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Re: [ARSCLIST] The ways CD's and DVD's can fail.
Mike,
Thanks for the information you provided. Your reply did seem a bit harsh,
considering I'm a newbie to the board. The document was never meant to be
a technical expose. Further comments in line below. I do not intend any
offense by these comments. I only mean to further explore the issue.
Sincerely, Ron
At 12/6/2006 10:46 AM -0800, you wrote:
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.
Key word is "can". I said essentially the same thing you did.
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.
OK, heres what the MAM-A website http://mam-a.com/ says about disc production:
For a CD:
http://mam-a.com/technology/manufacturing/index.html
The layers are, in order, polycarbonate, dye, reflective, lacquer,
coating. So, technically, the dye is put on the polycarbonate and the
reflective layer is put on the dye. So, the dye is adjacent to the
reflective layer and they touch each other. So, yes, one could also say
the dye is on the reflective layer once everything is put together. It
makes no difference. It's actually on the polycarbonate layer as well. It
technically would be erroneous to say the reflective layer is coated with
dye as I did.
Here is a quote from the MAM site: http://mam-a.com/technology/cd/index.html
"The polycarbonate disc is covered by an organic dye where the information
is stored. The dye will decompose under the effects of the heat generated
by a laser beam (wavelength of 780-790 nanometers). The dye blackens under
this effect and the information is created."
One could be forgiven for interpreting "decompose under the effects of the
heat" as involving evaporation. Perhaps it doesn't, though.
Here is another quote:
http://mam-a.com/technology/technical_papers/documents/why_mit_cdr.html
"Because our dye reacts in a "burst" mode instead of a "melt" mode as with
other dyes, sharper pit edges are created (see diagram below)."
This may not actually be a bubble, but close. I thought I actually read
the word bubble somewhere, but I'm not sure where.
Another quote: http://mam-a.com/technology/dvd/structure.html
"DVD-RWs employ the phase-change recording method."
...
"The recording layer's phase-change material becomes amorphous and thus
less-reflective during recording, and returns to a very reflective
crystalline state during data erasure."
I would agree with your statement about there being no dye in a CD-RW. I
believe they use a similar process to the DVD-RW described above. I don't
think many people use RW discs for archives. I haven't seen any longevity
tests for these.
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.
This may not be news to you, or some others in the group. However, it is
news to some in your group, other newbies perhaps. One person has already
written me thanking me for the information. It other circles, like the
records managers and archivists groups, it is even more news. I have had
several positive comments in those areas and the editor of one ARMA chapter
newsletter asked to use the article in their newsletter. Finally, in the
consumer arena, it is very much news. Gold discs are barely even available
in the major retailers. After visiting Wal-Mart, Target, Office Max,
Office Depot, Circuit City, Best Buy, and Fry's Electronics here in the
Atlanta area, I found only ONE similar disc, the Verbatim Archival
DVD. And, I don't even think they use pure gold.
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.
OK. Chemical degradations maybe. It was meant to be simplistic. Since
the discs have not been around for 100 years, we have only two ways that
I'm aware of to determine the viability of a disc. Accelerated aging tests
give a relative indicator of the probable life expectancy of a disc
compared with other discs. If, after environmental chamber exposure, the
error rate of disc A is substantially lower than disc B, it may be presumed
that disc A will last longer in a archive application. How much longer is
up for debate. Furthermore, after environmental chamber testing, some
discs are already above an acceptable error rate according to the relevant
CD or DVD standards. These would automatically be disqualified from
archival use. It is generally accepted that heat, light (mainly UV), and
humidity are disc killers. The best disc for archival use should be the
one that is most resistant to these things. From my research at this
point, the best dye for CD's for archival use is Phthalocyanine. They
don't release the name of their DVD dye, but I would still trust the MAM-A
brand name.
MAM-A publishes at least some of their accelerated aging tests, as shown in
these pages:
http://mam-a.com/technology/quality/durability.htm
http://mam-a.com/technology/quality/longevity.htm
http://mam-a.com/technology/cd/organic_dyes.html
The other way to determine the viability of a disc is to test for errors on
calibrated equipment before use and after use and during storage. This was
stated in the "Risks Associated with the Use of Recordable CDs and DVDs
..." article quoted in your archives.
