[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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