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.