Re: CD's, C64's and error-correction

From: John (
Date: 1999-10-08 10:38:34

Martijn van Buul writes:

>Let me put it this way. Suppose there's an error somewhere on a CD, say
>for 100 samples. The CD just can't read it, and has to interpolate, and 
>inserts data that won't sound too awful. A human mind won't notice
>the difference, since its a) a very short error (2 ms) and b) masked.

That's not how the samples are stored.  Audio CDs use an interleaved Reed-
Solomon error correcting code.  Bits belonging to one sample are scattered
over a wide area of the disk - nearby bits belong to different samples.
If you get a long run of errors (which is the most likely case for
scratches), you're going to lose a bit or two from each of many samples,
and the error correction can deal with that very easily (Reed-Solomon
is a class of codes which use redundancy to correct errors.  The more
redundant bits you add, the more error bits you can correct.  I don't know
off-hand the number of bits that the flavour used in CDs can correct).

Interpolation is only used when things go very very wrong.

A lot of the loss when going from audio to data is from giving the disk
more structure.  It's like a floppy disk - you can store a lot more on a
disk if you don't bother with sector headers and gaps.  There may be more
integrity checks as well, but an extra 15% in raw bits doesn't get you much
in the way of error correction.

John West
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