Disc Quality Scanning – PI/PO:
DVDs use an error detection and correction system (ECC) which is usually transparent to the end-user, but we can get an idea of the "quality" of a disc by performing Disc Quality Scanning, which shows how many errors the drive is detecting and correcting behind the scenes.
There are two layers or stages of error detection and correction on all DVD media; these are called Parity Inner (PI) and Parity Outer (PO). Data is arranged in ECC blocks containing rows and columns of user data with additional columns of PI error correction and rows of PO error correction.
An ECC block contains 32 Kbytes of user data with some added control data, scrambled and arranged in 192 rows and 172 columns with an additional 10 columns of PI error correction and 16 rows of PO error correction.
The Parity Inner stage is performed first, and up to 5 bytes in a row can be corrected. Any row with one or more errors is counted as a Parity Inner Error (PIE). Any row with more than 5 errors is considered un-correctable and is counted as a Parity Inner Failure (PIF).
The Parity Outer stage is performed next and will detect and attempt to correct any errors that are still left after the PI stage. Any column that has errors is counted as a Parity Outer Error (POE), and any column that has un-correctable errors is counted as a Parity Outer Failure (POF). If a POF occurs the drive can sometimes re-read the problematic spot and correct the problem; this happens only during normal reading and not during scanning, however.
Disc Quality scanning is influenced by the drive performing the test, and that’s why different drives report different results and even the same drive will report (slightly) different results when scanning the same disc again. Please note that PI/PO and Jitter scans only test some aspects of disc quality and that other important aspects are not revealed.
But what is a good scan? That is a discussion that we don’t think will end soon, as different drives report different amount of errors, some players are more picky about media than others, and so on. But as a comparison we present you with scans from two pressed DVD discs:
This scan above shows the results from a pressed, Single Layer DVD-Video disc (Flicka).
This above scan shows the result from a pressed Double Layer DVD-Video disc (I am Legend).
The Lite-On DVD burners used in this review report errors as follows.
PIE per 8 ECC blocks (rows with 1 or more bytes in error)
PIF per 1 ECC block (rows with 6 or more bytes in error)
We want to see as low error numbers as possible.
PIE per 8 ECC blocks should be no higher than 280.
PIF per 1 ECC block should be no higher than 4.
Both the pressed DVD-discs above are well within the standards.
If you want to look at the standards for yourself, download the ECMA 267 Standard for DVD-ROM, the ECMA 337 Standard for DVD+R/RW and the ECMA 338 Standard for DVD-R/RW at http://www.ecma-international.org.
Notice that there are other aspects such as disc reflectivity, tracking errors and so on that also will affect the readability of a DVD disc – but for this we do not have measuring equipment available.
Also, another note is that we have scanned the discs at 4X speed, by lowering the speed to 2X (DVD-R/RW)/2.4X (DVD+R/RW) or 1X the amount of reported errors may drop on some discs. We scanned at 4X CLV due to lower speeds taking too much time.
To see if there is a connection between the reported amount of errors and readability of the discs we also include the reading curve from a BenQ DW1655 DVD-Writer which by default is able to read DVD±R media at 16x speed. A small speed reduction near the end is still accepted on good discs, but serious reading problems or reading failures is a bad sign.
Jitter is a very complex subject and even more difficult to explain when we start to use optical drives designed for the home market to measure jitter values.
Let’s first look at the DVD specification for pressed DVD discs (in the bold part courtesy of Pioneer Electronics).
“The DVD design target is that when the worst-case disc allowed by the specification, considering the economics of production, is played using the worst-case pickup that can be produced in volume economically, the byte error rate after error correction will still be 1 x 10 – 20, which is good enough to be acceptable for computer applications.
Since the above target is for "after error correction," the error correction capability must be calculated. Considering the trade-off between error correction capability and the overhead of the added redundancy, the DVD format was set to one ECC block per 32 kB. This requires a byte error rate before correction of 1 x 10-2.
In order to achieve a good economy on both the part of the discs and the playback mechanisms. The current disc tilt specification was determined as a result of the efforts on both sides.
As will be explained hereafter, it is difficult to make the error rate a specification of the disc itself. Therefore, a jitter standard is set by the DVD specifications. A simple calculation based on a normal distribution requires that the jitter rate be under 15.4%, and experimental results indicate that jitter must be fewer than 16%, to achieve the required error rate. Since the disc tilt varies within a revolution, it was decided to adopt the design concept that jitter must remain within 16% at the instantaneous peak value of tilt. Since it is actually very difficult to measure the peak value, the concept became to measure the average jitter at fewer than 15%, and the byte error rate at under 5 x 10-3.”
What is Jitter?
In basic terms, we could say jitter is a product of “pit and land distortion” In other words, when the drive reading the disc has to compensate by means of a “tilt servo” which constantly tries to move and refocus the PUH lens for optimum tracking and tries to compensate for the imperfections of pits and lands on the pressed or recordable media. This is further compounded by the hardware used for recording and playback. Not only is the record and replay process limited by the resolution of the optical pickup, it is also horribly non-linear. In addition, the playback of the pits is subject to non-linear crosstalk from nearby pits in the same track, and also from pits in nearby tracks.
