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Re: compact disc longevity



>I am writing a paper about the changing perspectives on the issue of
compact
>disc longevity.  It seems that over the past decade the life expectency of
a
>CD has ranged from 8 years to over 300 years.  Therefore, I would like to
>ask for your current understanding of the issue and in particular, what
>source or sources lead you to this understanding?

In talking about longevity it is important to understand what is being
estimated and what assumptions underlie the estimate.

Most estimates of media longevity start with the assumption that media
degredation is a chemical process and, like all other chemical processes,
occurs faster when it is hot and slower when it is cold. The testing
process is complex, but basically boils down to heating CDs for various
lengths of time and then testing to see if they can still be read.
Various normalisations then give an expected lifespan under 'normal'
storage
conditions.

When interpreting such longevity statements, it is important to realise
that
they apply under the storage conditions specified. Colder, dryer, storage
conditions should extend the life of the media. Hotter, wetter, storage
conditions will reduce the life. So don't store your CDs on the back window
ledge of a car :-). More realisticly, I'd think about the effect of a fire
in your storage area. Even if not hot enough to warp or damage the media, a
fire could significantly degrade the life of the CD. Like the storage of
all
digital information, you should think seriously about off-site storage of
copies. Fortunately, making such copies is far cheaper with digital
information than paper.

Incidentally, this technique is standard for estimating the lifespan of all
media, not just digital media. How do you think they estimate the shelf
life
of 'archival' quality microfilm?

In discussing the lifespan of digital media it is unfortunate that the
information previously available from the (US) National Media Laboratories
Web site (http://www.nml.org/) seems to have gone away.

Having said all that, the longevity estimate is still based on the
assumption that the life of the media is limited by chemical deterioration.
Mechanical degradation may be more significant. One site (whose URL escapes
me at the moment :-) notes that minor scratches to the label side caused by
loading/unloading the CD may cause the CD to become unreadable long before
the 'chemical' life of the CD expires. A good argument for a separate
preservation copies and access copies.

Finally, I'd note that the major challenge for digital media preservation
is likely to be economics. That is, the media cost per megabyte, the cost
of storage per megabyte (generally the cost of the robot), and the cost of
management per megabyte. When we looked at the costs of a large storage
system (terabytes), tape came out significantly cheaper than CDs. Further,
there was very widespread industry experience with tape robotic systems
compared with the CD equivalents.

Andrew Waugh

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