CDs and music players

[Waterwolf]

You know when you play an audio CD on your computer, and the music player programs automatically retrieve the song details from the Web? How does that work exactly? I can see that there might be some kind of 'signal' present on a newer CD (though that's pure speculation on my part), but I've also seen it work with fairly old CDs. Can anyone shed some light on this?

[slimjim]

I think it just looks at the number of tracks and the length of each track and, in nearly all cases, this is enough to make an exact match.

[jenstar]

A CD has an index which tells the player where to find each track on the disc; it also contains the matrix number of the disc. The look-up program simply reads this number and returns the appropriate info for that disc.