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Posted by Richard Crowley on 10/08/07 14:48
"Rick Merrill" wrote ...
> Richard Crowley wrote:
>> "Rick Merrill" wrote ...
>>> My concern is that the generator is not keeping very
>>> good timing so the audio begins to lag over the course
>>> of a couple of hours recording. AFIK there is no time
>>> sync on the audio, just on the video.
>>
>> No equipment, even professional will hold sync over "a
>> couple of hours". Or even over one hour. They is why
>> we use gen-lock and timecode, etc.
>
> Call me suspicious, but if "gen-lock" signal is off by
> 0.1% then after 10 hours of recording there will be a 1%
> cumulative error.
First, "gen-lock" is not a signal. It is a method of interconnecting
equipment so that the sync signal of the "master" unit is
running the "slave" equipment. The slave equipment is said
to be "gen-locked" to the master.
And you are correct. If the sync signal is 0.1% off, then it
may be off by 1% after 10 hours. But remember that most
equipment drifts in BOTH directions so it is PLUS OR MINUS
0.1% and unlikely to have accumulated all in one direction.
But all equipment drifts to a greater or lesser extent. That
is why if you need to keep separate video and audio
recorders in sync, you "gen-lock" them together so that
they run at the same rate.
> It is the accuracy of the gen-lock that comes from the
> black-burst generator and that's what I wanted to know.
Most equipment is likely accurate to better than 0.1%
But likely NO equipment is absolutely accurate (except
occasionally by accident). The reason is that it is just
not worth the cost to make something that is absolutely
accurate. Nobody relies on the absolute accuracy of the
sync signal to lock together two different pieces of gear.
In places where agreement with wall-clock real-time IS
important (such as broadcast on-air operations, master-
control, etc.), they tend to use (nowadays) sync generators
that are themselves synced to a secondary reference such
as the constellation of GPS satellites, or WWV, etc. (in
the US, likely similar references in other places, etc.)
> In other words, does anyone ever check the accuracy of a genlock
> signal (e.g. one that was purchased on the cheap from ebay)?
Sure, most equipment of that kind can be "calibrated" by a
technician with the appropriate experience, information, and
equipment. Of course, it will almost certainly cost more than
you paid for it on eBay, which is why it is rarely done.
If you are looking for a solution to keep separate video and
audio recordings in sync over two hours, the absolute accuracy
of the sync generator is NOT the solution. Even if you had an
unlimited budget and could buy the most expensive equpment
ever made, people don't run open-loop and depend on the
absolute accuracy of the equipment. They interconnect the
equipment together ("gen-lock") to ensure agreement of speed.
Or else they just live with whatever "slippage" there is and then
simply correct it in post-production on their NLE timeline. That
is far and away the cheapest and simplest solution.
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