So what exactly is MP3? It's been closely linked with the Internet although it's not an exclusively Internet technology, and it seems that record companies talk about little else nowadays.

MP3 files are ordinary music tracks that have been digitised and stored on a computer. The beauty of MP3 is that it shrinks CD-quality music to manageable file sizes. Previously just one four minute pop song from a CD would have been around 40MB. With MP3 the file shrinks to 4MB with no appreciable loss of sound quality. And a 4MB file can be downloaded from the Internet in a reasonable time, effectively making MP3 the de facto form of digital music transfer.

At the heart of MP3 are mathematical algorithms that examine the music. If the left and right stereo tracks are identical, for example, MP3 encodes them as one track, saving file space (they're restored to stereo during playback). The algorithms also exploit weaknesses in the human ear. If a trumpet clashes with a violin, both are normally recorded even though the human ear will only hear a clash of sounds. MP3 encodes the 'clash' and discards the rest.

This might sound like the sound quality suffers. But it doesn't, and is to all intents and purposes indistinguishable from CD. In fact, some people (probably not those with gold-plated phono leads and green CD pens) say the sound quality is better because the fact that only the 'clashes' are encoded makes for a cleaner sound.

MP3 players allow you to play these files while out and about. They're nothing more than memory chips with circuits to convert the MP3 songs into audio which is then played through headphones. The players plug into a PC or notebook, usually by the parallel port, and music files are downloaded into them.

The benefits of MP3 players over, say, personal stereos or portable CD players are obvious. Aside from excellent sound quality, there are no moving parts so there's never any jumping or skipping. Without moving parts the batteries don't drain nearly as fast. Some of the players tested here work for around 10 hours or more on just a single AA battery.

We took a look at four popular MP3 players on the market. Note that we've reviewed two other MP3 players on this site - Diamond's newer Rio 500 and an earlier version of the Pine D'music. You might like to read those reviews too, and compare them with the products in this group test. Click the 'NEXT' link below to find out more.