[time-nuts] frequency (absolute) accuracy in sound recording/playback
Peter Gottlieb
nerd at verizon.net
Tue May 8 01:44:20 UTC 2012
We had time code to sync a number of separate A/V recorders so that
during editing you can cut from one to another seamlessly. I didn't
calculate or look at how tight the sync had to be. The mobile cams
could be out there for a while, maybe an hour or more, starting and
stopping to change tapes and batteries, but with the time code it just
worked.
On 05/07/12, J. Forster<jfor at quikus.com> wrote:
A movie may be 7000 seconds, and you may need a fairly stable timebase,
but every movie I've watched is made up of short (<300 second) scenes
that
are placed sequentially on the framework.
You are not meshing together a pair or multiplicity of 7000 second
event
sequences. E#very time you edit in another scene, you put the bricks
end
to end, so to speak.
-John
===============
> One area where accuracy is important is not because of pitch (nobody
can
> hear 1ppm differences), but because of the need to synchronize sound
> from different sources, particularly with video or motion picture
frames.
>
>
> 1000 seconds (20 minutes, give or take) with the sampler off by 1ppm
> will be 1 millisecond out of sync, which is probably hearable, and is
> 1/30th of a frame time. A 2 hour movie (about 7000 seconds) would be
7
> ms out of sync.
>
> Yes, we're not looking at needing Cs accuracy, but 10-20 ppm probably
> isn't good enough. So you're pretty much not going to be able to use
> the $10 oscillator in a can.
>
> So maybe a decent Rb, which is good to 1E-9 without doing anything
> special, wouldn't be a bad thing.
>
>
> And yes, there's a whole art to synchronizing stuff which was
recorded
> or filmed with incorrect sample rates, or ones that are "slightly
off".
> It wasn't too long ago that "quartz lock" for a motion picture camera
> was something that was a "special order" from the camera rental
house.
> I used to modify PC video cards for external clock input so I could
> adjust the refresh rate to match the camera speed (aka gen lock).
> There's a time nuts challenge... synchronizing something normally
driven
> off a quartz oscillator (however crummy) to a mechanical device (the
> movie camera shutter).
>
> And given the creative hierarchy on a set, it's going to be you that
> adjusts to them, not vice versa.
>
>
> There are directors who (for whatever motivation) also don't want
things
> like timebase correction used. Since I used to work for a physical
> effects company, I thought that these guys and gals who are hung up
on
> the "purity of the process" were wonderful, since they typically
wanted
> "real" special effects, not something composited in later by optical
or
> computer techniques.
>
> There's a whole industry supplying 24/48 Hz refresh hardware, as
> well. Well.. there used to be.. I'm not in that business anymore, and
I
> don't see credits for 24fps video as much, so they probably just
paint
> the screen blue or green or put registration dots on it and comp in
the
> images later. (Yes, I'm one of those people who watch all the obscure
> credits at the end for things like assistant hod carrier and such.)
>
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