It's generally accepted that regular MP3 music files compromise CD sound quality for convenience and portability. (Some listeners argue that even CDs are less than optimal.) Last year, Amazon and iTunes made concessions to upgrade the quality of their download tracks.

Some artists want the bar raised even higher. Metallica announced last week that its upcoming untitled album, in addition to being released on CD, will be available as a higher-quality digital download ($12) and on audiophile vinyl in a limited-edition $125 boxed set. It's due this fall.

In technical terms, one measure of quality is the amount of data per second of music; more is better. Standard tracks often are 128 kilobits per second; Amazon's and iTunes' are as high as 256 kbps. Metallica's tracks will raise the data rate to 320 kbps.