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Another off sync problem, this one w greenscreen

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Kim Brownlee
Another off sync problem, this one w greenscreen
on Jan 21, 2010 at 1:37:20 am

I have been working on a green screen music video for several months now, trying to get the footage to stay in sync. The client finally said forget it; he'd take the version w the green background, which I did.

But since I might want to use green screen in the future, I am not happy & want to know why it keeps going out of sync. It stays in perfect sync while in the Final Cut timeline, but once I compress it, whether in Compressor or Quicktime, (I've tried both), it starts going out of sync after about 30 seconds. I used FX Home Composite Lab Pro to do the green screen editing. And Final Cut just doesn't like it one bit. Any ideas on why this is happening and how to fix it?

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Robert Foote
Re: Another off sync problem, this one w greenscreen
on Jan 21, 2010 at 7:09:28 am

Im not sure about your program but is your audio at 32K or 48K? If not at 48K you could try changing it to 48K and reimport the audio, I have had the same issues and this worked. Good Luck

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david bogie
Re: Another off sync problem, this one w greenscreen
on Jan 21, 2010 at 2:53:22 pm

Audio drift is a function of only two possible things. The audio track is on another framerate or time base or your playback system is at fault and will not or cannot handle both at the same time
How you arrive at either situation is unknown.
In the olden days, we'd see posts everyday from folks capturing Canon's weird version of "DV." The only fix was to unlink the audio from the video, find the exact length of the video track (from oepning slate to end slate, event-to-event), write that down, find the exact length of the audio track (from start event to end event, not the physical size of the track since it's the same as the the video track yet it's out of sync), apply speed adjust to it, and enter the video track's length, render the audio, relink the two tracks.


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Dave LaRonde
Re: Another off sync problem, this one w greenscreen
on Jan 21, 2010 at 6:24:14 pm

Bogie's right: we have no way of knowing the cause of the drift. We can only speculate. That said, many people like to shoot with the Canon 5D MkII, which shoots ONLY at 30fps. Such footage must be conformed to 29.97 and converted to ProRes for use in FCP.

Here's another guess: was your FCP edit timeline set for 48 kHz audio? Did you get the music for the video off a CD, which is 44.1 kHz audio? Did you bother to convert the music to 48 kHz to match your edit timeline?

But I CAN comment on the green-screen clips of yours:
  • That green screen you used is WAY too small for your needs.
  • The green screen was wrinkly: very bad -- it was an inconsistent background. You may have seen light and dark green, but to the chroma keyer, they were totally different colors.
  • You need at least 6 feet between subject and background to cut down on green light spill. What did YOU have, a foot and a half, maybe?
  • Learn to light for chroma key! You can google helpful information!
  • Forget that FX Home Compositor or whatever it's called. Primatte, contained in Motion, is a darned good chroma keyer and you already have it. You simply have to learn how to use it.
  • Note for future reference: don't shoot people wearing green against a green screen, for reasons which have become painfully obvious.
  • The completed clips looked like the weatherman giving the forecast. Learn tricks like applying Light Wrap to integrate the subject more with the background.
  • The slo-mo stinks. More on that later.
  • If you're like most aspiring film makers, you have an HDV camera and a laptop, and that's about it. No lights. No reflectors. No good mics. No good way to capture video other than a stinkin' firewire. You need more stuff to attempt effects work, or abandon the thought of doing it.
  • Are you shooting at 23.976, AKA 24p and incorrectly referred to as 23.98? If you do shoot HDV, do you remove the pulldown before editing or upon capture?
  • If you capture from tape, do you log & capture individual clips only -- the proper way to do it -- or do you attempt to capture entire tapes, taking it as an article of faith that FCP will do what it's supposed to at time code breaks, but which in reality can screw up pulldown removal and even time code?
  • If you insist on doing effects work, drop the HDV camera like a hot potato and get at the very least a Panny HVX 200. Get P2 cards, they're worth it. With it, you shouldn't need a capture card to log & transfer to Prores, you can shoot 23.976 actual frames/sec, you can shoot 59.94 frames/sec for overcranking & smooth slo-mo, and it gives you 4-2-2 color resolution. More on that next.
  • HDV is the ABSOLUTE WORST thing you can shoot for chroma keying. The reason: God-awful color resolution, even worse than DV. What's color resolution, you ask? Spend 15 minutes watching this video podcast, where the importance of color resolution in chroma keying is discussed, and where DV plays the role of the example of Bad Color Resolution:

In short, the sync problems are just about the least of your worries, because a cursory inspection of the resulting clips reveals that you did just about everything wrong when you shot this extravaganza. Sorry for being so blunt, but that's the way it is. As you've discovered, a little knowledge can be a dangerous thing. Doing TV & film isn't exactly the most intuitive thing one can do.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA

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