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Mixing Camera-mounted External Mic + LAV mics = Audio Sync Issue

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Joao Luis
Mixing Camera-mounted External Mic + LAV mics = Audio Sync Issue
on Oct 30, 2015 at 7:53:35 pm

Hi,
I'm working on a project that involves shooting a lot of walking dialogues between two speakers, about 2 meters (6ft) away from the camera.

For stereo ambient sound I'm using Audio-Technica's AT8024 mounted and plugged into a GH4. For the speakers I'm running two Sennheiser lapel mics into a Zoom H4n.

When I bring the audio and footage into Premiere, both the AT8024 and lapel mics are in sync with the image (same audio settings on the GH4 and Zoom), but they're ever so slightly out of sync with each other, resulting in a "boxy" sound for the speakers' voice when I mix the audio tracks.

My guess is that this sync issue has to do with the lapel mics being closer to the sound source (talent) than the camera-mounted mic standing 6ft away and capturing both ambient and voice with a tiny delay, noticeable when mixed together.

This being the case, what is the best way to capture both stereo ambient sound and dialogue in this scenario? Should I just ditch the audio from the camera-mounted mic in these shots?

Cheers!


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Bruce Watson
Re: Mixing Camera-mounted External Mic + LAV mics = Audio Sync Issue
on Oct 31, 2015 at 1:16:35 am
Last Edited By Bruce Watson on Nov 1, 2015 at 10:48:18 pm

[Joao Luis] "This being the case, what is the best way to capture both stereo ambient sound and dialogue in this scenario?"

The best way is to capture dialog separately from everything else. The way Hollywood does it. You now know why they insist on doing it separately. And you've probably figured out that there really is no way to do a good job capturing both at the same time.

That said you can try delaying the lav tracks a bit. Sound in standard conditions travels at around 0.12 m/millisecond, so if you're a couple of meters apart, try a delay of 4-6 milliseconds. This might help if you're recording either outside, or in a large space. If you're in a small space, the boxy sound is "small room sound" cause by rapid reflections off room surfaces and all you can do about that is drop the "ambient sound" tracks from the mix. Sad but true.

[Joao Luis] "Should I just ditch the audio from the camera-mounted mic in these shots?"

Probably what you end up doing, yeah.

EDIT: Fixed my units. Only off by three orders of magnitude. Easy catch inside a DAW. But just using my brain, well....


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Joao Luis
Re: Mixing Camera-mounted External Mic + LAV mics = Audio Sync Issue
on Oct 31, 2015 at 2:41:30 pm

[Bruce Watson] "That said you can try delaying the lav tracks a bit. Sound in standard conditions travels at around 0.12 m/micro-second, so if you're a couple of meters apart, try a delay of 4-6 microseconds. This might help if you're recording either outside, or in a large space. If you're in a small space, the boxy sound is "small room sound""

I'm recording outdoors so no early reflections here. I did achieve some decent results delaying the lav mics track and fine-tuning the sync point. It seems like a viable option provided the distance between camera and speakers doesn't change too much. No "walking-up to the camera" shots I guess, but it's a chance at having the immersive (live) stereo mix I was aiming at, with cars driving past in the background, birds, feet walking on gravel, etc.

I'll test this out a bit more. I can always just use the lav (omni)mic tracks, maybe pan those left and right a bit.

Thanks for the help.


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Peter Groom
Re: Mixing Camera-mounted External Mic + LAV mics = Audio Sync Issue
on Oct 31, 2015 at 1:04:43 pm

Are your personal mics radios?
Peter

Post Production Dubbing Mixer


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Joao Luis
Re: Mixing Camera-mounted External Mic + LAV mics = Audio Sync Issue
on Oct 31, 2015 at 2:51:10 pm

[Peter Groom] "Are your personal mics radios?"

Yes Peter, a pair of Sanken COS-11Ds plugged into Sennheiser G3s.

At first I was convinced that this could be part of the problem. Either a radio delay(?) or the voices bleeding into each other personal mics - also being captured by the stereo mic mounted on the camera - with ill effects.


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Ty Ford
Re: Mixing Camera-mounted External Mic + LAV mics = Audio Sync Issue
on Nov 1, 2015 at 9:16:21 pm

Hello Joao and welcome to the Cow Audio Forum.

Bruce's answer is right, but I do the math differently.

Sound travels at 1100 ft/sec. So about one millisecond per foot. Your camera mics and lavs are creatingr time/space problem and it's causing phasing and comb filtering.

I guess I'd ditch the camera mic and split track the lavs, then pan 'em a just a bit in post to reduce the bleed, or maybe do a little more active mixing to pull out the mic not being spoken into.

Of course a nice cardioid on a boom might solve all of your problems. No phasing! Unless of course you can't cover the action with just one boom (and a qualified boom op)

Regards,

Ty Ford
Cow Audio Forum leader

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide
Ty Ford Blog: Ty Ford's Blog


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Bruce Watson
Re: Mixing Camera-mounted External Mic + LAV mics = Audio Sync Issue
on Nov 1, 2015 at 10:49:26 pm

[Ty Ford] "Bruce's answer is right, but I do the math differently."

Differently, and correctly. Sigh... I fixed my post. Thanks for the correction.


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Joao Luis
Re: Mixing Camera-mounted External Mic + LAV mics = Audio Sync Issue
on Nov 2, 2015 at 10:36:51 am

Hi Ty,

A boom mic would be ideal, but it's a one-man gig.
The camera mic is usable if the distant remains constant, i.e. constant delay, although it can be an unreliable element in the mix. I'm probably going to settle with split tracking the lavs and a bit of panning.

Thanks for the help.


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Ty Ford
Re: Mixing Camera-mounted External Mic + LAV mics = Audio Sync Issue
on Nov 2, 2015 at 2:31:16 pm

Hi Joao,

Heh, I read that as you are a one man operation, but at some point, the gigs will require more.

This is an excellent time to congratulate yourself on getting bigger gigs, but as you do, remember that the more complicated a gig the less of you there is to spread around.

I'd begin asking myself who I know that I could partner with to make sure you get what you want. I shoot, light, do sound and edit but inevitably, when trying to do all, something gets a bit lost.

Regards,

Ty Ford
Cow Audio Forum Leader

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide
Ty Ford Blog: Ty Ford's Blog


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Joao Luis
Re: Mixing Camera-mounted External Mic + LAV mics = Audio Sync Issue
on Nov 2, 2015 at 7:43:38 pm

Hi Ty,

I absolutely agree that one can easily be spread too thin taking on a complex project alone. Unfortunately I'm not in charge (yet!) and this being an all-around the world endeavor, the company can't afford another crew member on the road. Otherwise I'd definitely partner up with a sound technician, at least.

By the way - and sorry to steer off topic - I did a bit of research on the recorders we have at hand (Tascam DR-40 and Zoom H5) and came across your review. I have some doubts about the preamp quality on the Zoom so my idea is to send the lav mics' signal at a reasonably hot level from the receivers so I don't need to push the gain too much on the recorder to have it peaking around -12db.

Is this strategy correct?


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Ty Ford
Re: Mixing Camera-mounted External Mic + LAV mics = Audio Sync Issue
on Nov 2, 2015 at 8:55:59 pm

Hi Joao,

The best partners are those who - !. Get along! and 2. Have overlapping capabilities.

I am mostly a sound guy, but I also shoot and light and edit. I partner with guys and gals who usually are more videocentric and need the audio help. but I also help them with lighting, gripping, shooting and finding solutions to get the best out of what we show up with.

Regards,

Ty Ford,
Cow Audio Forum Leader

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide
Ty Ford Blog: Ty Ford's Blog


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