[mythtv-users] Working with DVD ISO images

[Brian Wood]

On Tuesday 16 December 2008 18:31:43 Alex Butcher wrote:
> 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.

I recall once reading a review of a DVD drive in some mag (I think it was a 
Plextor drive). The reviewer claimed the drive had "superior error 
correction". Not that I'd pay attention to magazine reviews much, but I guess 
this is why I thought it varied from drive to drive, though upon thinking 
about it the statement could be considered true no matter what (depending 
on "superior" to what), I suppose you could say that all drives had superior 
correction.

It certainly wouldn't be the first time such a review (and reviewer) were also 
full of something.

What you say makes sense. Even making recorded DVDs readable in most 
standalone players is something of a black art (though getting better), and 
full of seemingly inconsistent results.

In my own experience the media is a major factor, although it is not always 
the off-brand cheapies that don't work as well, I've had some that said Sony 
that were total junk, and some with only Chinese on the label that cost 
almost nothing work great.

Something a lot of people ignore: Make sure you have the latest firmware in 
your drive, if it is upgradable.

-- 
beww
beww at beww.org