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Date:	Tue, 17 Aug 2004 09:38:38 PDT7
From:	Peter Skye <pskye@peterskye.com >
Reply-To:	scoug-help@scoug.com
To:	scoug-help@scoug.com
Subject:	SCOUG-Help: CD Longevity

Content Type: text/plain

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Jerry Garren wrote:
>
> Does anyone have links to more information on this subject.

To continue this discussion, I ran across these CD Archiving
pages at Kodak while researching a different project:

http://www.kodak.com/global/en/professional/hub/museums/museumArchive/museumArchive.jhtml
http://www.kodak.com/global/en/professional/products/storage/pcd/techInfo/permanence.jhtml

Although Kodak claims that its CDs will last for 200 years
and quotes a commission as saying that magnetic tape will
last for several decades, my personal opinion is that's
grossly misleading and Kodak's marketing department probably
wrote such drivel.

First, the various other research I've read says 20 years,
not 200, under ideal storage conditions for CDs. Second, a
recent newspaper article noted that some CDs were lasting
less than 10 years even though stored properly.

The quote on the Kodak site about magnetic storage lifetime
being what I'll take as 50 years or sois also highly
suspicious. There are one heck of a lot of "master studio
tapes" that didn't last 10 years, and a few failed in less
than a year. (3M, perhaps the largest manufacturer of
studio tape, had a recovery procedure which required baking
some of these failed tapes in an oven. The chemical binders
decomposed and became fluid; by removing the fluids the
tapes were sometimes playable once again.)

If you truly want to save your CDs and DVDs for your
grandkids, making an occasional safety copy is a good idea.
Kodak _does_ say "it is prudent to create redundant copies
of stored digital data" and "having two copies in different
forms and in different physical locations increases the
chances that the information will survive", and on one of
the web pages above Kodak discusses "the principle of master
copies and derivatives".

The nice thing about digital copies is that if you have two
copies and both have errors, you can digitally compare the
two and (probably) figure out what bits to switch to fix
those errors. And if you have three copies then doing this
compare and correction is fairly easy (choose the two bits
which are the same as being the correct ones). For spanning
errors where one bit becomes two or two bits become one, you
can still do sectional pattern matching and most likely come
up with the proper corrections. (There are lots of other
techniques. Remember the floppy backup program Fastback?
It used the ninth sector on each track as an XOR of the
others so it could correct errors without you knowing about
them.)

- Peter

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