BMPCC audio sync issues with multi cam shoot

I just started editing some footage that I shot yesterday while using the BMPCC as a "B cam". My A camera is a Sony FS700. I captured audio using a Sennheiser ew100 G3 lav going into the FS700 via XLR, which is what I've been doing for quite some time now.

Upon starting to edit my first lesson (I do tennis instruction) I can see that syncing up the audio waveforms in FCP X gives me great results at the beginning of the video, but by the end of it (four minutes later) the audio is off by quite a lot for the BMPCC footage. I've run into this before years ago when using budget audio and video equipment to get started but never since moving up to more expensive gear.

I'm hoping that there might be some kind of fix for this but I'm guessing not. Any suggestions or ideas? Definitely disappointing as I purchased the BMPCC specifically to use as a B cam for my videos.

Thank you all in advance.

There are fixes. Pay somebody to sync your footage.
You are mismatching Frame rates if in four minutes your video and sound are out of sync.

To avoid this in the future use common time code source and feed ltc to one channel of each camera and use a separate recorder.

There are fixes. Pay somebody to sync your footage.
You are mismatching Frame rates if in four minutes your video and sound are out of sync.

Both cameras were shooting at 24fps.

23.976 or 24? Were both cameras on the same setting? Do you have any reference audio on the BMPC?

I'm almost 100% sure it's because the BMPCC was shooting in ProRes HQ right? I've heard there are some major audio issues with ProRes (HQ version).. this is what I would do.. export your audio as a blank MOV file in the format the Sony camera was using and then re-import the audio (from the new MOV file) and see how that works...?

23.976 or 24? Were both cameras on the same setting? Do you have any reference audio on the BMPC?

Both cameras were shooting 24fps. Yes, I shoot the BMPCC footage with a Rode mini shotgun mic so I could match up the audio in post.

23.976 or 24? Were both cameras on the same setting? Do you have any reference audio on the BMPC?

Both cameras were shooting 24fps. Yes, I shoot the BMPCC footage with a Rode mini shotgun mic so I could match up the audio in post.

I am pretty sure fs700 does not do 24.

23.976 or 24? Were both cameras on the same setting? Do you have any reference audio on the BMPC?

Both cameras were shooting 24fps. Yes, I shoot the BMPCC footage with a Rode mini shotgun mic so I could match up the audio in post.

I am pretty sure fs700 does not do 24.

Well I've been shooting with one for a while, and it does, heh.

Some cameras claim they shoot 24 fps, while indeed they do 23.97 - that might cause said problem.

Like Paul Golden mentioned, this honestly sounds like a 23.976 vs 24p issue. I know the BMPCC can do both 23.976 and 24p, but when I look on the FS700 product page, they only list "24p" which could mean true 24p or actually 23.976.

Is the Blackmagic footage slow or fast after 4 minutes?

Try time ramping the BMPCC footage slower or faster by a factor of .001 and see if it lines you back up.

Some cameras claim they shoot 24 fps, while indeed they do 23.97 - that might cause said problem.

Next time I shoot I'll try setting the BMPCC to 23.98 then. Hopefully that fixes it.

Easy fix. Chop the audio up into either 30sec or 1min pieces and shift/align each section up where it starts to slip. It's a cheap trick, but works for me all the time.

I never used FCPx so i cannot talk about this with much practical insight, but historically editing programs had problems with mixed audio sample rates. Did you record both with the same sample / bit rate? If not, match the sample & bit rates before importing to see if that helps (or try a different editing program like premiere, if the problem persists there).

Also it would be interesting to see how much they go out of sync as this might be an indicator if the problem is related to a 23.97 vs 24 fps problem or a sample rate problem - or something completely different.

you could select a 10min piece of both sources and check out the different in run length for that. When i have time i will try to do tests with my BMPCC vs. Fostex FR2-LE since this would be a serious problem.

cheers,
t.

Easy fix. Chop the audio up into either 30sec or 1min pieces and shift/align each section up where it starts to slip. It's a cheap trick, but works for me all the time.

Yup, that's what I've been doing. Gets the job done, but I'd obviously rather not have to continue doing it.

Did you record both with the same sample / bit rate? If not, match the sample & bit rates before importing to see if that helps (or try a different editing program like premiere, if the problem persists there).

Hm, hadn't thought about that. I don't see settings in either camera to change sample/bit rate for recorded audio. Maybe I'm missing it.

Some cameras claim they shoot 24 fps, while indeed they do 23.97 - that might cause said problem.

The FS700 does in fact state 24 but records 23.976.

Some cameras claim they shoot 24 fps, while indeed they do 23.97 - that might cause said problem.

The FS700 does in fact state 24 but records 23.976.

Reeaallly......well I'll be changing the frame rate on the BMPCC for my next shoot. Hopefully that fixes it.

There's always going to be some drift unless the cams are genlocked. But usually this drift is only a few frames per hour, so any noticeable drift after minutes most certainly is actually caused by different recording speeds.