Re: [AV Media Matters] How lossy?

[rohre@ARLUT.UTEXAS.EDU]

Jim,
If I said something about erasing, it was probably in the instrumentation
context, re bulk erasing, and the situation we found on a couple of
occasions over 30 years with Ampex instrumentation tapes.  In a 799,  we
once found factory test signals left on every other track which was their
test scheme.  They had forgotten to bulk erase that one reel!  We also
learned there is such a degaussing signature to some bulk erasers, that we
would test all the ones available if at a field site.  There was one at NASA
Kennedy that was much better than the others they had, and all of the same
model.  That was also true at Pt. Mugu where we used to do 14 track copies.
And among the degaussers here, I have my "best" one, which is not even the
one NSA approved for erasing!  My other one is better, and thus I degauss
sensitive stuff on the "approved" one, then use the "best"
 one to make sure it is clean of once around am effect on subsequent
rerecording!

Fun and part of the black magic, that kept us in business.

The enhancements were to drop in more gain in a particular part of the tape
bandpass to enhance part of the timing imbedded signal, or to de-emphasize
it when it got recorded too hot re the data on the same track.  We had a
fancy way of extracting the timing carrier wave with a bit sync as our PLL
and filter, worked really well, even though it was being used on an analog
direct recording where the timing was a sine wave recorded at low level on
the high side of the data band.  Yes, we also had to equalize and bias to
particular tape types.  But, once we standardized on 799 for masters, 79L
for long masters, and 797 for copies, we had very good repeatable results
batch to batch.  Over time, the recals took care mainly of head wear changes
in response.

We found 795 to have more slitting debris than 797, and could buy bulk 797
at commercial price cheaper than GSA 795.  There was a 795 E I believe, that
NASA sampled, and was noticeably cleaner than standard production.  Never
did see much of it, thus we went to 797 and it worked reliably.  We often
played copies many times, as we could data reduce only 4-5 tracks in
parallel when using the timing wave.  More than that, and edge to edge skew
would upset the timing reliabilty, as you were using the timing from only
one of the group of tracks, usually the center of a group.

Most of data reduction was at 20 times or more real time, ie high speed, and
had to be, as the slow speed Geotech real time speed was originally 15/160,
later we went faster with the Bell and Howell 1428 at a fast pace of 0.2
ips!  That was record side; on play we did not want to go so slow as to see
grass grow, and would reproduce at 15ips or around there.  Because of the
multiple customers for copies and multiple passes on each, we would mass
produce copies in a one week trip to KSC every six months during those
contracts.  One time we had four 3030's and two Racal Storehorses, all
teamed to produce both direct copy on 3030 and FM copies on the Racals at
same time from one Master play.  That time we had four of us to be able to
start all the recorders!  And all tapes had two IRIG time tracks, one odd
one even as well.
Stuart