Stereoscopic sync - tiny framerate difference

DDDyson wrote on 5/15/2012, 12:58 AM

I've been shooting 3D footage with a pair of identical cameras, but after importing the videos to Movie Studio I noticed their framerates were slightly different: 29.951 on the Left camera, and 29.950 on the Right camera.

And what do you know: when I zoom in to the start of the videos, the frame ticks are aligned. When I zoom to the end of the 35 minutes of footage, the frame ticks look staggered.

How do I get the frames precisely aligned? Quantize to Frames doesn't do it. I tried applying a tiny timestretch to the Right footage to get the ticks aligned, but I'm not sure if that's the right way to do it. I tried setting the footage's timecode to "Absolute Frames", but it didn't seem to do anything.

Has anyone else run into this problem? How can two identical cameras produce videos with slightly different framerates, anyway? How does Vegas detect this difference?

What I want to avoid, is Vegas blending between frames when rendering.

Comments

Chienworks wrote on 5/15/2012, 5:33 AM

Hold the Ctrl key down while you either stretch the shorter one out to match the longer one, or squish the longer one to match the shorter one. You should be able to get them to match exactly that way.

Personally, i'd also disable resample on both clips so that you don't get ghosted frames from blending.

DDDyson wrote on 5/17/2012, 11:04 AM

Many thanks for the tips!

I found that I have to disable "Quantize to Frames" and change the Ruler format to "Absolute frames". If I then enable "Snap to grid", the ticks get precisely algined when I stretch the video.

Chienworks wrote on 5/17/2012, 12:12 PM

Nice!

Good luck with your project. :)

DDDyson wrote on 6/5/2012, 2:50 PM

Thanks for your tips! :)

musicvid10 wrote on 6/5/2012, 9:13 PM

Are your cameras genlocked with a wire?

DDDyson wrote on 6/6/2012, 2:38 AM

Nope - and that's a problem. My cameras (2 x VIO POV.HD) don't support that.

I know that the GoPro 3D Hero kit has a synchronization cable, but I have a very specific mounting purpose because of which I must use bullet type cameras instead of "just any" 3D camcorder / kit.

The only thing I can do, and do, is use the same remote to start and stop both cameras. But this does not give correct sensor phase / frame sync. Yes, it's a bit of a problem.

Maybe, in the future, the VIO POV.HD developers can come up with a way to, say, synchronize the sensors with the wireless remote. If there is a way to reset the CCD phases via the firmware, that is. I don't really know if there is.

musicvid10 wrote on 6/6/2012, 9:01 AM

Sending a "start" signal to both cameras to precisely synchronize the first frame won't do it. Any two clocks will drift apart over time if not slaved to a pulsed reference signal, even if that drift is just a miniscule amount.

Even hardwired genlock can have small phase issues when the cable run is very long. The goal is to keep the differences as small as possible by chasing the reference. Ever listen to WWV when you were a kid?

DDDyson wrote on 6/6/2012, 10:20 AM

Drift apart they will indeed, especially since the framerates of those two devices differ ever so slightly.... (29.951 vs 29.950 fps). Hope there will be a solution for this for the POV.HD in the future.

Maybe the remote control could be set to send a regular signal to the POV.HDs and they would reset their CCD phases regularly... I have no idea if that's technically possible, but it ould be one way to do it.

What's WWV? (as in, I guess I haven't :) )

musicvid10 wrote on 6/6/2012, 1:07 PM

You had to ask ;?)

WWV is the universal audio time broadcast from the NIST atomic clocks (same ones that set the time on your cell phone).
You can hear them drift apart and catch up to each other every other minute or so.

Right-click and "Save Link As . . ."
https://dl.dropbox.com/u/20519276/WWV.mp3

DDDyson wrote on 6/6/2012, 2:49 PM

Ack jawohl. :]

Though, from what I've heard, the brain is actually quite tolerant of sync differences between the left and right eye. Luckily. Perfect sync would be ideal, of course. Hope there will be a way...