The EBU Specification 3244-E [24] defines the format of the transmitted RDS data, together with various applications which meet the requirements of several broadcasting organizations. Since their RDS transmissions conform to the specification any receiver manufacturer may produce receivers that will function correctly.
Broadly speaking, there are four main spheres of application currently envisaged.
In the first three applications the receivers will be fully integrated, in that the RDS decoder, the associated processor and resident software will all be fitted when the receiver is manufactured. For the message system the RDS receiver may well be fitted with an RS232 output so that it can be linked to a PC and function as a one-way modem in a business system. To provide for future expansion, there are a number of application areas as yet unspecified but these can be simply implemented by transmitting appropriate designation codes as detailed in Specification 3244-E.
The data is transmitted as 16-bit words, each of which is associated with a special 10-bit check word for error protection. Four such 26-bit blocks form a group as shown in Figure 11.1. Sixteen groups, each of which can be an A or B type, are allocated to cover the various applications, but to date only 11 have been specified, as indicated in Table 11.1. This allows for future developments. No additional bits are required for framing or other synchronization purposes as this information can be derived from the data stream itself, by the use of the special checkwords.
![]() Figure 11.1 RDS group composition |
An important feature of the RDS system is its flexibility. The different group types can be inserted in any order, to suit the requirements of the particular set of applications chosen by the broadcaster at any given time. Each group is an entity in itself and can be decoded without reference to any other group. The only overriding requirement is that the RDS data block containing the programme identification (PI) code must occupy the same fixed position in every group and be repeated sufficiently frequently to allow receivers with automatic tuning to operate, with a reasonable response time.
The main features of the message structure are shown in Figure 11.2. The key functions are as follows:
![]() Fig. 11.2 RDS message structure |
The first data block in every group always starts with the PI code.
The first four bits of the second block of every group specify the application of the group. Groups are referred to as types 0 to 15. Each type has two versions (A or B) defined by the fifth bit of the block. In A versions the PI code is in block 1 only and in B versions it is in both blocks 1 and 3.
The programme type code (PTY) and traffic programme identification (TP) occupy fixed positions in block 2 of every group.
The PI, PTY and TP codes can be decoded independently of any other block to minimize the acquisition time for this type of message and also to retain the advantages of the short block length (26 bits). This is achieved by inserting a special word, offset C, in block 3 of version B groups. The presence of this word indicates to the decoder, without reference to the fifth bit of the application reference code group, that block 3 is a PI code. Table 11.2 gives the recommended minimum repetition rates given in the EBU specification for some of the main applications, and Table 11.3 a typical group mixture to achieve it.
Table 11.2 Recommended minimum repetition rates for some main applications (EBU Tech 3244-E)
Applications | Group types which contain this information |
Recommended minimum repetition rate per second |
Programme identification (PI) code | all | 11 |
Programme service (PS) name | 0A, 0B | 1 |
Programme type (PTY) code | all | 11 |
Traffic programme (TP) identification code 11 | all | 11 |
Alternative frequency (AF) code | 0A | 4 |
Traffic announcement (TA) code | 0A, 0B, 15B | 4 |
Decoder identification (DI) code | 0A, 0B, 15B | 1 |
Music speech (M/S) code | 0A, 0B, 15B | 4 |
Programme item number (PIN) code | 1A, 1B | 1 |
Radiotext (RT) message | 2A, 2B | 0.2 |
Table 11.3 Typical group mixture to achieve the recommended repetition rates
Group type | Applications | Typical proportion of
groups of this type transmitted |
0A or 0B | PI, PS, PTY, TP, AF, TA, DI, M/S |
40% |
1A or 1B | PI, PI, PTY, TP | 10% |
2A or 2B | PI. PTY, TP, RT | 15% |
3A or 3B | PI, PTY, TP, ON | 10% |
Any | Optional applications | 25% |
Several potential applications for RDS have been specified in detail; most are termed optional. The compulsory signals, which must be included in order to comply with the EBU specification, are associated with the identification of the radio station itself.
These signals alone would allow the implementation of a simple automatic receiver, with display of the station name.
The programme service name (PS) is a string of eight
ASCII-coded alphanumeric characters intended for display on a
receiver. The simplest possible RDS receiver would make use only
of this feature, being otherwise tuned conventionally, but with a
positive display of the identity of each station.
