Digital Radio, DAB and RDS

Digital radio transmits sound as a stream of digital data rather than as a continuously varying analogue waveform. Digital Audio Broadcasting (DAB) is a widely used standard for this purpose. The audio is compressed, combined with error-correction coding and spread across many closely spaced carrier frequencies. This approach resists the noise, fading and interference that affect analogue AM and FM, so reception is either clear or, where the signal is too weak, silent, without the gradual hiss and distortion of analogue radio.

A single DAB transmission carries several radio services together in one multiplex, sharing the available capacity. Listeners select stations by name from a list rather than by tuning a frequency dial, and the broadcast can also carry text such as the station name, programme titles and short news items. Because services are bundled, a region may receive many stations from one transmitter block.

The Radio Data System (RDS) works alongside conventional analogue FM broadcasting rather than replacing it. It adds a low-rate digital data stream to the FM signal, inaudible to the listener, which carries useful information. This includes the station name shown on the receiver display and a programme-type (PTY) code that identifies the kind of content, such as news or music. RDS also supports traffic features: the traffic-programme (TP) flag marks stations that carry traffic news, and the traffic-announcement (TA) flag signals that an announcement is in progress so a receiver can switch to it automatically.

Frequently asked questions

How is DAB different from FM radio?

DAB sends audio as digital data with error correction across many carriers, giving clear reception or none, whereas FM is analogue and degrades gradually with hiss and distortion as the signal weakens.

What information does RDS add to FM?

RDS carries an inaudible data stream alongside FM that supplies the station name, a programme-type code and traffic flags, shown on the receiver display.

What do the TP and TA codes mean?

TP marks a station that broadcasts traffic information, and TA signals that a traffic announcement is currently on air, letting a receiver switch to it automatically.