[AV Media Matters] digital video TBC's in preservation copying

[amipa2@pobox.alaska.net]

Hey folks, One more time- I'm wondering- does the digital processing in a
TBC/noise
reducer concern anyone when making preservation copies? It can sure help
clean up an image, but it is of course an A/D/D/A pass in what would be an
otherwise analog path, and many archivists are against that in general.
Also, there is an audio slip of +1 frame against the analog audio, and not
many folks have audio delay devices (with their corresponding audio ADDA
pass for the processing).

Who has wrestled with that question and has any kind of policy (formal or
informal)?

Please reply- I'm stuck on this and we need to resolve our TBC issues.

Thanks,

Bob CJ
Alaska Moving Image

Moderators Comment:
I will take a short stab at this one. TBC's of one type or another are a
required element in making preservation copies. Exactly which TBC you use
and how you use them for tapes in different types of conditions sort of go
into an area where there will no doubt be differences of opinion, and
proprietary knowlege. In almost all formats TBC's were part of the system
design in the first place for high quality applications of the format so
there is no "ethical" issue here that I can really think of - but as a
practical matter you won't be able to make a tape that meets rs170a unless
you use one anyhow - so it is sort of a moot point. The quality of the TBC
is very important. For preservation work only the HIGHEST caliber device
should be used, but it is extremely important not to overprocess - so just
because a tbc has lots of bells and whistles does not mean that you will
necessarily use them, or want that type of unit. Basically you need to spend
the money to get one that has a front end capable of dealing with the
problems of older tapes which often exceed the windows and have issues that
other tbc's can't handle. For instance modern tbc's expect proper field
relationships - so when they get a signal from a skip field machine they
freak out. This is just one tiny example. The same goes for many issues in
non-standard or sorta standard formats - for instance animation done on a
sony 2500 1" C machine were C format - sorta.... lots of issues with
serration pulses. As an example one could have trouble playing a tape from a
2500 on a standard Ampex Zeus TBC  - it turns out that this TBC uses
serration pulses for timing - and the result can be a disaster. Most modern
TBC's expect signals that would come out of a modern machine - and when they
are not there or are severely distorted, they have all sorts of artifacts
that they add.So although a TBC may have an "infinate window" that may not
solve issues with older tapes which may have other issues. So you can't have
just one type of TBC depending on the type of work that you do you may need
many. I don't even know how many we have at VidiPax anymore - but in the
dozens. Some work better then others on different formats and in different
tape conditions with different formats.

As far as audio delay devices, they are readily available if necessary.
Actually delay units are built into many multipurpose audio devices used in
music mixing studios - just part of the box. I don't really have any
problems with the ADDA issues provided that the system is clean and
professionally wired - balanced, and so forth and high quality units are
used in the process (which is a big provided) - and again there really isn't
any practical option. As a practical matter on digital formats the A/D is
done in the tape recorder itself in most cases (although you do have an AES
option on SOME formats I don't believe that most people use it). The same
goes with CCIR601 imbeded audio stream.

AMIPA
Alaska Moving Image Preservation Association
amipa2@pobox.alaska.net
phone 907-279-8433
fax   907-276-0450