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Re: [AV Media Matters] SVHS Correction & Preservation
I was just trying to emphasize that a TBC, connected to a VTR or
internal, does not affect the recording. The TBC functions only on the
playback side, it does not change what is recorded on the tape. Many
people do not realize this!
Let's not get bogged down here with very strict definitions of a TBC or
framesync... For the purposes of modern VTR usage, the "TBC" is an
integral part of the deck, and it's functions (which include adjusting
playback levels, timebase correction, genlocking to a reference) are
simply part of the normal operation of the deck. A Betacam will allow
you to selectively blank lines in the vertical interval so that they are
not passed from the tape to the output. Whether this is technically a
function of the internal TBC, and whether the TBC is dealing with RF or
baseband video, is not really an operational issue.
-- Eric Wenocur
Lab Tech Systems
eric@lab-tech-systems.com wrote:
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".
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