Possible reasons for differing video quality

Regarding my earlier post (two down) on possible differences between US and Australian Blu-ray discs for quality, Sandra has written to me:

I was also present the night John talks of. There is an alternate hypothesis to encoding variations or imagination to explain these perceived differences. Is it possible that physical variations in the disks arising from differences blu ray disk production quality control standards or practices could lead to slight differences which are material for players i.e. which lead to visible differences for the user? I know the “digital is digital” argument regards 1’s and 0’s but it seems clear that some brand new disks sometimes have playability issues on some players. Could it also be possible that the tiniest variations in physical disks could result in slightly visible differences at the player level?Is this possibility being suggested in the following white paper from quality control specialists in the blueray production process? See:

http://www.media-tech.net/fileadmin/templates/BD_News/OTO222_p30-33_whitepaper.pdf

On p2 under the heading “Coating thickness uniformity: local defects” they state (2nd para):

“Bumps in the coating layer do not directly affect the signal quality. However, they may cause optical aberrations that lead to focus and tracking errors.”

One does not have to understand all the details of this highly technical paper, to get the point that bluray production is FAR more technically demanding than is standard DVD. This is after all new technology. I do hope I am wrong, as the implications are gruesome. I cannot help but wonder whether occasionaly the pressures on increasing yields could cause production houses to drop standards, or whether just natural random variation in the physical disk, even within agreed tolerances, can cause some players to playback ever so slightly differently even the same encode?In any case I agree with you that we need to do further comparisons, on the same gear, blind, to see if any variances can be repeatedly detected.

In the mean time, from your knowledge of bluray production and players, do you think this alternative hypothesis i.e. we just happened to strike a “dud” Australian disk, at all possible?

This is an interesting theory, but I have strong doubts about it.

There are two ways that data reading difficulties could, arguably, feed into performance. One is if the data itself is corrupted as a result, and clearly this could have major implications. This way is not in dispute by anyone, and I will return to it in a moment.

A more subtle way that is sometimes raised is that the data ends up not being corrupted, but only because the equipment is working harder at correcting the data. That in itself is said to feed back into the reproduction chain in some way, reducing quality.

I’ve always thought that this latter argument is not very strong at all, since the effects (for example, greater power draw by the processing chips) would be of a magnitude similar to other pseudo-scientific nostrums peddled in the hifi and home theatre arenas (eg. the £500. P.W.B. Quantum Clip).

So let us return to corrupted data. There is a major disconnect between the claimed corruption of data drawn from a Blu-ray disc, and the alleged effects on the screen. Intuitively it would seem that, say, the loss of some proportion of the data would translate to the loss of some proportion of the picture, perhaps fine detail or colour fidelity.

That’s how things work in analogue systems. But modern digital stuff does not work like that.

The data on a Blu-ray disc is not an analogue of the original signal. Through the compression systems it has been dismantled and repackaged in a completely different way that lacks all proportion to the original signal. Ultimately those proportions will be restored (more or less, because the compression is lossy), but while the data is being laid down on the disc, while it resides on the disc, and while it is being read from the disc, it has no apparent relationship to the original data, except via advanced mathematical transformations. Misplacing bits beyond the capabilities of the system to correct them will result in quite unpredictable errors.

When it comes to reading the disc, a significant number of read errors will cause the signal reconstruction to simply fail. What you will get is not subtle loss of detail, but gross errors such as macro blocking, dropped frames and even total signal loss.

That is how Blu-ray (and DVD) signal problems manifest. Not in loss of sharpness, detail, contrast and ‘life’ in the picture.

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