It's now many years since the first MP3 player arrived on the market, and we're increasingly seeing video and photo viewing features on what used to be simple portable music players. This convergence seems likely to continue, which is why we've put both types of player - music and multimedia - in the same category. There are differences, though, and in this guide we'll cover both separately.

We'll start with music players, still usually referred to as MP3 players even though most of them support other formats too, and some can't actually play MP3 files.

Since the runaway success of the iPod, hardware MP3 players have become big business and highly desirable. Yet don't be fooled into thinking you need to fork out the high price that Apple's after to enjoy a quality portable digital music solution. Far from it, in fact. MP3 players actually start from under £50, although before we talk price too much, it's best to go through the features that differentiate the various players on the market.

Let's start with capacity. The smallest players can hold around 64MB of data. If you assume that an average three minute song in MP3 format takes around 4MB (and we're being a little generous there), such players can hold 15-16 tracks at best. At this level, the units themselves are effectively flash drives, and thus double up as a portable storage solution too.

They also tend to be physically very small. At the top end, an 80GB player (which will measure the size of roughly one-and-a-half cigarette packets) contains a hard disk rather than a flash storage solution and can hold 20,000 tracks, which should easily eclipse most people's record collection! If you're spending around £100, at the time of writing, that should secure you around 4GB of storage, whereas you need to be forking out closer to £300 to get your hands on 80GB.

These capacities all assume you're using the MP3 format when storing your tracks, however, which brings us on to our next point. Bog standard players will take MP3 tracks, sometimes WAV (although the file sizes makes them cumbersome for portable music) and usually WMA (those created by Windows Media Player). Slowly emerging as the format of choice for digital music enthusiasts though is OGG Vorbis, and it's worth checking - if you're serious about your music - that any player you buy supports it. Not many do at the time of writing, and MP3 will be the mainstream solution for some time.

The interface is next. Aside from the very bottom-of-the-range players, your unit is going to have some kind of display and menu system through which you control it. That means you need to be looking at the buttons mounted onto the player as well, to see if you are comfortable using them. The screen itself is worth consideration, too. While most flash players have a perfectly adequate two or four line screen (and generally they're quite easy to read), the hard disk players store more content, and thus need to provide you with a better way of navigating through it.

Look here for functions such as backlighting and clarity of text, and see if you can see your player of choice in action to make sure that the menu system and means of control are to your liking. Good music is one thing, but eyestrain shouldn't be a price for it. It's perhaps of lesser importance on smaller capacity, flash based drives, but when you're potentially talking about thousands of tracks being stored, then a bit of research here is worthy of your time.

Battery is next. Flash solutions tend to run off standard AA or AAA batteries, and give you several hours of playback off just one of them. There's more going on inside a disk-based player though, and that drains power. So pretty much all of them come with a Li-Ion battery that gets you at least ten hours of playback. Check that figure before you hand over your cash, though.

And then there's software. At some point, you're going to have to hook your player up to your computer in order to get tracks across to it. While flash drives are happy just to sit in a USB port and have music copied across to them, with no requirement for further software (although an up-to-date version of Windows or Linux is a necessity), disk-based players should contain a decent smattering of software to help you rip and manage your tracks.

Music Match Jukebox is an excellent title to look out for (it's supplied with the majority of the Archos line), but then the simplicity of Creative's own Nomad Explorer for managing your music collection shouldn't be understated either. Try to get a player that supports USB 2.0 if your PC has it - it will genuinely save you a lot of time. Most now do, but there are some surprising exceptions.

Finally, there's aesthetics to consider. Only you know how important that is to you, but as the success of the iPod proves, sometimes good looks can justify extra cash. Suffice to say, the MP3 player market is flooded with designs to suit pretty much all tastes.

Now we move onto multimedia players, sometimes known as multimedia jukeboxes, portable media players or portable video players.

In a similar way to the advancements in audio that have brought us MP3 players and compressed music formats, we are now seeing compressed video gathering momentum. Running alongside developments in video media that have taken us from VHS to DVD and most recently, high definition video discs such as Blu-ray, are compressed media formats such as AVI and encoding standards like DivX.

A number of sites are now offering downloadable video content, often at high resolutions, and of course if you have invested in a digital video camera yourself you'll probably have a fair amount of content already. Viewing this digital media is usually a no-brainer; you simply copy it to your PC and watch through Windows Media Player. This is effective enough as a temporary measure but most of us would probably prefer to watch on a proper TV, either for the larger screen or to take advantage of the excellent resolutions now available on HD-ready displays.