The PI is a 16-bit code to identify the particular radio station, or network, originating the broadcast. All transmitters carrying the same PI code carry the same sound signals. A receiver can therefore search for a particular PI code and thus find a particular broadcast service. The first four bits of the code identify the country of origin, the second four the type of service, (local, national etc.) and the final eight form a serial number for the particular station or network.
The PS and PI features alone would allow the implementation of receivers which could find and display a station's identity automatically but the service can be considerably enhanced if further information is transmitted. For example, consider the case of a car radio as it nears the edge of the service area of a particular network transmitter. As the signal fades, the receiver will recognize that it must find another transmitter carrying the same programme. It could do this by scanning the band, searching for the same PI code, but this would inevitably take a few seconds, particularly as the receiver would need to stop on each receivable signal for long enough to decode the RDS bit stream. A receiver with two 'front ends' could be doing this all the time, and loading its memory with details of receivable signals as it encountered them, but the whole process can be greatly simplified and speeded up if each transmitter radiates information concerning the frequencies on which its neighbours can be found. The receiver then need only look at these specific frequencies to check for a signal at a receivable level. It is anticipated that a receiver with a single front-end will be able to achieve this quite adequately.
The RDS feature which accommodates this type of information is called alternative frequencies (AF). A list of up to 25 frequencies can be transmitted, and it is important to note that MF and LF channels can be included, thus catering for the situation where VHF-FM coverage may be incomplete.
A further improvement to the potential utility of an RDS receiver is provided by the other networks feature (ON). Consider the case of a receiver which is in use on a particular station, and the listener wishes to quickly tune to an alternative station. With the information so far available, the receiver would again have to scan the band from one end to the other in order to identify the strongest signal which carried the appropriate PI code. The time taken could well be too long.
This problem has been overcome by the introduction of an improved arrangement called 'enhanced other networks' information (EON). Transmitted together each station's signal is information about frequencies on which other services, referenced by their PI codes, can be found. This improved arrangement gives the receiver a sequence of PI codes and associated frequencies, for each of the other services available in the service area of the transmitter. An RDS receiver therefore has available in memory the station frequencies for the alternative stations.
Broadcasting these four types of information allows the development of truly intelligent receivers which not only find a station on request but can also tune to the transmitter for best reception as the receiver moves about the country. The receiver can also respond almost instantaneously to a request for a different station. This is achieved with no demands on the user for pre-programming, or indeed for any knowledge of the station frequencies involved.
The functional diagram of such a receiver is shown in Figure 11.3. The user selects the programme and the processor controls the tuning so that the appropriate transmission is received. The RDS data is decoded and all the relevant information concerning alternative frequencies is stored in memory. The station name and clock time will also be displayed. Multipath reception causes a low frequency AM component to be produced by the AGC system. The AGC system can provide a signal representative of signal quality, and this may be used to initiate the selection of an alternative transmitter.
![]() Fig. 11.3 RDS receiver with automatic tuning |
Traffic programme identification (TP) is an on/off switching signal to indicate to the receiver whether this is a programme which carries announcements for motorists. Traffic announcement identification (TA) is also an on/off switching signal which indicates when a traffic announcement is actually on the air, thus allowing automatic switching away from another radio station, or from tape cassette listening.
The operation of auto-tuning and a traffic information service is shown in Figure 11.4, which illustrates reception in a motor car moving along a motorway. At the start of the journey the vehicle would normally receive signals from the main transmitter (A). This transmitter carries in its RDS data the frequencies of all the other transmitters located at the same site together with those in the neighbouring sites (EON). Traffic announcements for the local area are radiated by transmitter (B). In response to a signal from transmitter (B), a traffic announcement (TA) flag is include in the RDS data of the main transmitter located at (A). This causes the receiver in the car immediately to change channel to that of the local transmitter (B) on which the traffic announcement is about to be broadcast. The TA flag could, in a suitably designed receiver, cause the cassette recorder (if in use) to stop, so that the radio would automatically receive the traffic announcement from station (B). At the end of the announcement the TA flag would be cancelled in the RDS data from transmitter (B) so that the receiver immediately reverted to the original programme from the main transmitter, or the cassette would restart.
This arrangement allows local traffic announcements, which are relevant to a particular location, and broadcast by a local station, to interrupt a national programme (or tape). It frees the national networks from the need to broadcast such information and provides the motorist with immediate information on local conditions. As the car continues along the motorway, it will reach the fringe of the service area of the main transmitters located at (A) but the AF codes in the RDS data enable the receiver immediately to tune to the same programme, but from the main transmitters at (C).
