[QuadList] Time Base Correctors

[Chill315 at aol.com]

TBC's are an interesting lot.  The Quad ones were all analog until the  AVR 
2 and 3.  There are advantages to the use of the analog type of  TBC.  
 
The video never is subject to the conversion to digital and back.  The  
conversion was not of a high quality in the early units.  Even the AVR 2  had 
issues and you had to tweak the converter to eliminate stepping.
 
The AMTEC and Colortec are a very stable unit.  The experience that I  had 
was that they never needed to be looked at.  I would do it anyway about  
once a year and never found them to be a problem.
 
The Velocity Compensator was the secret to a good playback.  It  integrated 
the errors over a head pass and thus made the color hue constant  across 
the length of a line.
 
Now what about a TBC.  Merlin was able to make several units  work.  Yes 
you get a very wide window.  If the tape could not be H  locked, then there 
was a problem that the analog TBC could not handle.   (Unless you had an AVR 
1.)  Most after market digital TBC's can not  handle segmented scan and the 
head switch.  This sudden jump in a line  placement is not what a TBC sees in 
every day life.  The result is usually  a displacement of the line.  There 
is usually a loop circuit to smooth out  the errors over several lines.  
That is the reason for the displacement. 
 
Most TBC's of the era were only 8 bit so the conversion is not as clean as  
a 10 bit.  I could walk up to an AVR 2 and see when the TBC needed  
adjustment because of the artifacts that 8 bit put in.  Just feed a ramp in  and it 
becomes very easy to spot.  
 
Would I change out the analog units?  My answer is no as I do not see  that 
there are going to be any better results.  The proper setup of the  
original machine is the secret of a good playback.  
 
Now for a possibility to play back the REALLY TERRIBLE tape.  The  machine 
can be played back in many different lock modes.  If you can not  obtain H 
lock, what about taking the output of the AMTEC and then feeding that  to a 
TBC?  Play the tape in any lock mode.  The AMTEC takes care of  the segmented 
scan and any guide errors.  Let a TBC external bring the  picture back into 
color and H lock.  Velocity errors might be an issue  because the AMTEC 
might not have the error integration across the line.   Any thoughts or has 
someone tried this?
 
Chris Hill
WA8IGN
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