| DAB vs FM sound quality22nd April 2007 Some people laughably think that DAB sounds better than FM! So this article will look at whether this ridiculous claim is true. If someone has good FM reception quality -- which means that they receive FM with a high signal-to-noise ratio (SNR) and without or with very little multipath distortion -- then what they're effectively listening to is the digital audio format used by the broadcasters to distribute their audio signals to the FM transmitter sites -- the FM system itself would be classified as "transparent", i.e. people would not be able to tell the difference in a blind listening test between the (correctly lowpass filtered and limited) audio going into the FM modulator and the audio coming out of the FM demodulator (think of the hypothetical situation where there's a wire connecting the transmitter and receiver, so there's no loss or distortion). The only possible difference that some ultra-eagle-eared person might spot is the stereo separation, which I'll come back below. The BBC uses NICAM to distribute the audio signals of its stations to the FM transmitter sites, and NICAM uses a bit rate of 728 kbps where 14-bit linear PCM samples are 'companded' down to 10-bit where large amplitude input samples only have 10-bit resolution but small amplitude input samples have the full 14-bit resolution -- the human hearing system is less sensitive to relative errors when the amplitude is high than it is to errors when the signal is small, and the SNR of high amplitude samples is also higher than for small amplitude samples, hence why NICAM uses a lower resolution for large amplitude samples and higher resolution for small samples. Basically, in a perfectly fair comparison where the same signal is being transmitted via 128 kbps MP2 on DAB and via FM with NICAM distribution to the transmitter sites, and where there was good reception quality on both DAB and FM, then if 128 kbps DAB ever sounded better than FM then I'm afraid that bears no longer shit in woods. Commercial radio stations use 384 kbps or 256 kbps with the APT-X audio codec to distribute its audio signals to the FM transmitter sites, and again, if 128 kbps DAB beats that then the Pope will have become a muslim. It simply ain't happening this millenium -- or at least not in a fair comparison... BBC Radio 3 is the only radio station in the UK where it's even arguable whether it sounds better on DAB or FM, and Radio 3 uses 192 kbps on DAB -- a 50% higher bit rate than the 128 kbps used by 98% of all stereo stations on DAB in the UK, and if Channel 4 wins the licence for the new national DAB multiplex it's planning to use 112 kbps for stereo stations, and you can bet that a lot of other stations on DAB that are currently using 128 kbps would follow suit. 128 kbps AAC vs NICAM would be a far closer battle, however, although I still think NICAM would win, but unfortunately the incompetent broadcasters and regulators who decided to use the old DAB system in the UK precluded the use of AAC on DAB -- although all of the BBC's stations will be available using 128 kbps AAC via Internet multicasting from this summer once the BBC iPlayer has launched. Back to the issue of stereo separation I mentioned earlier: Again, there is absolutely no contest between FM's stereo separation and MP2's laughable "stereo separation". The reason is that all radio stations that use 128 kbps use joint stereo, and joint stereo using the MP2 audio codec means having to use 'intensity stereo' where the left and right channel signal amplitudes are added together on a sub-band by sub-band basis and the ratio of the energies (the intensity) of the left and right sub-band signals is encoded. Intensity stereo destroys the 'signal envelopes' of the left and right channels, which is unfortunately the precise thing that gives the human hearing system its cues for which direction a signal is coming from -- d'oh! -- and intensity stereo really just amounts to pan-potted mono, so it doesn't actually even deserve to be compared on the basis of stereo separation. Comments
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Thanks for clearing that up!
Only trouble is that they plan to phase out FM and then quality loss will be compulsory! So much for choice and license fees eh?
FM switch-off
And I wouldn't worry about FM being switched off, to be honest, because the very earliest it could happen is 2020, and it'll probably be well after that IMO. The main problem the broadcasters face is that only 150,000 out of the 34 million cars and vehicles in the UK have got DAB car stereos, and the car manufacturers aren't exactly falling over themselves to fit DAB as standard.
I'm in the process of writing an article about this subject at the moment, which will be linked to from this page once it's finished if you're interested.
I actually don't think that there will be a permanent reduction in quality once FM is switched off, though. The BBC could provide its radio stations at the same or better quality as on FM via the Internet today if it wanted to. And Internet bandwidth is going to continue getting cheaper and more plentiful over time, so it's going to become increasingly difficult for the BBC to justify not providing its stations at high quality via the Internet as times goes on.