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Ed Denton

Phase alignment drifting

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I have some multitrack WAV audio delivered to me and I'm experiencing a weird issue regarding phase alignment. In Pro Tools I'm going through the audio and matching up the phase on the boom and lav tracks. I'm using the boom as the primary and matching the lavs to the boom. I'll sync up the phase on a clip at the start but then I'll move further down the clip and the phase will be out again! Its not consistent, sometimes its just a few sample forward, sometimes a few samples back, sometimes up to 1 or 2 milliseconds forward or back, all within the same clip recorded together in real-time. I know there is a delay involved with the wireless, but once synced I would expect the phase to remain in sync for the entire clip. Can anyone offer an explanation? Below is the recording setup:

 

Ch 1: Boom, Audix SCX1-HC, Ch 2: Sanken Cos11 -> SMDa -> UCR411a, Ch 3: Sanken Cos11 -> SMDa -> UCR411a. Recorded on Zoom F8 at 24bit/48 kHz polyphonic WAV. No record delays.

 

See below example of the drift after only 2 seconds.

Screen Shot 2018-04-30 at 6.30.16 pm.pngScreen Shot 2018-04-30 at 6.28.34 pm.png

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Looks like the boom is becoming later, so.... could the person speaking be moving slightly further away from the mic, ie mic is set for person sitting forward, and when they lean back they just doubled the distance between mouth and mic? Lav would stay constant as it is still similar distance from their mouth? sb

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I haven’t listened to your tracks, but from what you are describing, it would seem that it’s not caused by “drifting“, but rather by the physics of sound.

Unless the boom mic is attached to the chest of each speaker, right next to their lavs, the distance between the mics will vary, and therefore also the amount of the delay between them.

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1 to 2 ms is 1 to 2 feet in air. Is the boom moving around in relation to the talent? Is the talent moving their heads? Seems that this is the simplest explanation.

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Yep. People move. So do mics.

 

When I mix, for example, for a few takes I may use a bit of lav and boom in the mix and all is well. Suddenly there will be a small change. The actor might duck their chin or miss a mark, or the boom op will be a few inches higher or lower for one reason or another, and suddenly that mix of lav and boom at the levels that worked before don't work any more.

 

It's unlikely you'll be able to solve your issue clip by clip. You might just need to align and mix line by line to the edited footage, if your intent is to mix the boom and the lav together.

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Ok this makes sense, it is probably to do with the movement of the boom. I wouldn't have expected the effect to be quite so pronounced over such a small distance but I guess physic has other ideas. It's a real PITA to line up everything sentence by sentence to avoid phase issues. How to you post guys usually do it when mixing lav and boom tracks together when there are slight changes in phase. Sentence by sentence?

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There is interesting discussion about this issue in the sound editors round table discussion

in the General Discussion section that is worth a listen.

This causes me to wonder why it was not and issue when I was mixing multiple radios and a boom

to one track during my Nagra 4.2 days thirty years ago??

 

mike

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7 hours ago, mikewest said:

There is interesting discussion about this issue in the sound editors round table discussion

in the General Discussion section that is worth a listen.

This causes me to wonder why it was not and issue when I was mixing multiple radios and a boom

to one track during my Nagra 4.2 days thirty years ago??

 

mike

It's probably because things were mixed to one track. I've understood that there was generally only one mic on at any given time or the other ones were low enough not to cause phasing, just like the good mix tracks today. I mean, you had to get it right without phasing or it wasn't useable back then, so I guess people just got really good at it because they had to.

I've been thinking about using the delay function on my recorder to add latency to cabled sources to match wireless. I'm not sure how much it would help, but I guess it could be useful to have the boom always "dragging" and never "rushing" in relation to the wires. 1ms = about 32cm in the real world, so I suspect it might help significantly.

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