[mythtv-users] Working with DVD ISO images

[Alex Butcher]

On Tue, 16 Dec 2008, Brian Wood wrote:

> It's my understanding (please correct me if I'm wrong) that optical disks
> ALWAYS have errors, and even more errors are generated when the disk is read.
>
> CD and DVD drives have error correction of some kind, and as might be expected
> the quality of that correction varies from drive to drive.

Yes and no.

The error detection and correction is based on Reed-Solomon codes, so any
optical drive that complies with the relevant standards should implement the
same error detection and correction algorithm.

However, the mechanics and optics of various designs do vary - so
conceivably a disc that is marginal may read perfectly with no error
correction necessary on one drive, whilst being pathologically unreadable to
the optics of another - so bad, in fact, that there are so many read errors
in a given block that the ECC can't cope and can only return read errors to
the OS storage layer. Usually, things aren't as bad as that, and even a disc
with the odd light scratch can be read perfectly, even if it requires the
ECC to do so.

Furthermore, when we're talking about DVD playback software/firmware, it is
usually designed to be more or less tolerant of read errors. Poorer
implementations may crash, better implementations might have glitchy audio,
go blocky or skip a few frames, but otherwise carry on going.

Best Regards,
Alex.