On Fri, Jun 13, 2003 at 01:06:47PM -0700, Mark Knecht wrote: > >From my reading, which hasn't been that deep, all of these sorts of problems > add up to a few bad bits here and there. When we 'play' a CD using a modern > drive, it is my understanding that we read the disk one time and play what > ever we read. I'm no expert on the physical layer of CDs, but I've read something about it. There is error correction built into the mechanism of reading data from the physical medium of a CD. CDs have multiple kinds of error protection. Data mode CDs use a coding called Eight-to-Fourteen Modulation that uses fourteen physical bits to encode eight data bits. In addition, the CD data framing uses two interleaved Reed Solomon codes (CIRC). These codings allow CD-ROMs to always reproduce the exact same bits in a deterministic fashion. Audio sample are stored somewhat differently. The EFM coding doesn't really apply, but dual interleaved Reed Solomon codes are still used. The first application of RS is used to correct the kinds of burst errors you're talking about. The second application is used to correct random erros, and even problems with the first attempt at error correction. All said and done, CDs can replace something like 3500 lost bits in a row. Of course more bits than that, and you'll start loosing the bit-identical-ness of the CD. (Google for "cd circ" for some decent links) -- Ross Vandegrift ross@xxxxxxxxxxxxxxxx A Pope has a Water Cannon. It is a Water Cannon. He fires Holy-Water from it. It is a Holy-Water Cannon. He Blesses it. It is a Holy Holy-Water Cannon. He Blesses the Hell out of it. It is a Wholly Holy Holy-Water Cannon. He has it pierced. It is a Holey Wholly Holy Holy-Water Cannon. He makes it official. It is a Canon Holey Wholly Holy Holy-Water Cannon. Batman and Robin arrive. He shoots them.
