The Philips Pocket Expanium, like all its siblings, is an MP3 player which uses CD-R media. The difference is that this one is about half the size because it uses 8cm (21 minute / 185MB) CD-R media instead of the usual 12cm discs, and can hold about three hours of music in MP3 format.

If you've owned a full size unit, you'll know that they don't normally fit in pockets, which explains why this particular model is so interesting. Of course, for those with no pockets at all, there are also many solid-state devices using Memory Stick, Compact Flash etc., but CD-R media is much cheaper than Flash memory, and that means that you can burn compilations to CD and keep them, rather than return to your computer every time you want a different playlist. Currently, blank 8cm CDs can be bought for about £1 each. While this is about three times the cost of a normal blank CD, it's also about 300 - 500 times cheaper per megabyte than flash memory.

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So, the Philips unit is a neat compromise between unit size and media cost. It fits comfortably into most trouser pockets and shirt or coat pockets, and is not too heavy at 180g. The only down-side is that the main unit can't be clipped onto anything. If you want that, buy the Nike sports version which offers an optional belt strap.

The unit itself is very nicely designed, with well-organised buttons which have a positive click action. The dot-matrix LCD display is clear and quite well organised, although it isn't backlit and neither is the supplied remote, which we feel is a bit of an oversight for a device with as many options as this.

The Pocket Expanium 431 has a gold-plated headphone socket which doubles as a line-out, and the remote unit plugs into this. On the remote, you can select 'albums' (folders of MP3 files), tracks, shuffle modes and volume, as well as stop, start and resume tracks. The supplied headphones are the in-ear type and can be plugged into either the main unit or the remote; they provide quite acceptable audio quality. If you don't wish to use in-ear headphones, you can substitute other types and still use the remote unit.

Sound quality is good, and the unit achieves quite a high volume without distortion, although it might not please metal fans. Bass and treble can be digitally enhanced, but there are no graphic equaliser controls.

Rechargeable AAA batteries are included, along with an AC adaptor, and batteries can be charged in the unit. Battery life is rated at about 8 hours of playback time, although of course alkaline or spare rechargeables could be used when these run out, so you're not limited by any proprietary battery format. To achieve 8 hours playback time, the Expanium uses 'powersaving technology', which means that the 480 second anti-skip protection is reduced, but this is so good anyway that we were unable to cause the unit to skip at all, powersaving or not!

The Philips Pocket Expanium 431 is not entirely unique in the market. At the time of writing, we know of four other 8cm MP3 players, three of which are a bit cheaper than the Philips unit, while the fourth is basically a Nike-badged version of this unit. The build quality, style and smaller bulk of the Philips Pocket Expanium make up for the price difference, although you'll need to get your own MP3 ripping software as this isn't included in the higher price, whereas competitors Freecom and Napa do include some software.

The Philips Pocket Expanium 431 is a well-designed unit with some great features. Media costs of around £1 per 8cm CD-R mean that this is a great alternative to larger CD-R or Flash memory based units. It could do with a backlit display and some software for the price, though.