FAQ

Rant Update (for those in the know): Several folks, through personal mail and on Usenet, have pointed out that audio discs do place absolute positioning information for (at least) nine out of every ten sectors into the Q subchannel, and that my original statement of +/-75 sectors above is wrong. I admit to it being misleading, so I'll try to clarify. The positioning data certainly is in subchannel Q; the point is moot however, for a couple of reasons. The SCSI and ATAPI specs (there are a couple of each, pick one) don't give any way to retrieve the subchannel from a desired sector. The READ SUB-CHANNEL command will hand you Q all right, you just don't have any idea where exactly that Q came from. The command was intended for getting rough positioning information from audio discs that are paused or playing. This is audio; missing by several sectors is a tiny fraction of a second. Older CDROM drives tended not to expect 'READ SUB-CHANNEL' unless the drive was playing audio; calling it during data reads could crash the drive and lock up the system. I had one of these drives (Apple 803i, actually a repackaged Sony CD-8003). MMC-2 *does* give a way to retrieve the Q subchannel along with user data in the READ CD command. Although the drive is required to recognize the fetaure, it is allowed to simply return zeroes (effectively leaving the feature unimplemented). Guess how many drives actually implement this feature: not many. Assuming you *can* get back the subchannel, most CDROM drives seem to understand audio discs primarily at the "little frame" level; thus sector-level structures aren't reliable. One might get a reassembled subQ, but if the read began in the middle of a sector (or dropped a little frame in the middle; many do), the subQ is likely corrupt and useless. As reassembling uncorrupted frames is easy without the subchannel, and corrupted reads likely result in a corrupted subchannel too, cdparanoia treats the subchannel as more trouble than it's worth (during verification).