[QuadList] Time Base Correctors

[Trevor Brown]

If I can put an RCA spin on this

 

Quad errors are much faster than helical errors and so I would not expect
and external TBC expecting helical errors to respond to quad errors

 

The TBC's and servos also interconnect with error signal called THF and THA
( RCA words)to keep these small window TBC's in the centre of their
operating range.

That was the TCR 100 problem two decks, two servos but only one TBC  it had
to perform without THA to keep the TBC centered 

It drifted and failed to lock the only answer frequent alignment

 

Digital TBC's 

The RCA TR 600 used digital error detection, but Analogue Error correction,
perhaps they had A to D problems, 

Trevor Brown

videovault at sky.com

 

From: quadlist-bounces at quadvideotapegroup.com
[mailto:quadlist-bounces at quadvideotapegroup.com] On Behalf Of
Chill315 at aol.com
Sent: 08 December 2009 14:14
To: quadlist at quadvideotapegroup.com
Subject: Re: [QuadList] Time Base Correctors

 

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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