[QuadList] Cap Disc now Tape Speed

[Dave Sieg]

At the risk of aggravating an already well-worn subject to death, I've been
thinking about this the past few days since the thread began and have some
thoughts I just can't resist sharing.
*Ignore at will!*

The motors/pulley's, etc were probably initially machined in the mono days
so that a 60 hz drive pulled 15.0 inches of tape past the capstan.  In
record, the machine runs the capstan motor a sine wave locked to vertical
reference but with no feedback to know how much physical tape is actually
going past the capstan each second.  Its usually blank tape anyway, right?

Switch to color now.  If you used a 59.95 hz drive to the same motor, it
would only pull 14.9875" of tape past the capstan every second, if I did my
math right.  That doesn't seem like much difference but over the course of
an hour long tape it would amount to a 90" or 7.5 foot difference in the
physical length of an hour recording on tape.  (Again, if I did my math
right!) As I recall, in the earlier color machines, thats what is happening
in record.  If I am wrong and the capstan is being driven at 60 hz in a
color recording somebody more knowledgeable please correct me!

I honestly don't know if the SMPTE spec for mono vs color quad tape
addresses this or not.

Of course in playback, with the servo varying the capstan speed to whatever
it takes to make the tracks play back locked to reference vertical, it
doesn't really matter how many inches go by in a second as long as its
within the range of the servo.

So... changing the physical size of the pully shouldn't really matter in
playback, as long as its within the servo's lock range.  However, it would
make a huge difference in record, when there is no on-tape reference to tie
physical tape length to capstan speed.

Now, in the more modern machines where there were capstan tach pulses, it IS
possible to make the motor pull exactly 15.00" of tape through each second
in record.  Necessary partly because some of the capstan motors were DC...
but thats another story.

Of course I may be completely full of you-know-what here, but thats my
thoughts on changing the physical size of the capstan pullys.  Feel free to
correct me if I'm wrong.



-- 
Dave Sieg, President, ZFx inc.
www.zfx.com      www.linkedin.com/in/davesieg
www.davesieg.com
www.scanimate.net
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