![]() Fig. 11.4 Auto-tuning and a traffic information service |
The RDS data from all the programmes from the transmitters at (C) again all carry the AF codes for the main transmitters in adjacent areas together with the frequencies of any local transmitters. Again, if a traffic announcement is to be made from the local transmitter (D) covering a different local area the TA flag would be set and the receiver in the car would immediately tune from the main programme of the transmitter at (C) to the local transmitter (D) for the period of the traffic announcement and then switch back to the programme channel as before. The motorist is therefore always able to receive the best transmission for the programme of his or her choice and at the same time is able to receive any traffic announcements covering local areas through which the motorist may be travelling. As the receiver moves from the area of the main transmitter it is automatically reprogrammed with the alternative frequencies for the adjacent areas into which it may move, without any involvement from the user.
The RDS signal is digital and each of these codes represents no more than a single bit in the bit stream. It is thus practicable to repeat the codes frequently in order to give the required rapid response.
It takes very little of the RDS data stream capacity to transmit a digital code for the time and date. The broadcaster can derive the time information from a national standard, and manufacturers can implement clocks in radio receivers which are truly accurate, requiring no setting and automatically correcting themselves for changes, such as daylight saving time and leap years. They will also incidentally show the time as it is in the country of the broadcast, which could be useful in a journey across national boundaries and time zones.
Clock time and date (CT) consists of a binary coded representation of time and date using Co-ordinated Universal Time (UTC) and calendar date. A local offset is appended in order to allow the RDS receiver to generate an appropriate display of local time as required. Programme related data Different codes can be allocated to different types of programme, so that a receiver could search for the user's choice, or display the programme type.
A further code, decoder identification (DI), could be used to switch-in appropriate signal processing, for example noise reduction. The music/speech code (MS) permits automatic selection between two volume settings, adjusted by the user to suit his or her preferences. The programme identification number (PIN) is an unique code which facilitates unattended recording.
Text messages can be transmitted as radiotext (RT), and the RDS specification gives a format for a 64-character display at the receiver. Applications for radiotext include financial information, sports and news flashes. The specification also allows for the addressing of individual decoders so that such services can be private. The individual decoder address facility may also support a paging service, implemented on an RDS channel, Receivers designed to operate in conjunction with a paging service are similar to conventional fixed frequency pocket pagers, but with automatic station tuning. They may either display simply a telephone number or alternatively a message limited only by the size of the display and the associated processor. The tuning arrangements in such receivers involve a search through the VHF channel to find transmissions which are carrying an RDS signal having a recognizable paging code. The pager identifies the group code address and synchronizes itself in time. The pager then only responds at appropriate predetermined times, typically 6 seconds per minute, in the interest of economizing in battery power. The 'subscriber interface' is to the data management system of the paging service, which feeds the RDS coder located at the transmitter through an RS232 data link. The FM radio signal acts as a carrier and the service can be operated independently of the broadcasting operation.
A transparent data channel (TDC) has been specified and anticipates a requirement for downloading software. The in-house application facility (IH) is intended for the broadcaster's own use, for control or similar purposes.
The RDS specification determines the basic decoder requirements so that receiver decoder manufacturers are able to produce a product, in volume, suitable for a wide range of applications. As has been described, the facilities which are specified are very flexible, to suit the differing requirements of various organizations. A national broadcaster may wish to provide alternative frequencies so that a user in a car, for example, does not have to re-tune as the car moves between different service areas of transmitters. On the other hand, a commercial station may wish to operate a paging or information service, to generate revenue.
Variable data would normally be sent to the RDS coder at the transmitter over telecom links from the appropriate information centre. However, fixed data concerning transmitters is held in the coder in an EPROM.
A re-broadcast (RBS) decoder can be used to take the data derived from the multiplex signal of a receiver and reformat it into an RS232 feed which conforms to the RDS data update specification. This signal is then used as the input to the remote RDS coder. This arrangement assists the broadcaster to 'network' the variable data from transmitter-to-transmitter, avoiding the need for further telecom links.
Data can also be distributed to FM radio transmitters from a television network, using the teletext data channel. As the RDS data rate is quite low it does not impose a very significant overhead on the teletext service. The teletext decoders, located at the FM transmitter sites, decode the data and reformat it as the RS232 drive to the RDS coder. Individual station codes can be used as required so that only specified decoders will respond, thus confining the distribution of the data to specific transmitters.