If you're a bit of a computer guru you've probably worked out how to do this from your laptop, using either an S-video or alternative output to send content to an appropriate connection on your TV. This is often quite an awkward process, and in recent times we've seen a series of dedicated 'Multimedia' players that allow you to play back video, audio and view photos on your TV with almost no effort involved.

Set to continue to gain in popularity, these fairly simple devices are based around a simple hard drive along with decoding hardware that sends the video through a choice of outputs, including composite, component and S-video. As a result it's a simple matter to wire the unit up to the auxiliary port on your TV (if it has one) or, if you have a SCART converter box, through a regular SCART connection.

The popularity of such devices underlines a growing interest in a straightforward solution to get the most from compressed digital video. You can pick one up fairly cheaply, but should make yourself aware of the capabilities of such devices before you buy.

The first and most obvious thing to look for is capacity, since this most directly relates to the price of the product. Some designs behave as external hard drive enclosures, allowing you to buy a standard 3.5-inch or 2.5-inch drive and fit it into the unit yourself. Others include built-in hard drives that can't be swapped out, but are often available in a range of capacities. Which one you go for obviously depends on how much digital content you want to store on the device. If you're looking to save cash you can pick up something at the lower end of the scale, say 40GB, and just chop and change the content via your PC as and when you want to watch or listen to new media.

But probably the most important consideration, one that's frequently overlooked and often buried among the specifications of the player, is the maximum resolution supported. Different makes and models have varying maximum resolutions, and any content you try to play back that's been encoded at a higher resolution simply won't work.

With high-resolution HD becoming more widely available you'll ideally be looking for a machine that's capable of least 720p/1080i. Anything lower and you'll run the risk of finding future content that won't be compatible. Sometimes product specifications quote data resolution figures, such as 720 x 480 or 1280 x 720. In these instances the second figure relates to the figure used when quoting HD resolutions.

If you have a collection of media files on your computer that you're expecting to play back through a multimedia player, view the properties of the file to find the resolution of the video and ensure that the specifications quoted on any multimedia player you're thinking of buying are at least the same if not higher.

For reference, 720 x 480 is DVD resolution, 1280 x 720 is standard quality HD and 1920 x 1080 is Ultra quality HD.

Support for high resolutions is no good unless the device can send the video out at a decent quality. Most players come with a choice of connections. Composite is most common and all multimedia players will support this standard, but for higher HD resolution signals you should look for component connections as well, provided of course your TV supports them. HDMI outputs are the best and also support high quality digital audio. In terms of audio, your multimedia player will ideally have an S/PDIF output for handling surround sound and high quality digital, although standard composite audio is more common.

Moving onto file support, just about all multimedia players are compatible with AVI video formats, plus MP3 and JPG for photos. It's worth checking for compatibility with other standards such as MPEG-2, WMV or RMVB if you have files encoded in these formats; obviously the better the file support the fewer problems you may have in the future.

Usability is more difficult to judge when looking around for a player to buy. If you have the opportunity to arrange a demo or test a product, you're well advised to do so. The majority of players use a simple menu structure to help browse and play back audio and video content or view images, but this, along with the settings that allow you to configure the player's behaviour, varies from simple and informative to downright frustrating. For the most part this is a minor point but a worthy consideration nonetheless.

Buying a dedicated multimedia player such as this isn't the only way to watch or listen to compressed digital content. Some manufacturers of portable video players now supply accessories that'll help you port the video content out to a TV. The Archos 604, for example, comes with an optional DV-station that allows you to not only view video content from the player on your TV, but even record television into a digital format to watch on the portable player later. If you already own a portable video player, check with the manufacturer to see if they provide any DV solutions that'll allow you to do the same.

Remember to check to see what's in the box before you buy. If you end up having to purchase additional leads such as a component video cable because there isn't one in the box, you can end up spending quite a bit extra to get the best from your player.

All multimedia players should include an infra-red sensor and remote control; this is a must and you should avoid anything without one. Some have extra features such as memory card readers for easy viewing of photographs, network support or wireless connectivity for streaming content. Others are capable of upscaling video content to higher resolutions. It's worth considering paying more for these sorts of features if you think you'll use them.

And that's that. There's the odd exception to the guidelines above, but this should give you the grounding you need to make your choice. One point to finish on, as usual, and that's to read reviews of your player of choice. This guide, by its very nature, is quite generic, so if you can chase down some specific write-ups of your favoured model, than all the better.