Multi-cam sync, flash, rolling shutter

smhontz wrote on 1/23/2013, 8:41 PM

The situation: I have a two-camera shoot. Canon XF 105 & 305, running 1920x1080 30p.

Camera A: Has medium shot of speaker, receiving direct feed from house mixer
Camera B: Has wide shot of speaker, just using camera mics

I sync'ed both up by matching up the audio waveforms as much as possible and "nudging" Camera B back and forth until the audio echo disappeared. We then used only the Camera A audio feed from the mixer for the sound.

We then proceeded to use multi-camera to cut back and forth between the cameras - some 70 cuts.

Looking at the rendered output, my partner thought the audio seemed out of sync. Since the cuts that use Camera A also use the mixer feed that went into camera A, those ones seemed in sync, but Camera B was slightly off.

Fortunately, someone was taking pictures during the event, so I was able to see a camera flash on both cameras. But it appears in an unusual way:

Timecode:
1:14:28 both pictures normal
1:14:29 Camera A picture slightly brighter, Camera B normal
1:15:00 Camera A normal, Camera B shows full frame flash
1:15:01 Camera A shows only the top 1/3 frame with flash, Camera B normal
1:15:02 both pictures normal

The fact that Camera A showed only 1/3 frame of the flash leads me to believe I'm seeing a rolling shutter effect. So, I'm not sure exactly how I should correct this. It appears that Camera B is one frame ahead, so I should move it back one frame. Does that make sense?

Also, is there any way to move all the events related to just one camera in a multi-camera track? There are so many cuts, and we used pan/crop to reframe shots. I'd hate to have to start over from scratch.

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Comments

musicvid10 wrote on 1/25/2013, 12:06 AM

The maximum frame variance between two cameras would be 1/2*1/29.97, or <1/60th of a second, assuming NTSC.

ddm wrote on 1/25/2013, 12:34 AM

there's also 24p and 30p to consider, as long as we're digging in the weeds.

musicvid10 wrote on 1/25/2013, 8:47 AM

OP is running 30p.
So the maximum frame boundary offset is +/- 1/60th sec, not 1/30th.

smhontz wrote on 1/25/2013, 10:26 AM

I really, really appreciate everyone's feedback on this issue and I want to ask just one more question so I can make sure I make the best decisions going forward as I film more multi-camera events. These (so far) have been a single speaker or a musician/musical group performing.

Given this typical setup:
- Two cameras that have the capability to sync timecode to each other
- Camera A: Wireless lav mic on Ch 1, On-camera mic on Ch2
- Camera B: House mixer feed on Ch1, On-camera mic on Ch2
- Zoom H4n near stage as a back-up audio recorder

Here is my suggested process:
- Link the cameras to sync timecode; if possible, leave them connected during the event
- Start both cameras running, take a picture with a flash so I have a clear visual reference point that each camera can see
- When editing, sync based on the visual flash
- Use the best single audio feed I have (probably the house mix)/mix feeds as necessary, sliding audio to match up if necessary (when mixing multiple feeds)

Does that sound okay?

EGS wrote on 1/25/2013, 12:21 PM

"...sliding audio to match up if necessary (when mixing multiple feeds) Does that sound okay? "

I'm a long-time audio pro. I think trying to combine audio tracks taken from different cameras is asking for trouble. Even if the cameras are TC synced, the audio will very likely be off enough to cause audible phasing. TC is not as tight as a digital clock sent via wordclock. I'd suggest recording all audio sources to the same machine, like maybe a 4 or 8-track Sound Devices recorder or similar. I use a portable Pro-Tools rig. All audio sources are recorded on separate tracks in Pro-Tools. The cameras record the on-camera mics - only as a reference, never to be used in the final edit. If necessary, the audio tracks can be time-aligned in Pro-Tools. I recently recorded an outdoor band gig, 4 audio tracks in Pro-Tools - 2 tracks from the PA board and my 2 mics positioned about 25 feet from the stage/PA speakers. The 2 mic tracks were late, of course, as compared to the board feed. In Pro-Tools, I zoomed-in on a solid snare hit, slid the 2 mic tracks back a little, and it sounded great. Export this time-aligned audio mix into Vegas, and go to town. My 2 cents !!!

