Clover Systems DVX-4, Руководство по эксплуатации

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DVX Instruction Manual 45
End Users
Critical Applications – If your application has a high cost of failure, you can increase your confidence level by testing
the discs.
Archiving – The initial quality of a disc profoundly affects its potential lifetime. A good disc has greater tolerance for
the effects of aging than a poor one. If you are archiving either DVD/CD-Recordables or replicas, you will want to
select the highest quality discs to archive. This is possible only if you have a way of measuring disc quality.
Avoid long access times – As discs degrade, access times increase dramatically.
HOW TO MEASURE DISC QUALITY
Traditionally, many disc producers evaluate the quality of disc copies by comparing the copy with the original. In some
cases, this can be useful, but data comparison does not measure disc quality. For instance, a bad disc can still verify.
The DVD/CDs error correction can make a bad disc work, but it may be about to fail, or may not work on all readers.
The easiest way to measure disc quality, however, is to measure error rates. This technique is effective because any
serious problems on the disc will affect error rates. A meaningful quantitative measure of disc quality can be obtained
by measuring the quantity and severity of errors generated on playback. Important information can be gleaned by
looking at the size of the errors as well as just the number. BLER (BLock Error Rate) only counts how many errors
occurred; it does not tell you anything about how severe those errors were. It is quite possible to have a low BLER, but
have serious uncorrectable errors. DVX measures six different sizes of errors, and graphs them by increasing radius,
providing a detailed picture of disc quality and defects.
Errors are not necessarily physical features on the disc, but are a measure of how well the total system (disc plus player)
works. In fact, all discs generate errors on playback - even the best discs produce thousands of errors. The errors are
partly due to the fact that playing a DVD or CD is a difficult and complicated process. The width of the pits on a disc is
smaller than the wavelength of the light used to detect them, so players are operating at the limit of physical laws.
Playing a disc is not wholly deterministic, but is rather a statistical process. Therefore, the results will not necessarily
be the same each time.
Moreover, the objective (the lens that picks up the laser beam that reads the disc) must stay focused within a range of
less than 4 microns while the disc is moving both horizontally and vertically. The pickup must follow the spiral track
of pits as it moves to an accuracy of much less than one micron (one millionth of a meter). Several highly tuned and
sophisticated servo (control) systems are used to maintain focus, follow the track, control the spindle speed, and handle
timing issues related to reading the data. These servos are very sensitive and work only within a certain range. Thus,
some of the errors occur as a result of the narrow tolerances within which the equipment must function.
Two primary features of the disc itself that can cause errors are poor pit geometry and physical defects. Pit geometry
refers to the depth, width, length, and wall slope of the physical pits created in the disc. Although recordable discs do
not have pits, the recording process produces areas on the disc that function like pits and are subject to imperfections
that cause errors. Physical defects include pinholes in the reflecting layer, black spots, bubbles, and scratches. Poor pit
geometry and physical defects can make it difficult or impossible for the servomechanisms to read the data properly.
A determination can often be made as to whether problems are caused by pit geometry or local defects. A burst of large
errors confined to a small part of the disc is most likely caused by some kind of local defect. If many large errors are
found over the whole disc (or a large part of it), then the problem is most likely poor pit geometry.
It is extremely difficult to measure pit geometry directly. An easier approach is to play a disc and look at the signals
produced by the pickup. The way the light beam reflects back is influenced by the size and shape of the pits. By
observing the pattern (referred to as an “eye pattern”) created by the playback signal, it is possible to measure features
such as pit depth and length. DVX provides a simple way to make pit geometry measurements using the optional
Analog Measurement Module, or oscilloscope.