by Whet Moser
That spiral track of information is very small on a CD. A traditional album side, 20 minutes long at 33 rpm, has a spiral that winds around the record about 650 times; stretched out, the groove would be about the length of a football field. A CD groove, by contrast, is about half a micron wide. (Human hair is about 40 times as thick.) It would stretch out to about three miles.
The problem with that tininess is the number of things that can go wrong on that scale. The most minuscule particle embedded on those dots, the most microscopic flaw in the disc itself, any scratches on the outside of it--all of these things can disrupt the information flow, by eliminating not just a few but thousands and thousands of "bits." Such disruptions are inevitable, and in fact appear by the dozens on any CD: the CD has to compensate. That's what the error encoding does.
It actually does two things: it gives every 16-bit word (actually it's two 8-bit words, now; to make things easier, they're split in half) something called an "error code." That's the Reed-Solomon part. And it rearranges the data into a nonsequential order--that's the Cross-Interleave.