ddm wrote on 1/25/2013, 1:06 PM

Wait a sec. if you're not using timecode to sync cameras in your video editing program, which vegas won't do anyway, I don't think, then why bother having matching timecode. You'd be better off just genlocking the cameras, that way, the flash method will work fine. Matching timecode in multicamera is essential in programs like Avid's Media Composer, it actually looks at the timecode and syncs everything automatically, no flash or plural eyes needed, you can stop and start cameras etc. but vegas won't use timecode like that, so it's kind of a moot point.

OldJack wrote on 1/25/2013, 4:04 PM

@EGS...Have you ever used Pluraleyes software to sync audio?

musicvid10 wrote on 1/25/2013, 6:49 PM

If you are going to have a card/tape change, it is important to run the same audio to both cameras, if those serve as your primary recording devices.

Genlock starts the frames together, at the same time. Genlock and timecode sync are two different things.

Vegas uses SMPTE timecode to sync the takes using the Multicamera options.
This is not speculation. This works unless there are garbage frames at the beginning of a take.

It's nice to have a audio/visual sync frame as a backup. It doesn't need to be a camera flash. A hand clap by non-equity talent works just fine ;?)
I use a Seiko metronome that has a loud tick and red LED.

The same points have been repeated enough times that I'm going to bow out of the discussion. Let us know how it turns out!

ddm wrote on 1/25/2013, 8:55 PM

>>Vegas uses SMPTE timecode to sync the takes using the Multicamera options.

Excellent, did not realize that. That makes things very easy if you're using cameras that have timecode in. No reason to bother with a flash or plural eyes, if you have matching timecode then the pictures will be in sync. (always good to have a plan B like musicvid says)

Byron K wrote on 1/25/2013, 10:20 PM

Sorry to jump into this thread so late in the game but here's 2 methods I use when editing video from different cams that are out of sync w/ out time code.

Hopefully the audio and the video of the cam that is out of synch is in sync but just doesn't line up with the other camera's audio. If the camera that out synch has audio that is not in synch w/ it's own video then that's a much more challenging issue and will probably require a lot of manual aligning.

Fortunately, my cams stay pretty much in synch because I use the same brand cameras but sometimes i get video clips from other people who are taking video w/ their cams and their clips are not in synch w/ the my cams but the audio and video of their camera is in synch.

Method 1:
Do a rough split of the video of the camera that is out of synch by pre splitting of the video that's out of synch at various sections then synch w/ Plurals eys then use Vegas multi cam to do rough cuts. Then tweak the cut points

Method 2:
Use Plural Eyes to synch the videos from the start and solo one audio track, use Vegas multi cam to get rough split points. At this point the video will be out of synch but no worries this will be fixed in the next step. Exit multicam and expand the video on separate tracks. Use Plural Eyes to re-synch the split video clips to the master audio track (in your case the house mix) and then tweak cut points. Mute or delete the audio track of the cam that is out of sync.

MikeLV wrote on 2/6/2013, 11:26 PM

I always sync my two cameras up based on the audio tracks. I start recording on both cameras and then just clap my hands and match up the wave forms. I wouldn't even know where to begin with syncing video if my cameras are shooting different things... I leave Quantize to Frames off permanently because there's a lottttttttt of space between frames when making adjustments. Nothing ever happens at frame boundaries.

smhontz wrote on 2/7/2013, 8:37 AM

We solved the problem on our most recent shoot by hooking the cameras together just before the shoot, syncing the timecode, and then disconnecting them. We also shot a few test flashes from a still camera to verify that the timecode stayed in sync during the show. It all worked beautifully, and I was able to have the multicam feature in Vegas just sync everything by timecode, so there was no manual syncing to be done.