Stability of Digital Betacam

Lydia C. Egunnike <l.egunnike [at] snark__slq__qld__gov__au> writes

>I was wondering if anybody has any in-depth information on the long
>term stability of Digital Betacam videotape.

For me there are two issues in this question:  the physical
stability of the video format Digital Betacam (sometimes known as
DigiBeta), and then the suitability of DigiBeta as an archival
format.

First, all magnetic media has one major physical problem: the binder
that holds the magnetic particles to the polyester tape deteriorates
due to hydrolysis.  Even though binder formulations have improved
over the years, it seems likely that some tapes made today will
begin to experience some deterioration of the binder within our
lifetime.  Unfortunately, there is no current published research on
the life-expectancy of videotapes.  The most recent published
prediction of the life expectancy of videotapes was in 1995 with the
publication of Magnetic Tape Storage and Handling, by John van
Bogart of the National Media Laboratory.  Nonetheless, it is my
thinking that videotapes manufactured today, even by the most
reputable manufacturers, will experience problems in the ten to
thirty year span that is referred to in that publication.

Having said this, it is my recommendation that regardless of the
format, always choose a well-known  manufacturer as your tape
supplier. Furthermore, it is essential that any tape library have a
well-thought out plan for the migration of materials to more stable
tapes and formats over the years. Finally, there is no red flag
system to know when your videotape is deteriorating.  It is possible
that the videotape will sit on shelves for years and then not play
back.  You will have no way of knowing (though there are some
strategies, like having a bellwether tape in your collection that
will be played back occasionally to see if there are signs of
deterioration).

Digital formats have several additional problems that make me
question their use as a sole  preservation master.  It is true that
digital materials can be copied without degradation of the image
unlike analog formats, and there are handy features in the digital
realm that tell you when the tape is having a high error rate;
however, damage to the physical substrate that carries the
information can be catastrophic in that once it is physically
damaged, its information is gone, and if the tape is sitting on your
shelf without being played you won't find out if there is a high
error rate.  In analog formats it is sometimes possible to recapture
information.   All formats are also in danger of being rapidly
replaced by newer and better formats.  DigiBeta, despite its
popularity, will not be around in the way that an archive or library
would expect.  If the physical deterioration doesn't get your tapes,
format obsolescence will.

DigiBeta has an additional problem (from an archival standpoint) in
its use of compression.  As an acquisition format it has a
good-looking image, but when you get into using DigiBeta to copy
film to tape or tape to tape, the DigiBeta quantizes the image at a
4:2:2 level (meaning that the color difference signals are sampled
at half the rate of the luminance signal; the whole sample is done
at around 8 bits), and compresses per field at 2.3:1.  That means
that there is a lot of information being thrown out that other
formats would keep, or that the original will always have. It might
look o.k. or even very o.k., but it worries me that the total sum of
information is not being passed on.   [And then we get into the
complications of actually making excellent copies, but that is
another topic].  The bottom line is that compressed copies are not
adequate preservation masters.  There are uncompressed digital
formats, and there are other options like making two copies.

Most importantly, why not keep the originals, especially the film.
There is artifactual value in the film (especially some of the older
formats), in addition to its informational value.  In not too many
years, there will be artifactual value to many video formats too.

Sarah Stauderman
Conservator
VidiPax, the magnetic media restoration company

                                  ***
                  Conservation DistList Instance 13:6
                  Distributed: Wednesday, July 7, 1999
                        Message Id: cdl-13-6-001
                                  ***