DAB radio provides audio quality equivalent to FM broadcasts when using a 'bit rate' of around 160Kbps. (A lower bit rate provides inferior audio, a higher one provides superior audio quality).
Here are the bit rates currently in use by principal stations:
Radio 1 > 128
1Xtra > 128
Radio 2 > 128
Radio 3 > 192 (or 160 for some programming)
Radio 4 > 128 (or 80 for some programming)
Radio Five Live > 80 (Mono)
6 Music > 128
Radio 7 > 80 (Mono) or, I believe, 128 (Stereo)
World Service > 64 (Mono)
Classic FM > 160
Absolute > 160
TalkSport > 64 (Mono)
Chris