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RDS-TMC is a proposed new RDS feature for FM Radio (VHF) which provides a means for conveying travel information in a densely coded digital form, along with a conventional VHF-FM programme signal, such that it is independent of the normal sound programme. Using TMC, special RDS receivers employ decoding circuitry and digital storage to retrieve the messages and store them for future interrogation by a listener. Messages can be delivered in various ways- for example, a voice synthesiser could be used, in the language of the listener's choice.

RDS-ALERT was one of about seventy projects within the European Community's research programme, DRIVE, and was concerned with the development of RDS-TMC. Work, carried out in the period 1989-90, had a primary aim to build a consensus amongst broadcasters, traffic authorities and receiver manufacturers in order to establish a future standard.

The Â鶹ԼÅÄ participated in RDS-Alert to ensure that TMC would serve the needs of public service broadcasting in the future and to provide advice for the most efficient use of the limited RDS data capacity. Field trials were made in the Wrotham service area in early 1900 to establish the system performance, using two early versions of an ALERT coding strategy (A & B). Evidence was gathered on message reception reliability and system performance using a novel approach. Error statistics of RDS reception in a mobile receiver over a wide range of routes and RDS transmission characteristics were collected. Subsequently these were used in the laboratory to simulate the whole transmission chain and allow different coding proposals for TMC to be tested and analysed under identical conditions.

It appears that, with an RDS deviation of _+2 kHz, an RDS-TMC Service might be provided using a data rate of just one RDS group a second which is less than 10% of the total RDS capacity. This could be accommodated in existing Â鶹ԼÅÄ transmissions without prejudice to the integrity of the existing RDS Service. To achieve this, TMC messages must be kept short to ensure adequate reception reliability and the number of messages broadcast must be limited, probably to a few hundred at a time. With lower RDS deviations and/or where ARI signals are simultaneously present, RDS reception reliability may be only marginally adequate to support a TMC service.

A draft new coding proposal, ALERT C, has been developed as part of the work of RDS-ALERT and is based on the best aspects of the earlier A and B proposals. Although not yet formally adopted, ALERT C is suggested by the EBU and ECMT as a basis for future work. Further full-scale field tests of ALERT C are needed before deciding whether RDS-TMC would provide a useful traffic information service. These tests should also help to clarify the other, non-technical, features associated with such a service for example, the cost and acceptability of receivers, integration of the information with that provided by other broadcast travel services.