[QuadList] Cap Disc now Tape Speed

[Guy Spiller]

All this discussion brings to mind a question I have pondered for quite 
a while....
On a PAL quad, is the tape speed actually 5/6 of 15ips or were there 
mechanical changes in the mechanics of the capstan drive or the shaft to 
bring the speed back up to 15ips..more or less.. to maintain sufficient 
video track spacing?
Guy

Chill315 at aol.com wrote:
> 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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-- 
Guy Spiller
email: quadruplex at verizon.net
phone: (804) 379-2050
website: www.GuySpiller.com
Midlothian, VA