peteh,

if what you say were true there would be no need for drives to re-read data which has been incorrectly read. i can assure you that most 'drives' be they cd-rom or hard do indeed have to re-read data when incorrectly read at some point or another.

i'm not sure of the exact specifications of the rs coder used for cd-a but as a system rs is better at correcting 'burst' errors (errors which occurr one after the other) than randomly distributed errors. obviously this will deal with physical defects on the disk better than random electrical noise induced data errors.

an example would be if you were using an rs system where for every 255 bits of data you used 32 bits as parity information, and had 8 bit words then you'd be able to correct up to 16 individual errors so one 50 bit 'burst' of errors would be fine but if those 50 incorrect bits were randomly distributed through the 232 bits of real data then there would likely need to be a re-read. also if the errors occur during the reading of the parity bits then you also run into problems.

another thing is that you've got to remember that cd's are 20+ years old. the amount of processor power necessary to impliment rs decoding is not trivial so red book probably uses a low number of parity bits and so won;t be able to rescue many 'words'.... i'm winging it now so i'll shut up.

cheers

julian