The idea behind digital radio, as with digital TV, was to broadcast more stations in the same bandwidth but it was never going to work. Analogue broadcasting makes very efficient use of bandwidth. The down side is that there is no possibility of error correction so noise cannot be distinguished from valid data. To maintain data integrity in the face of signal degradation you have to digitize - and up goes the bandwidth.
Consider the bandwidth of CD quality audio. 44,100 samples per second at 32 bits each consumes over 700 kHz - and that's without error checking! The same information could be broadcast using SSB in a 44 kHz slot. FM, as actually used by analogue radio, is more wasteful but it's nowhere near as bad as 700 kHz. The same problem arises with digital TV and the solution is the same; compress the data.
Analogue TV consumes about 6 MHz per channel. That's enough bandwidth to transmit digital data at 12,000,000 bits per second. Now look at DVD where you can get eight hours of TV, complete with error checking, onto a 4.7Gbyte disk. That's about 1,400,000 bits per second and yet the quality easily outstrips analogue TV. That's why you can broadcast six digital TV channels and several radio channels in a single UHF slot.
Audio does not compress so easily. MP3 at 128 kbits per second is just about good enough for the average listener but it takes 65536 kHz of bandwidth to transmit - and still no error checking. Audiophiles expect better from their new digital radios so let's try MP3 at 320 kbits per second. With error checking on top you're pushing 200 kHz per channel which is on a par with analogue FM. Yes, you can have noise-free radio but you can't get more channels in without sacrificing quality.
If you want high quality radio at home you can get it through your TV digibox. If you want a vast choice of stations you can get them on the internet. If you want high quality radio on something you can carry around then, and only then, do you consider DAB. More than anything else, two factors have killed off DAB: the receivers have been grossly overpriced and quality has been sacrificed big time. I'm pleased to say that I never bought one.
