[QuadList] What is a Proc Amp

[Chill315 at aol.com]

We have had a lively discussion of a proc amp.  so here is a little  
background information that most of us probably know.  Please feel free to  add 
anything you can.
 
A Processing Amplifier is a device that is used in the video chain to  
adjust levels and do other corrections to the signal.  RCA originally  called it 
a Stabilizing Amplifier.  
 
The unit takes an incoming video signal and strips sync off.  This is  
regenerated to provide clean pulses with the proper rise times, levels,  etc.  
This can be done via a gen-lock technique with a very fast time  constant or 
just strip, stretch, reshape and reinsert method.  (The cheaper  ones do the 
latter.)
 
The video is treated as a separate signal.  Video is usually clamped  
during the horizontal interval to remove any DC variations or AC hum.  It  is 
then adjusted for correct levels, black, white, and Chroma.  After this,  the 
sync portion of the signal is reinserted back into the video with  
adjustments for levels, burst phase, and timing of sync to video.  Some of  these are 
internal so you can not do it without the green screwdriver.  
 
For a transmitter, the processor must do several additional steps.   First 
is White clip to prevent negative modulation of the transmitter.   This is 
usually done about 105 IREs.  Because Chroma can exceed that level,  the 
white clip is done in two ways.  The hard clip is done in the separated  
luminance channel.  Luminance was separated to allow individual adjustment  of 
Chroma and Luminance.)  Soft clip does it to both luminance and  Chroma.  Soft 
clip is set higher.  Then the linearity corrector is  added to allow for 
correct of the transmitter gamma cure to make it a linear  system.  
 
In addition we use a Proc Amp at the output of an edit suite to do a couple 
 of things.  Fist is to provide a way to reinsert house sync directly from  
the sync generator so that the SCH phase remains constant.  Also it keeps  
the quad machine from having H shifts upon the camera or source switch in 
the  studio.  We also use it to provide a white clip to prevent the over  
deviation of the modulator in the quad.
 
This is the simplest definition of a proc amp.  I hope it helps.
 
Chris Hill
WA8IGN
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