Re: [AV Media Matters] SVHS Correction & Preservation

[eric@lab-tech-systems.com]

Do keep in mind that a TBC is a playback device.  That is, it only
affects how the signal leaves the VTR, regardless of what is actually on
the tape.  So, the vertical interval data may be on the tape, just not
leaving the deck (and therefore not recorded on a dub).

This is also true for VTRs with internal TBCs (basically all
cassette-based component or digital formats).  Most of these machines
have menu settings which control what lines are blanked during playback.
  In some cases lines are also selectively blanked on incoming video as
well (not a "TBC" function, but a function of the record circuitry).


-- Eric Wenocur
Lab Tech Systems

Moderators Comment:
Well I am not quite sure what is meant in the first paragraph here. To
try to clarify..

In the classis sense a TBC really does nothing regarding how a signal
leaves the VTR other then from a timing point of view - and then it
depends on the TBC/Syncronizer and its design. So a TBC is not capable
of telling a VTR to "blank out line x of video" - it all comes out of
the VTR as RF all mixed together. A pure Synchronizer in fact does
nothing at all to the playback device - they were specifically designed
for applications where you cannot control the device (like a signal from
a satellite for example). TBC's in the classic sense can control the VTR
only in terms of a phase locked loop for head/drum type timing for servo
control - but they do nothing in terms of controlling the deck to tell
it to not send any specific lines of video. 

In certain formats the machines were not capable of taking any type of
signal from an external TBC. Consumer VHS machines for example have no
ability to gen lock or to servo lock to an external TBC - they don't
need to do this. This has been true for MANY VTR's over the years - so
in order to get these machines to work in a system you need to use a
Syncronizer because you have no control of the VTR.

As the technology advanced it became essentially as inexpensive to make
TBC circuits as Syncronizer circuits and most modern TBC's are closer to
Syncronizers - at least in how you think about how they work.

When you conceptually think of how a VTR works - think of it recording
an RF signal. There are no real lines per se - only amplitudes that
exist at a certain time intervals or base ( - time base corrector). Some
VTR's in certain formats that  have a built in TBC can allow you to
blank out different lines if you wish, and do other things, but a VTR is
mostly an RF recorder and does not really know much about "lines".


>
> Moderators Comment
> I believe that some TBC's allow you to keep the vertical interval in
> some lines - I am almost certain that with some of the DPS TBC's (now
> Leitch) you can choose not to replace some of the lines that contain
> this information. It is a dip switch selectable thing that is not
done
> from the front panel - but it can be done (then again you are talking
> about an expensive TBC - not your 300 dollar variety). There used to
be
> a difference between frame syncronizers and TBC's - one of them was
that
> Sync's would entirely replace the vertical interval, and TBC's
wouldn't.
> I would bet very good money that you can find an older TBC that will
do
> the job. It may not be a component TBC - but it will pass vertical
> interval information. You will probably have to test them one at a
time.
> For instance I bet some old microtime TBC's will have no problem at
all
> (not sure if they made PAL ones though).
>
> jim
>
>

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