[QuadList] Cap Disc now Tape Speed

[Chill315 at aol.com]

We are all missing the point.  There are some things that are correct  and 
some that is a misunderstanding.
 
The SMPTE specification is what sets the track imprint on the tape.   ANSI 
Standard C98.6-1965.  The specification for the distance between four  
tracks is .0625.  (Four tracks are used because of the possibility of  
quadrature.)  This is the only thing that counts so that interchange will  be 
possible.  There is only one specification and it does not care if the  signal is 
color or black and white.  Anything that is done to a machine  must still keep 
the footprint on the tape the same.  Tape speed is defined  by ANSI 
standard C98.4-1963.  It is based upon 60 Hz vertical.  The  note states that the 
change to 59.94 will result in a proportional change.
 
Record mode requires the head and capstan be locked together.  The 240  
from the head is used to lock the capstan in record mode.  The 240 from the  
head is used to make a control track.  This is recorded on the  lower part of 
the tape.  The frame pulse is recorded to tell where  vertical is located.  
 
The diameter of the capstan will determine the amount of tape that is  
pulled through each second.  It is made to a .0001 tolerance so that the  amount 
is 15 inches every second.  It is also this that sets the distance  between 
the head tracks.  Thus the need for a very precise capstan.   If two 
capstans are different sizes even within the .0001 tolerance, over an  hour they 
will pull a slightly different amount of tape through.
 
Tape speed is governed by the diameter of the capstan shaft and  pulley.  
Its rotation is set by the incoming video.  The only thing  that determines 
how much tape is pulled through is the mechanical sizes.   Nothing more.  If 
any pulley diameter or capstan diameter is modified,  the machine will no 
longer record signals to the SMPTE specification.  This  is unacceptable to 
maintain the SMPTE specification and interchange.
 
The bandwidth of the servo allows for the change from monochrome to color  
recordings with out a problem. Since the head is locked to the vertical, and 
 thus the capstan is locked to the head, then the machine could care less 
what is  going into it.  
 
There has never been an issue about color and monochrome capstan  sizes.  
Since the very first days, the machines from RCA and AMPEX have  been used to 
record both color and black and white.  There was never a  change to the 
VR-1000 or the TRT to change out the capstan for color.  I  had the TRT and am 
aware of every modification that was done to the  machine.  
 
Playback will make the tape speed be what it was in the record mode.   The 
machine is now locked to house and the head and capstan are locking tape to  
the house.  Thus any minute speed variations will be compensated for.   
Time base correctors or their windows do not have any influence on the  speed.  
 
Lock up modes.  There are a series of steps that he machine goes  through.  
The head requires a start up mode so that it can come up to  speed.  Then 
it changes to different modes as it goes through the  process.  One of the 
things that is also done is to have a boost applied to  the reel motors.  This 
will affect the tape speed during the boost  cycle..  This boost is 
released after a fixed time.  
 
The real answer is to ask Charles Anderson what the story  is.  Maybe Tim 
can do this and we can finally put this one to  bed.
 
Chris Hill
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