Each Album and Track is converted to a number of different mp3 formats.
.The highest quality, 320kbps, is what we deliver to the users who buy the Tracks and Albums.
We also create a 128kbps variant for mobile downloads and a 96kbps variant for the previews.
Keeping the previews in a low bitrate format allows them to be delivered very quickly and efficiently to the users, with very short buffering delays.
The quality is a bit of a tradeoff: good enough to listen to and enjoy the songs; not good enough to download without paying the artist.