[mythtv-users] Working with DVD ISO images

[Rod Smith]

On Tuesday 16 December 2008 07:57:14 pm 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.

I don't know about "always," but both CD and DVD media specifications include 
error correction facilities, so that if a disc DOES have errors, they should 
be handled seamlessly by the hardware, up to a point. Of course, there are 
limits to this error correction.

> So - just because a disk or an image seems to work OK in one drive, does
> not necessarily mean that the disk is "OK", it simply means that the errors
> on the disk can be coped with by the system (drive, software etc.) being
> used to play it. Change any of these parameters and it might not work.

Correct. There are all sorts of compatibility issues. I've even seen drives 
that can't read the discs they burn, although other drives can.

> Different viewers have differing tolerances for problems. I have seen what
> looks to me to be horrendous crap on a DVD playback, while the person
> sitting next to me, looking at the very same screen, claims to see nothing
> wrong.

This is likely a different issue from media errors. Media errors are likely to 
produce pretty awful problems -- an inability to begin playback, seriously 
corrupted video, pauses in playback, or the playback tool quitting entirely. 
(This assertion is based on my personal experience with dodgy media and 
failing drives.) What you're describing here sounds more like different 
peoples' sensitivity to the lossy nature of the video codecs in use. MPEG-2 
(and MPEG-4 and other common video codecs) deliberately throw away data, 
albeit in a controlled way, which can result in momentary blockiness in scene 
shifts or periods of high motion, color artifacts, and other problems. Most 
people don't notice these issues up to a certain point, but how bad it has to 
be before a given person notices a problem varies from person to person.

Consider this: Actual data errors, beyond what the error-correction circuitry 
can handle and that would result in a corrupted MPEG-2 stream would also 
result in corruption to a data DVD, which is recorded in exactly the same way 
as a video DVD. Such corruption would be unacceptable in most data DVDs, such 
as a Linux install DVD. The Linux installer would flake out or report errors. 
Clearly, such problems aren't universal, although they do occur. The point 
being that DVDs often do produce 100% correct data reads, after the 
error-correction circuitry has done its job. If you routinely see video 
problems that the person sitting next to you doesn't see, then I'd suspect 
differing ability to perceive MPEG artifacts rather than DVD playback errors.

The sort of CONSISTENT corruption the OP is reporting isn't likely to be a 
result of the sort of data errors (or flakiness of the drive) that would make 
the disc useless for data storage, which in my experience would cause the 
viewer to terminate or freeze up. The symptoms the OP is reporting sound to 
me like bog-standard DVD encryption when viewed without the benefit of 
decryption software (decss). As others have pointed out, using decss to copy 
DVDs is illegal in the US, thanks to the DMCA, and discussing it here is 
inappropriate.

-- 
Rod Smith