jonathan.rawle.org

Readers could be excused for not knowing that a new digital radio multiplex launched in the UK today, containing 18 new stations. There has been little in the way of fanfare; in fact, there has been virtually no media coverage at all. Take-up of DAB radio continues to be poor, thanks mainly to two factors. One is that people don’t see the need for, and are not interested in, the additional stations available. The other is that the sound quality fails to live up to what was promised: better than FM quality.

A quick glance through the list of stations shows that the stations are largely more of the same from the same big commercial broadcasters. More Absolute radio, More Kiss, more “chill” and “smooth” pop music stations. These are hardly going to have people rushing out to buy a DAB set. There are also stations aimed at particular religious or ethnic groups: Muslims, Christians, the Asian community. Surely these are only going to attract a small number of listeners? There is also a children’s station, plus Jazz FM makes a welcome appearance on national terrestrial radio (having only previously been available in London or online).

Sadly, when it comes to sound quality, it’s the same old story we have become familiar with on DAB. Most of the stations are in mono, and have bitrates or 80 kb/s for music, or 64 kb/s at a lower sampling frequency for speech stations. Once again, rock bottom sound quality in order to squeeze in the stations, and that fantastic improvement of stereo FM to mono DAB. There is, however, one interesting new development. Three of the stations are being broadcast in the newer DAB+ format, using the HE-AAC v2 codec (also known as aacPlus). What’s more, these stations are in stereo! Before we get too excited, though, the bad news is that the DAB+ stations are only being broadcast at 32 kb/s. Now, while HE-AAC is much more efficient than the MP2 codec used in old-fashioned DAB, is it really that much more efficient? The people who developed HE-AAC claim 48 kb/s gives similar performance to MP3 at 128 kb/s. But we only have 32 kb/s. Do these new DAB+ stations sound even as good as the few existing 128 kb/s DAB stations such as Classic FM?

A quick listening test immediately shows the answer to be no. Tuning to Jazz FM on its new DAB+ channel, the sound quality leaves a lot to be desired. There is that clear rasping sound, an immediate sign that the audio has been encoded using too low a bitrate. Switching to the 128 kb/s MP3 internet stream, the difference is stark. A much wider soundstage, clearer, deeper, smoother tones, none of the artefacts due to low bitrate. Even if anyone thought the DAB+ version was OK, I would challenge anyone to say they couldn’t hear the difference in the MP3 version.

Turning now to Classic FM on DAB, at 128 kb/s MP2, although the sound is always slightly harsh, it doesn’t have the same raspiness that belies a low bitrate. Switching to internet radio, 128 kb/s MP3, there is still an audible difference, but it is more subtle. The sound is more pleasing to the ear, but it’s harder to place quite why. The conclusion is that 32 kb/s DAB+ does not sound as good as 128 kb/s DAB. But then, it only takes up a quarter of the bandwidth.

It would be interesting to know why Jazz FM chose to go down this route. Were they desperate to broadcast in stereo? Their London broadcast is only mono. Or did they want to save money by going for DAB+, and stereo is just a by-product of that? The argument against switching to DAB+ in the UK is that many people have DAB sets that can not decode DAB+. However, I’ve long doubted that to be significant. Radios sold for a long time now have been DAB+ ready (although some Pure branded sets apparently didn’t actually include the HE-AAC codec to save on licensing fees, making a complicated download process necessary!) Surely only very early adopters, and then only those who have hung on to their by-now rather antiquated DAB sets, wouldn’t be able to pick up the new DAB+ stations? In any case, the naysayers have been proved wrong, and DAB+ has come to national digital radio.

Jazz FM and the other two DAB+ stations should be considered experiments. If they attract a significant number of listeners on DAB+, that should make the case for other stations to swtich to DAB+. The trouble with digital radio is always that these is commercial pressure to squeeze in the stations, and that means poor audio. By switching from DAB, to DAB+, you can have the best of both worlds. An 80 kb/s mono DAB station can become a DAB+ at 56 kb/s – the highest possible bitrate for HE-AAC – providing higher quality, stereo sound in 70% of the bandwidth. That way, it would be possible to improve the sound quality of all of the stations on the original national commercial DAB multiplex, while leaving room for four or five additional stations. Everyone wins except a few people with old DAB radios, and if they are early adopters, they will have enjoyed the original DAB transmissions at 320 kb/s, and will likely have thrown the radio in the cupboard in disgust at the degradation of the service many years ago.

The current choices made for the DAB+ broadcasts are a poor demonstration of the technology, just as squeezing the DAB bitrates have made those stations unpleasant to listen to. Yet if, despite this, DAB+ listener numbers are seen to be on par with DAB, and it results in an industry-wide switch to DAB, something good could yet come of it, and the UK may eventually have digital radio broadcasts that live up to their original promise.