Multicam audio echo

Hey everyone.

This problem's driving me crazy and it's probably a simple fix.

Quick facts. Have searched forum. FCPX is updated. Both cameras are the same model - jvc 100s, lavas too are the same sony models.

Situation. Two camera shoot. If i put a lav on one camera, and a lav on the other camera, in post the sound has a slight echo as if slightly out of sync -turn off one audio track, echo goes away.

Images are in sync.

when i try moving one audio track 1 subframe to the left or right, echo grows worse; more frames even worse.

IF i put both lavs on ONE camera, no problem. Okay so that's a solution until i have three lavs! (I don't want to pull off sound to say a Zoom)

AND it does not happen continuously. We have shoots where for the first minutes there is no echo, but then it arrives, stays a bit, then goes away. Trying to cut these bits up and moving their subframes does not fix the drift, the echo.

Moving from stereo setting to dual mono has no effect.

I tried deleting the video, making a new .mov out of both tracks, then mutlicaming just the audio tracks. Echo still there and subframing doesn't help.

Is this a multicam bug?
Am i doing something wrong?
Could this be a camera fail?

Anybody else run into this?
Does anyone have a solution?

thank you
Hoss

Are you using exactly same camera settings before shooting?

Yes, we were concerned about that as well. we worried that there might be an internal audio setting that was different between cameras. Nothing there but then I'm far from an expert in theses matters.

Thanks for responding.
Hoss

I'm far from an expert in theses matters.

me too, but:
you expect two devices being perfectly in synch ... except cams triggered with ext TC support, this isn't possible. Just a faction off the 'tic' you notice this 'reverb' ...

I'm not familiar with sub-frames, all I know they exist, but I was told, it goes down to sample-frequency ...
Have you tried to put two clips in a timeline (not MC) and manually shift one's audio track?

Option #2, too simple … Use/'open' one audio track at a time ... ?

Not sure this is really a sync problem but the fact that echo is sometimes there then disappears and then returns is puzzling. Sounds more like you might have audio from both lav mics on in the project. For this type of 2 cam shoot, once your multicam is in a project, I would suggest toggling the dialog channels on and off when the respective talent is speaking. In other words when the interviewer is asking questions, disable the audio channel for the interviewee and vice versa. The echo could be coming from each lav picking up sound from both people. If there is overlap in speaking, then you keyframe parts of the necessary audio so both persons are heard, but generally disabling the audio when a person isn't speaking and keeping only the speaking persons lav enabled is a better way to go. This assumes of course that each audio channel when selected on its own sounds good and is in sync with its video frame.

I think dgwvideo's answer is most likely correct.
The in and out most likely due to subjects directly facing each other or being closer then farther away so both lavs occasionally pick up sound. You might detect this in the clips.
His suggestion to mute one channel may very well reduce the echo.
The other possibility is that the mics are picking up sound reflections from hard surfaces. Then more difficult to reduce the echo. There are threads here discussing how to reduce this or prevent it, and what plug-ins might help.

Hope this helps, Greg

Thanks to all who responded.

In the end, that was the conclusion we came too, as well - cameras picking each other up. So, we're going to move to offloading the sound to say a zoom or something when 3 lav are in play and never separating lavs to two cameras if we don't have too.

Thanks all.