[QuadList] Cap Disc now Tape Speed

Wed Jan 27 10:54:39 CST 2010

-----Original Message-----
From: Chill315 at aol.com
To: quadlist at quadvideotapegroup.com
Sent: Wed, Jan 27, 2010 9:04 am
Subject: Re: [QuadList] Cap Disc now Tape Speed

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.

Chris, the diameter plus the input from the servo determines the speed and position of the tape.  The servo is also a PLL which will put the tape in the exact position it needs to be in.

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, who is Charles Anderson, that name rings a bell, was he at Ampex??

NMY TNX  GK

Chris Hill

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