Posted by David McCall on 02/14/06 16:25

<blackburst@aol.com> wrote in message
news:1139931655.211071.134400@g44g2000cwa.googlegroups.com...
>
> Most, if not all, of the 3/4" decks have a "steering sync" or "advance
> vertical sync" input that does, indeed, steer the servo in that deck,
> but the input signal for that connector is usually a dedicated output
> of a TBC. I'm guessing that it would not work as you hope.
>
> On the other hand, if you have a deck that takes a blackburst input,
> that might work.
>
All of the 3/4" decks that were designed for linear editing would
have sync inputs. IIRC the advanced sync is an output from the
timebase corrector you will be feeding the deck into. This output
is created by the timebase corrector and will be in sync with
"house sync" except that it will be a few lines early. That way the
TBC will have time to buffer the video and get it to the switcher
at the right time. Without advance sync, the TBC will have to buffer
the whole frame and put it out a frame late. This is assuming that
the deck is being fed sync at all. If the deck is free wheeling (no
sync input), then the TBC will wind up doubling up frames, or
dropping frames as needed to keep the output in sync.

Prosumer switchers work this way all of the time because there
is no sync between the deck and the switcher, so the decks are
always free wheeling. The TBCs in these units are actually frame
synchronizers. In fact most modern TBCs are frame synchronizers.
That means that they have a buffer that can hold an entire frame
and release the video in sync with the "house sync" no matter
what the source is doing. Frame synchs usually have the capability
of doing a freeze frame too. Early TBCs did not have this capability.

Advanced sync was essential on early TBCs because the buffers
were very small. 16 lines was fairly common. The output of a tape
deck will not be completely stable on it's own. Even if they are locked
to an external sync, there will be errors involving minor fluctuations
in speed. A TBC, even with a small buffer can take out these minor
imperfections in speed as long as they are within the bounds of
what the TBC can handle. If the errors exceed the buffer, then the
image will completely fall apart.

David

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