[QuadList] Cap Disc now Tape Speed

Wed Jan 27 10:40:00 CST 2010

Oh yeah...forgot about the "H" factor.  Now it makes sense!
Guy

Don Norwood wrote:
> 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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>
>
>
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>

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