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I've taken a look at things and the problem is that the UK jumped the gun with DAB in my honest opinion.
They utilised a codec (compression/decompression) spec that was derived from a listening panel in 1990. This meant a MP2 layer 2 codec. Freeview uses the same codec!
To cut a long story short, this meant that a bit rate of at least 192kbps (kilo bits per second) was needed for the sound quality to perceived as excellent. Sadly only Radio 5 does this.
DAB missed the boat in that it didn't use the MP2 layer 3 codec which meant the same perceived sound quality at 128kbps.
What the UK should have done is wait a while for the DAB2 standard to be mandated.
The digital radio streams are multiplexed (muxed) together. The radio stations share this mux meaning the available bits have to be shared amongst them.. As more stations are added to this mux, then the bits per station reduce.
They can use smoke & mirrors to give more bits to the parts of the audio that need it but there is only a finite amount of bits to go around.
There are more modem codecs available such as the AAC family, but these are only available in the DAB 2 standard.
Where they have shot themselves in the foot is that the DAB standard is not compatible with the DAB 2 standard.
To make matters worse, OFCOM have allowed broadcasters to reduce their bitrates if they buy better MP2 encoders but unfortunately as the saying goes, "you can't polish a turd..excuse my French".
This is why listening to the radio stations online is best because they can avail themselves of the best codecs and bitrates; FM is the second best; and then DAB & Freeview come in a lazy third.