Re: [AV Media Matters] Re-Evaluating Tape Mastering Stock

[Jim Wheeler <Jimwheeler@aol.com>]

Stuart

On June 1, you said;
<<On backcoating, we have seen Zero failures in over 25 years.  As I
understand it, backcoating should reduce print through as it
provides some
spacing between the layers of two winds of tape. We have never
had a print
through problem with back coated tapes.  We DEFINITELY saw an
improvement in
winding and tape pack, when our data tapes are cycled frequently
over certain
sections.>>

Backcoating was developed by 3M in 1968 to eliminate the problem
of shifting
packs during shipment.  Backcoating is only 50 micro-inches thick so
thickness is not much of a factor regards print-through.  The
back-coating
has carbon in it to decrease electro-static buildup during high
speed shuttle
so I suspect that it is the carbon that helps decrease print-through.

<<...but it seemed that backcoated tapes always rewound better
for us than
non back coated.>>

A properly made back-coating will have large spikes that grab the
next layer
and prevent it from shifting.  This prevents pack-shifting and
results in a
smooth pack.

<<Backcoated tapes were the choice for Masters and long duration
unattended
recordings.  I believe the JPL Galileo recorder uses backcoated
79L or 799
Ampex tape.  It is still functioning.  They did have a head
stiction problem
which was in a batch of tape which I also had, but this seemed to
be a sticky
shed situation, and was overcome by jogging the recorder when
starting, ie
doing a forward and reverse to "unpark" the tape from heads.>>

I was involved in solving the tape problem on the Galileo tape
recorder.  It
was not a sticky-shed problem.  It was a problem with an ultra-smooth
sapphire rod in intimate contact with a worn section of tape.
The resulting
Van der Waal forces were strong enough to prevent the tape from
moving.  The
problem was solved by not parking the tape in an area that was well-used.

<<To clarify here, I consider that sticky shed is a different
artifact, and
we see it only on the OXIDE side of tapes.  And only on some tapes.>>

I have heard of sticky-shed occuring on the backside--apparently
because the
oligimers ooze out to either surface.

<<Tapes with older formulations do not have it, and most recent
tapes have
not developed it with decent storage.>>

Most tapes made before about 1968, did not use polyurethane
binder.  It is
the polyurethane that is susceptable to breakdown due to moisture.

<<...and Jim Wheeler or someone would have to comment on whether they are
different from audio backcoatings.>>

Each tape manufacturer has their own formulation for
back-coating.  It is top
secret so no two tape manufacturers will have the same back-coating.

<<Besides coating problems, which seem to be fewer with modern
controls of
the batch line, a critical step is the slitting.  Slitters eventually get
dull, and can ruin a batch of tape.>>

Slitting was a speciality of mine at Ampex.  It's not a matter of
becoming "dull".  It is a problem of "runout".  An arbor of slitter
blades cost around $50,000 and the precision is in maintaining a
runout of less than 50 micro-inc hes.  As a reference, the
wavelength of green light is 18 micro-inches.

<<With smooth tape, you also have the issue of brown stain in dry cool
climates. This is a microscopic deposit that may form on heads, reducing
signal level by spacing loss according to the Wallace equation. >>

The "brown stain" is caused by the ferrite video head and the tape
binder. At humidities above around 40 percent, the binder/ferrite is
washed away. Below about 30 percent RH, the binder/ferrite stays on
the head and forms a layer.  This layer prevents the tape and the
head from wearing but it also creates a thickness and seperation of
the head and the tape. This seperation decreases the signal output
as per the Wallace equation.

Jim Wheeler