters can provide dynamic RDS features because they are connected to the BBC NICAM distribution system, which sends high-quality digital audio to the transmitter and sends RDS update data to the RDS encoder to change the normal cycle of transmitted groups [4].

The following features are included in this category:

Travel programme (TP)

Travel announcement (TA)

Programme type (PTY)

Programme item number (PIN)

Radio text (RT)

Both TP and TA are simple 'flags' which can be set on or off, and they form the core of the RDS travel service.

TP signifies that this programme carries travel information at some time. A radio can detect it so that on arriving in a new area, with no prior knowledge of the radio stations there, a search tune could be invoked to find a station with a TP flag set on; the listener will then hear travel information during the programme.

The TA flag is more specific and indicates that travel information is being transmitted now. Several resultant radio responses are possible. At the lowest level, volume can be raised to ensure the announcement is heard; if cassette listening is in progress at that moment, then a switch back to radio listening will again ensure the information is received.

However, another application of the ON feature permits an even more refined RDS travel service to be constructed, so that whatever programme you are tuned to, you can hear the local travel information from the nearest BBC local radio service. It is possible to reference all the other BBC services and inform the radio about TA flags being switched on for BBC local radio services. Thus, one can be listening to a national network service and the radio will 'find' a travel message from the nearest BBC local radio station, giving relevant, timely and, with any luck, accurate information about the local travel conditions.

Buried in this process is a lot of data communications between the travel announcement studio, the national network transmitters, the local radio transmitters and the central RDS computer in London. The BBC is pioneering methods of effecting this cheaply. This is achieved by update data being labelled with a service number so that RDS data can be received off air at the local radio transmitters, to command them to switch their flags, without the need for additional data circuits.

Returning to some of the other dynamic features, programme type (PTY) allows broadcasters to label their programmes with a code which the radio will in the future be able to use for tuning purposes. For example, if one wants to listen to news, the radio could be asked to search for a programme with a News PTY code. If none is available at the time, then the radio could be set to switch to news as soon as it became available. This PTY feature has both speech- and music-based codes, which are also standardised with the European radio satellite codes to ensure greater user acceptance.

The programme item number (PIN) code uniquely identifies each programme during the day and could be used to switch the radio on at exactly the programme start time.

The radio text (RT) feature can provide up to 64-character-long messages, and would be particularly useful on a tuner to give the phone-in number for a programme or the address to write to for further information.

Clearly a vital ingredient in the development of RDS has been the development work needed by the radio manufacturers around the world. Initially, the car-radio manufacturers have been deeply involved: probably every one is developing a radio, simply because of the enormous advantage that the automatic tuning aspect gives to these products. At least they have to decode PI and AF to achieve automatic tuning, and most also provide station name from the PS.

We can already see that a number of different implementations are possible for displaying the PS; some receivers use dot-matrix displays, some use starburst displays, some are back-lit for increased visibility. For the user, dot-matrix displays have the added advantage that upper- and lower-case characters can be displayed, which aids station name recognition if the broadcaster is using both. BBC Radio has studied display recognition, and uses upper-case for the national network stations but finds that a mixture is best for local radio, where the slight limitation of only eight characters is easily overcome — e.g. Radio Cambridgeshire is seen as 'Cambridge’.

The display features of RDS are very important, but the radio manufacturer has to use RDS for control functions if automatic tuning is to be implemented; the processing of the incoming data stream demands considerable complexity to decode with enough accuracy to act on the data in a logical way. This needs a great amount of development to achieve sensible responses from the radio in the variable reception conditions that will exist as it is moved about. The car-radio manufacturer had another challenge to pack all the extra electronics into a very small DIN/ISO case; thus, large-scale integration has been a must from the start.

One may assume that any of the static RDS features could also become dynamic on a network transmitter, and the BBC is working on this. In the crowded FM band, the BBC has been forced to make maximum use of the allocation over the years by putting different programmes on FM from those on medium and long wave at various times of the day. Perhaps one of the best known examples is the schools broadcasts, which during school term are carried on Radio 4 FM while Radio 4 LW carries other programmes. With RDS, the listener who has just tuned to Radio 4 FM in the middle of a schools broadcast has a good clue to what is going on from the PS name being displayed as 'BBC R4Ed\

RDS is a complex subject for the manufacturer and the broadcaster so it must be treated very seriously by them if the listener is to rely upon the automatic responses which have suddenly become possible in the receiver. Maintaining perfect sympathy between the listener and broadcaster has been a vital objective of the EBU, and the BBC has given strong support to that idea. But, for the listener, RDS must be very simple, so our complex discussions must eventually be distilled into a very easily used product. Indeed, with the first-generation RDS receivers we are finding that it really is quite boring if all you want to do is listen to Radio 2 all the way from Newcastle to Newquay, for 'BBC R2/T is all you will see on the front of the radio — but unobtrusively it will have retuned at least six or seven times on the way and, incidentally, told you, by means of the PS display, that Radio I can sometimes be found on that FM frequency.

Manufacturers are now making radios, especially car radios, with RDS and many other features, such as cassette decks. They are complex and inevitably quite expensive. Like all technological developments, the prices are expected to fall progressively. There are already some compatible tuners about, and it seems likely that RDS will be incorporated into more in the future. When CMOS technology becomes available, portable receivers can be expected to take advantage of RDS too.

Acknowledgement: I am indebted to the Director of Engineering of the BBC for permission to publish this article.


1 'Tech. 3244-E: specifications of the radio data system RDS for VHF/FM sound broadcasting', EBU document, 1984

2 CCIR recommendation 643 (Dubrovnik, 1986)

3 PARNELL, S.J.: 'Decoding RDS1, Electronics & Wireless World, 1989, pp. 148-152

4 MARKS, B.: 'What is happening to RDS?', Electronics & Wireless World, 1988, pp. 1096-1100 129

Originally published in IEE Review, April 1989 (Volume 35, Issue 4)

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