The things that causes jitter divide into three main types.
Variation in pit length and width.
Crosstalk from nearby pits in the same track.
Crosstalk from pits in adjacent tracks.
Variation in pit length and width:
The recorded pits themselves are not perfectly accurate. Anything which causes variations in the sizes of the pits will produce jitter. A prime culprit of this is sudden variations in laser power (laser noise). If laser power varies, then the laser beam itself changes and will vary in intensity and possibly focus. This will cause the pit length and width to also vary and we now have jitter.
Crosstalk from nearby pits in the same track:
If the pits are not totally accurate, then the laser beam spot may overrun a pit and gather data from the adjacent pit in the same track, or if the “land” is to short, then the laser beam spot can be influenced by the adjacent pit and this is called inter-symbol interference. Inter-symbol interference is worse at low recording velocities, because the pits are shorter and closer together. And it is the cause of "deviation" of the pit lengths.
Crosstalk from pits in adjacent tracks:
Crosstalk between pits in adjacent tracks is caused by the laser beam spot being larger than the width of the track. It is a largely random contribution and is worse at lower recorded velocities, because the highest frequency components of the readout signal in the wanted track, with which the crosstalk is competing, are weaker.
Some other factors to consider:
There are many aspects to consider when we add Recordable DVD media into the mix. We are now dealing with an organic dye, which is inherently unstable. We must also consider the equipment we are using to measure jitter is aimed at the home market. So we must also take into account variations between drives that we are unable to calibrate for such tests.
Now let’s look at some of the hardware limitations of the drives we are using to measure jitter.
If there is no tilt, then the jitter value includes components from light source noise, circuit noise, disc noise, standard interference between symbols (inter -symbol interference), and some small amount of crosstalk from the neighboring tracks.
Next we consider manufacturing variation in the circuitry.
Variation due to the circuitry have noise-like characteristics, and increase the minimum jitter level, but are thought to have a very small effect on tilt margin. Factors such as offset in the servo circuit, however, both increase the jitter level and decrease tilt margin.
How we will measure jitter.
We will be using a Lite-On DVD writer to conduct these tests along with Nero CD-Speed. In the screen shot below we can see a PI/PIF scan including a jitter test (the purple graph in the lower window) we carried out on a single layer DVD+R media.
Now, let’s find a reasonable average jitter level. Experimental results indicate that 8% average value or less is a desirable figure, based on the DVD specification. That does not mean that jitter average values above 8% are bad. In fact, many optical drives will quite happily read recordable DVD media with jitter values of more than 14% average without any problems. Other drives, including standalone DVD players may begin to struggle reading discs with average jitter values above 10%. So there is a fairly wide range of acceptable values. One must test our own playback devices to see what they can cope with.
However, for the purpose of having a basic guideline we can use in our reviews, we present a rating system for average jitter values.
Less than 8% (average) = Very good
8% – 9% (average) = good
9% – 12% (average) = average
Above 12% (average) = poor
Here is an easier explanation on how to read the test results
Maybe this got too technical, and you are wondering what to look for in your Nero DiscSpeed Quality Scans?
Use this as a guideline for good discs:
PI (Parity Inner): No larger areas on the disc should exceed 280 PI-8 errors, do not worry too much about high single spikes that exceed 280.
PIF (Parity Inner Failures): No larger areas on the disc should exceed 4 PIF-1 errors, do not worry too much about high single spikes that exceed 4.
And as always: the lower is better.
DVD+R media compatibility and write quality
In these tests we will be using a Lite-On DH20A3P drive along with Nero CD-DVD Speed to measure the disc quality. We will provide you with an image of the Quality Scan, Transfer Rate Test (TRT), and a Table which gives the pertinent details for the burn and quality scan to include the number of samples taken in the Quality Scan. We will also be using the BenQ DW1650 along with Nero CD-DVD Speed for our read-back tests.
The Sony DRX-S70U burned with very good results with the Taiyo Yuden media.
Here the Sony DRX-S70U burns this media with good results, however the PIF errors were higher than we would like to see, and a jitter average of 7.17% is Very Good.
The Sony DRX-S70U burned this media with very good results with a jitter average at the lower end of our “Average” rankings.
Here again, we find this media from Verbatim burned with very good results and a low jitter average.
The Sony DRX-S70U did good overall when burning this media.
The Sony DRX-S70U burns this test media from FujiFilm with very good results with a very respectful jitter average of only 8.00%.
This test disc from ProDisc burned with very good results; however, the Sony DRX-S70U would only burn this media at 6X speed.
DVD+RW media compatibility and write quality:
We used the same test procedures as in our DVD+R tests.
Below are our obtained results.
The Sony DRX-S70U burned this media with overall good results.
The Sony DRX-S70U writes DVD+R with generally excellent writing quality and proves to be one of the fastest slim-line DVD writers we have tested. Writing quality with our tested DVD+RW was ok but could be improved.
Now let’s look at DVD-R/RW performance and quality on the next page…..