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.
You're right about CD's. Regarding DVD's, see this link:
http://mam-a.com/technology/technical_papers/documents/bonding.htm
During testing while dropping discs on the edge from a 4 ft. height, 30% of
competitor's discs started to delaminate. While it may not be a common
problem, I would certainly rather have the discs with the heavy duty
bonding system.
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.
Having personally copied several old video tapes onto some Maxell DVD-R's,
I can attest to the fact that touching or rubbing the disc in any way with
anything, even one's finger, is subject to create a scratch. Recordable
DVD's seem to be much more susceptible than commercial movie DVD's. I
haven't done much with recordable CD's so I don't know about them. As to
the number and type of scratches required to create unrecoverable errors, I
don't know and don't want to. I'd much rather have scratch protection so
it isn't an issue. My personal non scratch protected DVD-R's are getting
an add on D-SKIN after recording. Granted, in a low use archive setting,
it's probably less of an issue. But, copies are used for access purposes
by numbers of people, it becomes more important. I plan to copy the rest
of my 200 video tapes onto MAM-A gold discs as soon as I save the money for
the discs. In that quantity, it starts to be a non trivial cost for me,
particularly since it takes three discs to copy two 6 hour tapes and still
maintain good picture quality.
Also, the "Risks Associated with the Use of Recordable CDs and DVDs ..."
article quoted in your archives says, regarding scratches:
"It is preferable that no repairs or polishing is undertaken on archival
optical discs as
these processes irreversibly alter the disc itself. However, if the disc
surface (reading
side) shows scratches that produce high level errors, repairs which return
the disc to a
playable state may be allowed for the purposes of transfer."
In other words, if polishing is absolutely necessary, it should be done
carefully, then the data should be read and transferred to new media.
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.
My main point was that some of the name brands, maybe even my Maxell's
(don't know for sure), are manufactured by different facilities at
different times in a contract manufacturing style. This makes it very
difficult to control the quality of the production process. Not only that,
if the manufacturing facility changes, the quality may change, so you don't
have dependability for the brand. Also, the lowest bidder usually means
the crummiest product. There is no possible way that a product with a
substantially lower price can meet the same standards as one with a mid to
upper price, assuming the higher priced ones put the extra money into R&D
and production facilities. I currently drive a 25 year old Mercedes with
about 185K on the odometer (can't afford the new ones). Everyone tells me
it easily has half it's life left in it. The reason it's still around is
that the quality was designed, and priced, in when it was made. Many, but
certainly not all, cheaper cars of this vintage are in the scrap heap. Not
only that, many government contracts, etc. automatically reject the lowest
bidder. Anyway, I certainly DON'T want lowest bidder discs for my archive,
or for any of my clients. I want mid to high grade, or even pro grade
materials, even if they cost more.
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.
I'm glad to explore more sources, and will do so over time. But, I don't
think the MAM-A rep gave me any substantially wrong data. I intentionally
went searching for the best product on the market for my own videos and for
my clients, and I ended up there. And, there weren't that many errors in
my email. All the essential points of the email were valid. For maximal
protection and longevity of your data, archival CD's and DVD's should have
1) gold core, 2) premium dye, 3) excellent bonding (if applicable), 4)
scratch protection, and 5) single factory world class quality control.
Mike
--
mrichter@xxxxxxx
http://www.mrichter.com/
------------------------------
Ron Frazier -- P.O. Box 2284 -- Cumming, GA 30028 -- 770-205-9422 (O)
Email: rwfrazier AT macdatasecurity DOT com (replace the AT and DOT by hand)
I am an independent consultant interested in exploring ways to archive data
over long periods of time.
Recordable DVD's & CD's can fail in 2-5 years. Don't let that happen to
YOUR data.
Get your GOLD Archival Grade DVD's & CD's from
http://macdatasecurity.com/ today!
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