[QuadList] Cap Disc now Tape Speed

[Don Norwood]

Hello Guy:

Nope, it's actually 15.625 ips (NOMINAL) because the head is running at 250 
rps instead of 240.

Don

----- Original Message ----- 
From: "Guy Spiller" <quadruplex at verizon.net>
To: "Quad List" <quadlist at quadvideotapegroup.com>
Sent: Wednesday, January 27, 2010 10:42 AM
Subject: Re: [QuadList] Cap Disc now Tape Speed


> 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
>
>
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