There are three main types of MP3 players, which differ mostly in terms of their method of data storage. Each has its own pros and cons.

For reference's sake, the average MP3 file is somewhere between 3MB and 5MB at 128Kb/s quality, and runs for three and a half minutes. So 128MB can store around 32 tracks, or three albums, or nearly two hours. 256MB can store 64 tracks; 512MB 128 tracks and 1GB 256 tracks, or 15 hours.

First off the ranks were solid state devices. Current models sport 128MB or 256MB of internal Flash memory, and some of these include expansion slots for memory cards like Secure Digital (SD) or Memory Stick. As solid state storage devices contain almost no moving parts, they are theoretically less prone to failure, and don't suffer the dreaded 'skipping' that CD-based MP3 players are renowned for. The limitation then, is one of storage, and the cost of flash memory, which is far higher than the other media. The USB-based players can also function as thumb drives, which is an added advantage.

Then there are CD-based MP3 players, both for full sized 12cm CDs or smaller 8cm CDs. The storage capacity is limited only by the CD media itself. 8cm discs store up to 200MB, and the full sized discs are available with up to 800MB of storage, but are usually limited to 700MB. These devices play standard audio CDs as well as CDs containing MP3 audio files, which is an added advantage. Despite various anti-skip measures, CD MP3 players don't have the mobile durability of the solid state players, so jogging with them can be a breakbeat experience.

The other type of MP3 player houses a micro hard drive, with models ranging from the original 6GB Creative Nomad, through to the new 40GB Apple iPod. However, with size, comes cost, and these devices are still at the top end of the price bracket. One advantage of some hard disk-based players is they double as portable storage. Just plug it into a USB or FireWire port, and it comes up on most new OSes as a removable drive.
Despite the various storage methods, however, all players use the same technique to actually generate the audio. Essentially, a Digital Signal Processor (DSP) applies a decompression algorithm to an MP3, or equivalent, file, and the file is then passed to the Digital to Analogue Converter (DAC), which changes a stream of numbers into a sound wave. A small processor within the player handles things like the user controls, reading and displaying track information, and so on.

Different players produce different quality of output, depending on the components used in their production. They also include different features, and as well as MP3, support different audio standards. Taking that on board, we tested these players using a standard set of criteria.

Storage
We looked firstly at storage capacity, including any expansion options, but we rated this taking into account the price of the player. It wouldn't be fair to simply compare a device with 128MB of available storage to something with 20GB or more, unless you consider the additional costs such storage adds.

Sound quality
We chose a range of tracks, encoded at rates from 128Kb/s to 192Kb/s, across a range of genres. From hard rock to classical, including techno and even a Gregorian chant. To listen for background hiss, we included a spoken word track.
The same files were tested across each player, as we listened for depth and strength of bass, clarity and smoothness of mid tones, and sharpness and detail of upper tones. We took into account any available EQ presets, and tested with both the bundled earphones and a pair of Sony MDR-7506 studio quality cans.

Installation/ease of use
What good is a portable music device, if you need a degree in engineering and 12 fingers to operate it? We took a look at simplicity of navigation; size and position of controls; and readability and amount of information on the display.
In terms of installation, we considered the steps required from opening the package to listening to music. Are there onerous software installation requirements, or can you just plug it in and copy your files over?

Other features
As this round up is about MP3 playback capabilities, we took into account any other features that were included, but didn't focus on them. An FM tuner is a nice inclusion in many players, and considering the miniaturisation and low cost of radio components, it is surprising that even some of the latest model MP3 players do not feature this.
Voice recording capability is also heavily featured, but would have narrow appeal in the context of mobile audio, but is useful for business or students.

The features we were more interested in were specific to MP3s. For example, the level of support for ID3 tags and any bundled MP3 creation software (which surprisingly few catered for).

Whilst looking at these players, we also took into account the build quality, the aesthetic appearance of the device, user guides and so on.

Scores
We used the new PC Authority Labs scoring system, where we score each player for Quality, then find the average score across all players and see whether each individual player is above or below average. We then make a score of 100 the average, so if a player scores 115, that means it's 15 percent above average in that department.
We then do the same for Features, then combine the scores at a weighting of 60% quality, 40% features, and divide by the price to determine the Value score. This means a player with a lower price will get a high Value score. The Overall score is just an average of the other three scores.