Downloaded music comes in a variety of formats, and not all are compatible with each computer and audio player. The most common formats - MP3, WMA and AAC - are described as lossy and compressed, which means that some sound quality is lost in order to reduce file size.
MP3 (MPEG-1, Layer 3)
MP3 is the most universal format, compatible with almost every player and desktop computer. But the major record labels have tended to reject it as it does not offer DRM (digital right management) - the ability to control how tracks are used by consumers. In addition, Microsoft and Apple have preferred to push their own formats. However, MP3 remains popular with listeners and is backed by many independent labels.
WMA (Windows Media Audio)
WMA was developed by Microsoft and is widely used on Windows PCs. It is compatible with most music players though not Apple's iPod. Standard WMA files can be opened on Macs using free Flip4Mac software (www.flip4mac.com), but WMA files sold online usually include Microsoft's DRM protection and can only be opened with Windows.
AAC (Advanced Audio Coding)
AAC is a successor to MP3 and is widely used by Apple software and on iPods. It comes in two flavours - protected with DRM and unprotected. Most music purchased through the Apple iTunes Store uses the former and can only be played on particular audio players: iPods, Motorola's Rokr phone, and Apple's new iPhone.
ATRAC3 (Adaptive Transform Acoustic Coding 3)
Sony's digital music format. Its use is restricted to Sony devices and it can only be bought through Sony's Connect store.
The following are offered by some specialist sellers:
Closely related to AAC.
Vorbis is an open-source format and therefore not subject to licensing fees. It often beats other lossy formats in audio tests.
FLAC (Free Lossless Audio Codec)
This open-source format uses lossless compression and so allows smaller file sizes without losing quality. Alternatives to FLAC include Apple Lossless and WMA Lossless compression.
WAV files are not compressed so no quality is lost – but the files are large.