[QuadList] Servo Modes

[Chill315 at aol.com]

There are a number of servo modes on the Ampex Intersync and the RCA  
machines.  Which one is the best to use?  Here are my thoughts.
 
Ampex the best is full Automatic.  This insures the following  conditions 
are met.
 
Vertical lies up with house so there is no switching glitch when the  
machine is taken.
Vertical lines up with house to keep SMPTE time code coherent with Vertical 
 on Tape if using a house generator.
        (Editors will burp a lot if  the vertical and the SMPTE Sync word 
is not within the proper timing  window.)
Proc Amp can reinsert House Sync to keep correct RS-170A on output.  
        (Note that this does not  prevent 170 nSec shifts in the picture.)
Burst insertion can be house to reduce jitter.
House Sync does not have the residual jitter of the Proc Amp Sync.
 
This requires that one does the proper setup of the servo and H Phase both  
on the Intersync and AMTEC centering.  This is done to center the video  
correctly with respect to house sync.
 
It is the setup used in most editing situations to get better edits.  
 
Ok What happens if you do Horizontal Mode.  
Slightly faster lock time.  
    Who cares in what we do?
Better recovery if the tape breaks up due to bad switches on the  tape.  
    It has to be a really bad tape to consider this  mode.
Proc Amp is in flywheel mode for sync
AMTEC is in External mode.
More jitter in both Sync and burst.
 
What about Vertical?
Tape has to be real bad or you have a digital TBC in the machine.
AMTEC is in flywheel mode and only picture straightening is done.  
        (The Intersync could not  have the H Stability Controls adjusted to 
get out the error of the H  lock.)
 
If one has a digital TBC should we not run the machine in full lock to  
reduce the off tape jitter?  Thus less velocity errors that the TBC has to  
correct.  (Not the Velocity Errors that the VelComp took care of but the  ones 
that because the Horizontal Sync is moving so much and many TBC's have a  
slight flywheel of the clocks to eliminate noise errors at the write to memory 
 stage.)
 
I have never used Preset modes but they are the sloppiest servo  setting.  
This is a playback with no vertical or Horizontal lock.  The  basic servo 
that was on the very first VR-1000 system.  I have no idea  why someone would 
use them for playback unless the tape was extremely  bad.
 
My Conclusion is that all playbacks should be done in full Automatic  mode. 
 All other modes have compromises.  Many times the issues will  not bite 
you but you never know.  
 
 
 
RCA offered four modes of lock, Pixlock, LineLock, Switchlock, and TW  or 
Tone Wheel.
 
Pixlock was the same as Automatic in the Ampex Machines.  Linelock  starts 
in full automatic and then after lock reverts to the same as Horizontal,  
Switchlock was vertical lock, and TW is the original servo system.
 
RCA did have the advantage with LineLock that it would always lock  
vertically for a clean switch and then if there was a problem, have a fast servo  
recovery.
 
The only time we used anything other than Pixlock was during  maintenance.
 
Switchlock was used in the editing mode but I did not have an editor to  
have any experience with this operation.
 
I can not remember how the Sync and Burst were treated in the different RCA 
 modes.  Most of the early machines used regenerated sync.  Maybe  someone 
has the book and can tell us.
 
Chris Hill
WA8IGN
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