[QuadList] Cap Disc now Tape Speed

[georgenann at aol.com]

-----Original Message-----
From: Dave Sieg <dave at zfx.com>
To: Quad List <quadlist at quadvideotapegroup.com>
Sent: Tue, Jan 26, 2010 10:38 pm
Subject: Re: [QuadList] Cap Disc now Tape Speed


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?  

The cap is derived from head tach which is derived from either ref or incoming video, depending on if you are in rec or play. Tape was slower. Lets don't forget the reason for Drop Frame Time Code.

Tnx , GK


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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