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Re: [ARSCLIST] The ways CD's and DVD's can fail.
My comments are interlineated. Note that I am an (advanced) amateur in
these matters.
Ronald W. Frazier wrote:
A commercial DVD of a movie, as I understand it, is PRESSED from a glass
master disc. Thus, the reflective surface of the disc actually has
little pits in it which the laser beam reads. This type of disc can last
a very long time.
It can - or it may not. Its failure modes are not the same as those of
recordable discs. Its one clear advantage is that it is inherently
stable where all recordable optical media are inherently unstable.
A recordable disc does not have any real pits. The reflective surface of
the disc is coated with a dye which either evaporates and changes color
or creates a bubble when the laser beam writes to it. This type of disc
may not last nearly as long for the reasons below.
Quite imprecise, I'm afraid. Yes, it has not pits or lands. No, the dye
is not on the reflective surface. No, it does not evaporate. No, it does
not form bubbles. Yes, it may not last as long as a pressed disc - or it
may last longer.
Note that there is no dye in a CD-RW - an erasable CD. Nor, of course,
are there pits and lands.
I have talked to a rep at the factory that makes archival grade discs.
He explained several ways that a recordable DVD (or CD) disc can fail.
Factory reps can - at least in theory - be sources of accurate
information. There are more consistently reliable sources which are
often referenced in this group and may be found in the archives. A good
starting point may be http://www.mediascience.com/ and, of course, the
CD-R FAQ.
Failure mode # 1: OXIDATION - I found out that the plastic part of the
disc is not waterproof, despite what we might think.
Well known and well documented - not news.
Failure mode # 2: DYE FAILURE - The chemical dyes used in recordable
DVD's intrinsically go through chemical reactions over time that change
their color and reaction to the laser beam.
Not strictly "chemical reactions". For the rest of the paragraph, the
treatment is simplistic at best. The validity of accelerated life test
is not proven; the results of such testing are proprietary and not
disseminated; dyes which may seem to decay more slowly under some
conditions may fail quickly under other conditions.
Failure mode # 3: BONDING FAILURE - As mentioned above, the DVD is
produced by bonding two plastic discs together with the reflective
surface, the dye, coatings, etc.
At best, of limited relevance. The layers of a Compact Disc are not
"bonded"; the DVD bond has not been found to be a failure point. Note
that there are bonding issues in LaserDiscs, which may be the reason for
this issue being raised at all.
Failure mode # 4: SCRATCHES - If you've used recordable DVD media very
much, you probably know they're extremely susceptible to scratches.
In general, a non-issue. Readability will be maintained through
remarkably severe scratching and certainly will not be compromised in a
well-run library or archive. Spills are more of an issue, particularly
those with polycarbonate solvents. Note that wear-and-tear scratches can
be polished out with relative ease if somehow they appear on an archived
disc.
Failure mode # 5: PRODUCTION QUALITY - I was told that many name brand
disc sellers bid the production out to the lowest bidder.
We flew to the moon in hardware produced by the lowest bidder who met
the specifications. A modern, efficient production facility may provide
better quality at lower cost than one which is poorly maintained. No one
here is recommending flea-market media, but excellent manufacturers make
high-quality discs at reasonable prices.
Overall, I suggest that you can readily find better information sources
than a representative with a vested interest. Browsing through this
list's archives and the papers referenced there is highly recommended.
The results may not offer the simplicity you have found, but they are
likely to have fewer errors.
Mike
--
mrichter@xxxxxxx
http://www.mrichter.com/