Introduction to Digital Audio Broadcasting (DAB)

The DAB system was designed in the late 1980s, and its main original objectives were to provide radio at CD-quality; to provide better in-car reception quality than on FM; to use the spectrum more efficiently; to allow tuning by the name of the station rather than by frequency; and to allow data to be transmitted. DAB fulfills most of these objectives, but with one rather important exception: DAB sounds worse than FM.

Why is the sound quality so bad?

The main reason why there is a problem with the audio quality on DAB is due to the broadcasters using bit rate levels that are too low to provide good audio quality. The reason why they're using insufficient bit rate levels is due to DAB using the inefficient MP2 audio codec, which needs to be used at bit rate levels of at least 192 kbps to provide good audio quality -- FM provides an audio quality which is equivalent to 192 - 224 kbps MP2. Unfortunately, 98% of all of the stereo stations on DAB in the UK are using a bit rate level of 128 kbps, hence the audio quality is poor. This problem of using low bit rates doesn't only affect the UK, either, because the handful of other countries that are trying to promote the old DAB system -- Denmark, Norway and Switzerland -- are also using low bit rate levels.

The reason why such low bit rate levels are being used is because the broadcasters have decided to launch quite a lot of new digital-only stations, but as there is only a limited amount of spectrum available for DAB to use, the broadcasters decided to use low bit rate levels in order to fit these new stations onto DAB even though they knew full well that the audio quality would be lower than on FM.

The broadcasters and Ofcom try to make this out as being a "trade-off", but the reality is that audio quality was sacrificed in order to provide more stations. For example, the broadcasters decided to use 128 kbps for stereo stations, and this allows 9 stations to be carried in a DAB multiplex. If they reduced the number of stereo stations to 8 rather than 9 then half of the stations could transmit at a bit rate of 160 kbps, which would provide a significant improvement in quality, albeit that it would still sound worse than on FM.