Most of the criticisms levelled at digital download services have been about price and digital rights management technology, and while both issues are important – for example, the Legacy Edition of Jeff Buckley’s Grace is £15.99 on iTunes and comes with copy protection, whereas the CD is £12.99 in Amazon and my local supermarkets – I reckon there’s a bigger flaw: sound quality. To be blunt, the sound quality of legal downloads is terrible.

For many people it isn’t an issue – the popularity of MP3s at a frankly unlistenable 128Kbps never fails to amaze me – and the headphones supplied with most music players are awful anyway, but when you start listening to your digital music collection on decent speakers or through decent headphones the problem couldn’t be more obvious if your iPod displayed “this sounds crap!” in fifty foot letters of fire.

The problem is compression: whether you’re buying from iTunes or Napster, the music you get is compressed to make it a small file size, so downloads are typically 1/12th of the size of the original track. That’s great for portability, but file sizes aren’t really an issue now that even the cheapest MP3 players come with massive amounts of storage. And what you gain in portability, you lose in sound quality. To make the files smaller, some of the information has to be thrown away; the lower the bitrate the smaller the file, and the more that’s chucked out.

Different compression technologies work in slightly different ways, so for example there’s a noticeable difference between songs encoded with Windows Media technology and songs encoded with Apple’s AAC. But what they both have in common is that when you buy downloads, the songs you get are encoded at 128Kbps – which means that there’s a stark difference between the download and the CD version. There’s noticeable “swooshing” in the high frequencies, and if you like your music hard and heavy, you’ll discover that walls of distorted guitar sound positively weedy. It’s a problem with dance tracks, too: with downloads, the all-important low end thump is conspicuous by its absence.

I mentioned Jeff Buckley earlier. I did an experiment using his music: I downloaded Grace and Last Goodbye from the iTunes Music Store, and I compared them to the MP3s I’d already ripped from my CD (192Kbps, variable bit rate encoding). Compared to the MP3s, the iTunes downloads sounded positively anaemic: tinny, metallic and painful to listen to at high volumes. If you’re into making music, the best way I can describe it is that it sounds as if someone’s slapped on a compressor and turned it up to eleven.

The problem isn’t the format – it’s the quality. Both AAC (Apple) and Windows Media (Microsoft) formats are better file formats than MP3, so a 128Kbps iTunes or Napster download will sound better than a 128Kbps MP3. However, no matter which of the three file formats you go for, 128Kbps still sounds rotten. I’ve found that 160Kbps MP3s are better, but higher bitrate MP3s are better still.

There’s no reason why iTunes, Napster et al can’t offer higher bitrate music, and apparently Microsoft is promising to do just that with its MSN Music Store. I hope Apple does the same: now that high capacity MP3 players and broadband connections are commonplace, there’s no need to squish music files down to the point where sound quality suffers. Until then, I’ll keep buying CDs from Tesco and ripping them into iTunes, boycotting the discs whose copy protection won’t let me play them on my PowerBook.