With digital its all to do with the number of bits used to encode the sound we hear.

Just to reiterate (from another thread) the number of bits used in digital sampling just sets a resolution limit on the dynamic range i.e. the loudness, that is recorded (although note that the bit-depth mapping to dynamic range can mean that the most quiet of sounds may only use two or three bits of the total bit depth - if it's very quiet then the most significant bits will all be zero). The upper frequency limit is dictated by the sampling rate alone and is not dependent upon the bit-depth of the sample.

I'll just add that DAB, like MP3 and Ogg Vorbis, is a lossy codec and the bit-rate doesn't have a linear relationship with either the depth of the sample or the sampling rate. Neither DAB (in any of its variants), MP3 or Ogg Vorbis can be regarded as Hi-Fi (where Fidelity is the key word).

However, the primary issue, when comparing analogue and digital music sources, is not one of resolution, either in sampling depth, or directly, in the sampling rate, but is one of phase coherency.

According to the Nyquist-Shannon sampling theorem, any frequency can be accurately sampled if the sampling rate is at least twice the frequency of the sampled signal. So far, so good. However, frequencies higher than half the sampling rate are not just degraded but may be regarded as completely random; they're not just a bit wrong, but totally unreliable.

Because you can't vary the sampling frequency as you record the original music to ensure that you're always sampling at a high enough rate, because if you're sampling it you can only know the frequency after you've sampled it, a fixed sampling rate is used and frequencies above half of the sampling rate are filtered out.

The real issue arises because the filters needed to remove the frequencies that are too high for the sampling rate have an adverse effect on the phase of the signals passing though them, with the result that the phases of different frequencies are shifted by different amounts; although all the different frequencies go into the filter 'in-phase', when they come out of the filter they are no longer so.

This, and the quietness problem, are the biggest problems that audiophiles have with digital sound sources.