Earlier in this blog I’ve shown you an FM signal, so now it’s time to show you the newer digital equivalent! Digital Audio Broadcasting (DAB) is becoming popular in the UK, and most modern radio sets sold have DAB reception capability. DAB offers a greater choice of radio stations than available through the FM or AM broadcast bands, and these stations are available in alphabetical order rather than having to remember a frequency.
DAB works by assembling a group of radio stations into a single signal, called an ensemble or multiplex. Each ensemble signal is owned by an organisation licensed to broadcast by the UK’s radio communications regulator, OFCOM (Office of Communications).
The BBC, Digital One, Digital Radio Group and Switch Digital are among several DAB licencees. Radio stations contract with one of these licencees to carry their content on their ensemble. Digital One owns a UK-wide licence, so stations can reach a national audience. There are regional and local ensemble licencees too.
DAB works technically by transmitting a data stream of bitrate 1184Kbps (kilobits per second). Each radio station’s audio is compressed using the MPEG2 audio codec. Depending on the quality of the sound required, the compression can be vary from 192Kbps (BBC Radio 3 when playing orchestral music in high quality stereo) down to just 48Kbps for some ‘talk radio’ stations. Most stations are given a bandwidth of between 80 and 128Kbps. The higher the bitrate, the better the perceived sound quality
The bitrate is changeable at any time (BBC stations on their National DAB ensemble alter bitrate throughout the day depending on each station’s programme content) but the total bitrate for all the stations together on one ensemble cannot exceed 1184 Kbps.
Here are four DAB signals side by side as received through my HackRF software-defined radio and SDRSharp software here in north London:
If you look at a DAB signal close up, you will see that it is formed of what looks like a ‘sawtooth’ – these are the signal peaks of 1,536 separate carrier signals called subcarriers that are transmitted in parallel with each other. Each of these subcarriers are spaced 1KHz apart, so the whole ensemble signal takes up 1536 KHz of radio spectrum – the spread of signal form left to right in the image. Here is a close-up of the BBC National DAB signal:
Further reading: