Sunday Times Describes the Audio Quality on DAB as being "Dire"

10th June 2005

David Hewson, who writes in the Doors technology section of the Sunday Times, wrote the following last weekend about the audio quality on DAB:

"DAB radio is in the same sad state as ever. Like just about everything else in the digital audio field, it is technically and aesthetically inferior. DAB works by compressing audio - taking out bits and hoping we won't notice. It does this not using the old standard of MP3 but the even older one of MP2, and the results are dire."

Sad but very true, I'm afraid.

It's interesting to note that the BBC have installed new MPEG audio encoders for their DAB stations this year, and yet the audio quality of the BBC music stations is still dreadful. These new encoders were hoped by many in the DAB industry to significantly improve the audio quality on DAB, and they have improved the audio quality on the BBC's 128kbps music stations, but you have to consider just how poor these stations sounded before the encoders were installed, because they really had to be hears to be believed just how incredibly poor they sounded. So, although the new encoders have improved the audio quality a little they still sound dreadful on average, and literally jaw-droppingly-bad at worst -- unfortunately, it is jaw-droppingly-bad for a significant portion of the time.

One of the "quirks" of highly-compressed audio such as that used on DAB is that the audio quality varies dramatically over very short periods of time. This variability is caused by the fact that the audio quality depends on how easy or difficult the source material is to encode. You can hear this variability on most tracks that stations like Radio 1 play, because if there is one instrument or vocals in the foreground while the other instruments are faded into the background then the encoder handles this pretty well, but then when all the instruments kick-in again there's simply far too much information that needs to be encoded, and the encoder fails miserably, with the result being little better than you'd expect on AM radio.

A peculiarity of the audio codec used on DAB (MPEG Layer 2, or MP2 for short) is that it uses the intensity stereo method for joint stereo encoding, and joint stereo is used on all of the low bit rate music stations -- which account for 98% of all stereo stations on DAB in the UK. Intensity stereo encodes all of the higher frequency part of the audio signal by adding the signals from the left and right channels and encoding a value that signifies the direction from which the sound comes from. Unfortunately, this totally destroys the relative-phase (time-difference) information between the left and right channels, and because it is this relative-phase information between the left and right channels that creates the stereo image (because it is the relative time-difference at which a sound reaches your left and right ears that the brain interprets as the sound coming from a certain direction) the stereo image on low bit rate DAB stations is either very poor or absolutely non-existent, and this exacerbates the problems with the lack of definition on anything that isn't very simple to encode, because all of the ill-defined instruments combine together in the centre of the "stereo image" to form something which is best described as being a horrible mess. A good example of the kind of problems that I've described above that you get on DAB can be heard in this audio sample that I recorded off Radio 1 today:

Radio 1 audio sample (560 KB)

The problems with the audio quality on DAB described above are all caused by the bit rates being used are simply far too low. The first problem alluded to above where any audio that isn't simple to encode sounds very poor would be hugely improved if higher bit rates were used because it is exactly at the times when the encoder requires more bits to encode the audio that the audio quality degrades by so much. And the second problem of using intensity joint stereo coding is also helped because at higher bit rates the encoder can choose whether to encode the audio using two discrete channels where there is a large difference between the left and right channels, and only resort to using intensity joint stereo when the difference between the channels is small and using joint stereo would be beneficial to the overall quality -- at low bit rates all higher frequencies are encoded using joint stereo and the encoder cannot intelligently choose which is best, because there simply isn't enough bits available in the first place.

Unfortunately, the MP2 audio codec is now about 14 years old, so it is a very well understood codec, and because most improvements in efficiency (that is, the bit rate required to provide a given level of audio quality) occur mainly at the beginning of the codec's lifecycle and you get diminishing returns as time goes on, then we are very unlikely to see much, if any, improvement in the audio quality that 128 kbps can provide. And this shouldn't be surprising, because the MP2 audio codec was designed to be used at higher bit rates such as 192 kbps to 256 kbps, and it is virtually always used at these higher bit rate levels for TV channels' audio channels (e.g. BBC 1, 2, 3 & 4 all use 256 kbps for their audio channels), so using MP2 at 128kbps was always pushing it beyond its capabilities.

The truth is that FM provides the best audio quality out of all of the ways you can receive radio in the UK at the moment, and this is mainly because the audio quality isn't degraded by the data compression as DAB is, so if audio quality on the radio does matter to you, then the best advice is to buy a decent FM tuner such as the Denon TU 260L which can often be found on ebay.co.uk going for incredibly low prices such as £20 - £30, which is an absolute bargain for such a great sounding tuner, and use a better aerial than the piece of wire that usually comes with tuners and hi-fi systems. Such aerials can be bought for only £15 from places like Maplin Electronics (e.g. £15 FM dipole kit, search for product code XP09K) or CPC Farnell (£9.60 for an FM dipole, search for FM1081), and the main advantage of such aerials compared to the usual wire aerials you get free with tuners and hi-fi systems is that the aerial doesn't have to be right next to the hi-fi system, and you can place the aerial in a better reception position in the room -- the best position will usually be close to a window, and FM aerial is on my window-sill. The best place to install an FM aerial is on your roof, or failing that in the loft, but, of course, many people cannot or don't want to do that. But such a modest investment of £15 on a new aerial can dramatically improve FM reception, and it's a shame that rather than giving good advice like this, broadcasters such as the BBC will just push everybody towards DAB even though the majority of people will just listen to the same stations on DAB as they have been doing on FM, but these stations are at a lower audio quality on DAB.