Longevity of CDs

Mark Clarke <markey<-a t->gn< . >apc< . >org> writes

>My question is: Music CDs can play when damaged as the player
>*interpolates* to replace missing data - in music there is a lot of
>redundancy, the music in one  fraction of a second being
>substantially the same as in the previous and next fraction of a
>second. But with data, this is surely not the case. Can one scratch
>could damage a file enough to be unreadable, turn a picture into
>gibberish or whatever? Or would it only affect a tiny portion (lose
>a couple of words, for example)?

There are several ways that information may be encoded or written on
a CD-ROM, and this encoding system will largely determine whether
total or partial recovery from a loss of data due to a physical
defect is possible. In conversations of this sort, it is important
to distinguish the media itself from the way that the information is
encoded on the media. Sometimes there is industry agreement that
there is only one way to record or encode the information on the
media.  With most modern media, however, there are several agreed
upon standards that vary in terms of the way in which the data is
written to the media.  Since these standards differ, the ability to
recover from an error of one type or another can also
differ--sometimes dramatically.  Most of the newer formats have
several "built in" error correcting systems that try to prevent a
catastrophic failure.  These methods include redundancy of data,
check sum digits and similar schemes to verify the accuracy of data
read, the physical separation of data so that redundant data is not
physically located in the same area (so that in case of error the
risk of total loss is less), error concealment (primarily for lossy
compression schemes), and other technique.  Different hardware and
software systems from different manufacturers may also differ in
this regard.  If one were to bring a severely scratched music CD to
a "high end" audio store it is probable that some of the systems
will be able to recover better than others.  Similarly some computer
CD readers are more "robust" in this regard.

There are certain physical areas of CD's that are more vulnerable,
such as the index track area which tells the system where all of the
other information is and how it is organized.  A scratch in this
area can be particularly bad.  Also, scratches on the veneer or
label side of CD'S are likely to be much more serious than a scratch
on the "playing" side due to the relatively small amount of
protection to the aluminum substrata that actually has the
information recorded on it.  Similarly markers that use solvents
that could damage this veneer layer should be avoided because they
could reduce the protection the aluminum disk has from oxidation.

There are many different types of CD media as well, and some of them
are very different from the CD-ROM type media used for mass produced
music recording.  DVD for instance are multi-layered media with a
very high recording density, and although DVD-R, DVD-RAM, and
DVD-ROM are just starting to appear on the market, the ability of
these media to recover from an error of one sort or another may be
dramatically different from the music CD's and CD-R that we are so
familiar with.

Jim Lindner
VidiPax
The Magnetic Media Restoration Company
vidipax<-a t->panix< . >com
New Address and Phone #:
450 West 31 Street - 4th Floor
New York, N.Y.  10001
212-563-1999
Fax: 212-563-1994

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                  Conservation DistList Instance 11:23
                Distributed: Tuesday, September 9, 1997
                       Message Id: cdl-11-23-009
                                  ***