Most music downloads are compressed - which means their file size is reduced so that they don't clog up hard drives or internet connections. The most popular formats - MP3, WMA and AAC - use lossy compression. This means some sound detail is lost in order to reduce size. The resulting quality largely depends on the bitrate - how many bits of data are used to describe the sound. The higher the bitrate, the higher the quality - and the larger the file. Bitrate is measured in kilobits per second (kbps).
Some common MP3 bitrates are:
- 96kbps – FM radio quality
- 128kbps – decent quality
- 192kbps – good quality; difficult to hear imperfections
- 320kbps – very high quality; close to lossless
Some formats offer higher quality than others at the same bitrate. WMA and AAC files encoded at 128kbps typically sound as good as 192kbps MP3s (though not to everyone's ears - this remains an area of endless debate for audiophiles).
In addition, not all MP3s are the same. Some use 'LAME encoding' which is superior to standard MP3 files of the same bitrate.
As a rule of thumb, you should aim for MP3s of at least 192kbps, and AAC and WMA files of at least 128kbps - but use higher if available.