Wave farewell to AM radio, say experts

The Telegraph talks of "the higher quality of digital broadcasts". Compared with? FM? Erm... potentially, yes; unfortunately, most stations on DAB are streamed at a bit-rate nowhere near enough to do them justice, especially for the codec (MP2) that DAB uses.

MP2 (more precisely MPEG-1 Part 3 Layer 2) is a cousin of the de facto audio encoding standard MP3. The bit-rates of stations on DAB vary from 192 Kbps (if you're lucky) down, with 160 Kbps or 128 Kbps being typical values. Some stations are encoded at 64 Kbps.

According to this article, MP3 was designed to achieve the same perceived quality at 128 Kbps as MP2 manages at 192 Kbps. And to audiophiles, MP3 at 128 Kbps betrays a noticeable drop in sound quality. To achieve FM-quality audio with MP2, a bit-rate of 256 Kbps is required (source: www.digitalradiotech.co.uk), although you can get away with 224 Kpbs or (at a pinch - and cost) 192 Kbps. In August 2006, 98% of stations broadcasting in stereo on British DAB were encoded using a bit-rate of 128 Kbps (Ibid), equivalent to MP3 at 85 Kbps. It's significantly worse than a good-quality FM signal sans hiss.

Fortunately, a successor to MP2 is around the corner. AAC+ (or MPEG-4 HE-AAC) manages to achieve near-CD quality at 128 Kbps, and will be progressively introduced into DAB sets. So you might want to hold off buying a new set (or replacing your old one) for the time being. The logical conclusion to all this is that MP2 broadcasts will be phased out.

A note to manufacturers: consider designing sets that can automatically download and install new codecs as and when they become available.