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Re: [ARSCLIST] Re print-thru
Hello, Lou, Will, and Aaron,
I started this this morning and then was going to post it on my blog
and now that Goran and David Breneman have posted, I thought I'd send
off part of what I wanted to say.
This is a very interesting discussion and I think there are some unknowns here.
First, I agree with Lou that the tension in different modes depends
on the machine. The rule that a play wind is better than a fast wind
came from the days of Ampex 300s and 350s, I think. As you may
recall, these had play and then flat-out fast wind. While the torques
were adjustable with those hot slide resistors, these were constant
torque machines. More about constant torque later.
As we moved forward (and perhaps backward) we have seen single-motor
consumer machines (including the Ampex 600 et al) as well as
servo-controlled, constant tension studio machines.
The consumer machines produced horrid fast winds but many tapes would
fast-wind well on later professional machines.
A second thing to consider is the slitting of the tape and how air is
entrapped between layers in how it winds. Remember the German folks
especially used pancakes in radio and did not use reels. U.S.A. tapes
did not perform as well in that environment. However, at least one of
the things which caused those tapes to have fewer popped strands in
addition to the superior slitting -- at least as I understand it --
was that the surfaces were a bit rougher at a microscopic level.
However, at least some tape researchers in North America felt that
the added roughness, while it contributed to smoother tape packs
during high-speed wind due to better air removal, also raised the
level of asperity noise. Some might argue that asperity noise is a
non-issue with AC bias and erase, some might argue that it still is a
factor. I think I come down on the second side of this as the phase
of the AC bias can't be linked to the particle size.
One of the challenges in winding tape is that there are conflicting
--It is good to get a smooth pack.
--It is good not to wear out heads to get the pack.
--It is good NOT to have too much pressure on the outside as
that can lead (especially on 7" and smaller reels) to hub
compression and thence to spoking.
Here are some thoughts I've posted over time to my BLOG concerning
winding and tape packs. These are from the oldest to the newest.
Wind tape at low speed (library wind) entirely onto one reel after use.
As you can see over twi years there has been a progression of
thinking as I learned more about tape structure.
If we look at print through for a moment--and I had pulled Mallinson
"The Foundations of Magnetic Recording Second Edition" to do it right
but just don't have the time--, there are two factors which can
effect its creation and/or its severity. One is the percentage of
magnetic domains that are very sensitive to being magnetized and the
other is distance between the two layers of the tape. Heat may play a
part in this.
If some magnetic domains are very sensitive to being magnetized, then
they can be magnetized by the adjacent layer. This can be made worse
-- much worse -- if there is a bias field (perhaps even the earth's
magnetic field, but usually larger that causes these domains to be
ready to flip.
So these annoying domains have taken a magnetic "print" of the layer
below (or above) and have a small shadow of that magnetic signature.
Winding through the tape does several things, one is that it sets the
tape down against a different part of the reel and perhaps that helps
self-erase this faint image as it tries to pick up the "new" image of
its new neighbour. Print through does increase over time, and I have
heard arguments both ways concerning whether there is a practical
limit or not. The more I think about it now, I think there might me
as given a particular storage environment (temperature, humidity, and
magnetic "bias") at some point all of the weaker domains that are
available to flip will, and then there will not be enough energy to
flip the stronger domains.
I think we look at the distribution of the "sensitivity" of the
domains along a bell curve. There are some that will flip with the
slightest trigger and there are some that may not flip. Most will
flip in the middle say third.
I think the bell curve is linear but our hearing is logarithmic, so
even a very small number of domains at the weak end of the bell curve
flip, we'll be sure to hear it if it's not masked by louder noise.
I wish Eric would work on the skimming circuits of his A820. I would
be VERY interested in the results of that. Goran, have you ever used
skimming? The archive would have to be OK with this. I would suggest
doing a transfer with the tape "as is" and getting as good a transfer
as possible with the crosstalk, and then try the skimming. The risk
is that the tape will be partially erased and that high frequencies
MAY be erased more than lower frequencies.
I hope this helps -- I don't have this well enough presented yet for
my blog. Someday.
Richard L. Hess email: richard@xxxxxxxxxxxxxxx
Aurora, Ontario, Canada (905) 713 6733 1-877-TAPE-FIX
Detailed contact information: http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.