Solution of the jitter problem in scans obtained with Plextor drives, eureka?

Have you been perplexed by the jitter scans given by the Plextor drives and hoped the results to be reported in some reasonable unit? It has not been possible to make sense of the numbers returned by the drives as displayed in PxScan, perhaps that is why PlexTools does not even show any numbers for jitter. It has just occurred to me that there may be a solution. I came by it through reverse thinking prompted by a hint. In view that many earlier versions of PlexTools err in calculating PIE averages, it is not unthinkable that Plextor has also made some other mistakes. Have interest to see if you can find the kind of miscalculation that would give those weird numbers? It is surprisingly simple.

Hopefully we come to the same answer: By definition, jitter (%) = σ ÷ T × 100. Now if computed as T ÷ σ × 100, you will get 1250 (in %) when σ is 0.08 T, for example. Thus, Plextor drives appear to inadvertently report jitter as its inverse in %. To get jitter values correctly is then simple arithmetic. Please note that data-to-clock (D.C.) jitter is adopted by DVD specifications, whereas TA test measures data-to-data (D.D.) jitter, which is also employed in CD specs. D.D. jitter (the avg. given by PxScan in TA test) is generally somewhat higher (commonly 1.2 - 1.5 times) than (D.C.) jitter for DVD, though the differences vary from case to case.

Although I am confident that the mystery has been cleared up, helps from persons capable of deciphering the codes of Plextor’s firmware are needed for final proof.

This is rather intriguing!

Have you been in touch with alexnoe about this? The example shown for pxscan on his homepage certainly is compatible with your conjecture, in particular since lower values mean higher jitter on Plextor dirves.

G

muchin: Use 2 different drives to scan the same disc, and you’ll see that it can’t be that easy …

Different BenQ drives can show jitter that differs more than 1.5% on the same disc, and so can LiteOn drives (using the new DVDScan utility), in my experience.

Could the explanation be a combination of [B]muchin[/B]'s theory and some variance between drives?

This is rather intriguing, indeed!

But I have another question for you all. I have seen jitter tests performed with professional CD/DVD analyzers, and the results are shown in nanoseconds (with results for both 3T pit and land). Any idea how this correlates (with the results from the Plextor or BenQ drives)?

-FRANKe

alexnoe: I know it is too easy to believe right away, but don’t immediately reject it just because it is simple.

Consumer drives lack standardization, and jitter is affected by several parameters, such as tilt, period of the PLL clock, asymmetry, etc., so it is understandable that different drives do not give identical results, just as DrageMester has said. Even with Pulstec-based devices, bottom jitter (tilt-compensated) will vary a bit, though not much, between different units. I have made some studies with such machines under the guidance of a friend with more than ten years of expertise in the optical storage industry. That is one of the reasons why I am confident in what I have said.

Standard deviation (σ) is given in nanosec. It is conventional for CD jitter to be reported as σ of 3T to 11T each.

muchin: A disk that shows like 1100 for jitter in my PX-712 shows about 900 in my PX-716. I remember reports from people owning those drives as well that their result is vice-versa…

Even if your explanation is correct, you could not really use it for obtaining a real jitter value, unless you calibrate the scale for each drive.

Another reason why pro tester need callibration.

The theory sounds reasonable to me. But I do think it can be questioned how usefull it is with the differences between the drives if there is no callibration.
Plextor jitter reporting has known to be quite drive dependant from day one.

Thank you for pointing out that relationship. A similar statement in the web pages for PxScan has been in my mind and finally led to the thought that Plextor may have made a mistake.

alexnoe:It seems that your PX-716 is somewhat faulty or that disc is an exception. It is up to the manufactures to tune their optical drives to behave about the same in testing disc quality. As to the variations between models, it is common for a newer model to be a better reader, as reflected in reporting lower jitter and PI/PO errors, than its predecessors in general.

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Even if your explanation is correct, you could not really use it for obtaining a real jitter value, unless you calibrate the scale for each drive.
You have asked for too much from consumer drives. For CD, some well tuned Plextor DVD writers may have come close to report “real jitter”; whereas for DVD, it is not possible to determine “real jitter” with any present-day drive, including professional units. [/font]

Since there is no way for the user to calibrate consumer drives, the PI/PO errors as well as jitter measured are always drive-dependent. We have to live with that uncertainty if we want to know something about the quality of the disc. You can’t solve the problem by providing PxScan, neither can I do anything about it.

PxScan in conjunction with Plextor Premium has been my favorite for assessing the quality of CDs. I have marveled with your skill in reverse engineering. Without your program to provide the numbers returned (after being miscomputed) by the chipset, it wouldn’t have been possible for me to find out the mistake. On the other hand, as you probably do not have access to professional equipments, it is not possible for you to have a proper grasp of jitter and its measurement without help from experienced persons. I hope that you do not feel bad about it.

To my knowledge, most professionals would not even look at the scans obtained by consumer drives. But people except very few have no choice unless not to do such tests. I certainly cannot solve the problems of Plextor drives.