Phase cancellation to remove noise

Hi guys, Obviosly phase cancellation is something we are all familiar with, M-S stereo, 1-3 rule, cheesy sci fi effect. Seriously though I did a job today and the producer told me they had had problelms with their audio and wanted it cleaning/sweetening. On one interview they had been at an airport and obviosly had to deal with planes taking off. The guy at their post house suggested simultanously recording a seperate track of the planes using a distant mic (with a distant mic) so that he could invert the signal and mix it with the interview track in post therefore cancelling out the plane noise.

Is this something that would work well and therefore worth doing in the future... I'm thinking this would also effect the frequencies not only of the plane but the dialogue.

[scott]

Having not tried this I can't say definitly but my experience and logic would say this would not work at all. For this to work at all the two mics would have to be very close together, the phase and timing of the sound you want to remove has to be exactly the same at both capsules for them to cancel when one is reversed. But I have seen things more counter intuitive than this work so who knows.

[Simon]

I think this may be classified under the "Knowing just enough to be dangerous" category. I've alos heard this "Technique" being attemted to get rid of echoes from a particualr location. It does not work !

If an airplane only had very low frequencies, and you kept the 'noise' mic fairly close to the 'voice' mic (but not close enough to pick up much voice), it's conceivable you could catch plane noises in nearly the same phase on both mics, and then lose the plane by flipping it in post. Though it would be a lot easier to just throw a filter into the voice mic channel.

But airplanes are broadband. The only way that two mics could catch every frequency in the same phase would be if they're exactly the same distance from the plane, with exactly the same acoustical path. If there's any difference in the paths at all, flipping and inverting one mic will cancel some frequencies and reinforce others.

[philper]

Ask the post guy if has ever really tried this and how did it sound?

Philip Perkins CAS

thanks for yor responses. I will find out more from the post guy... laters

[olrg]

what about shoot the plaine that evryone will know where is noise come from.the neks can be another good thing.reporter mike in the frame can do the job .the last eskape is the telephone about bomb in the eria or a folt declaraition about no money for workers in the field(strike in 5 min),i think all this things will work better than the phase cancelaition :shock:

[oleg]

what about shoot the plaine that evryone will know where is noise come from.the neks can be another good thing.reporter mike in the frame can do the job .the last eskape is the telephone about bomb in the eria or a folt declaraition about no money for workers in the field(strike in 5 min),i think all this things will work better than the phase cancelaition :shock:
Oleg Kaizerman

[Angelo D´Angelico]

There are same good tools to remove far airplane noise in the mix, such as the Hum Rem. Plug in, or the cedar, or even good notch filters.
But the Problem is the permanent change of frequenz from a passing airplane.
Maybe the guys in the post won´t do a " phase cancellation " ( I don´t think, anybody will really think about this ).
Maybe the guys just want to have the second mic to use the sound as a clear " finger Print " of the frequenz , to feed the outboard.
It did the experience in the mix, that passing airplane noise is not a broadband noise.
Bút this stands not for starting or landing airplanes ! This is white noise broadband !

[Chris Price]

It seems to me that this technique if it is to work at all has to be applied exactly. In other words the noise cancell channel has to be very near the mike which is recording the used audio channel. Loweer frequencies can be coherent with the cancelling mike further waway.
Then there is the question of where is the noise coming from and thirdly the cancelling channel should hear as little of the dialog as possible.
In principle that is what a hypercardioid or shotgun mike does anyway
I would tend to close mike the person(s) being interviewed with a radiomike turned right down at the transmitter end so there is no limiting in the radio and if two people are talking record two channels so that you can edit the dialog AB so that will halve the background atmos compared with a 2 mike mono recording. A 416 just on the edge of frame from underneath is also a great improvemnt as long as a plane does not fly over head during the shot!

Chris Price
Ambient: info@ambient.de

[Steven]

2 words: CEDAR DNS-2000 :D

I have one at YELLOW CAB Studios, and it sometimes REALLY helps in that sort of situation. The phase cancelling "trick" is about as effective as... er... well it's not actually :?

Cheers,

Steven GHOUTI
YELLOW CAB Studios
Paris, France

Has anyone actually tested the "phase cancellation trick" and can he/she describe the process in details?
Branko

[verrando]

It's never been tried because it's so impractical.
1. A device to minutely adjust the phase between two sources (basically a delay) would be required on set or in post. On set, who has time to monkey with it? And the moment one of the mics moves, you'd have to reset the phase!
2. A good mixer has a multitude of means, through tools or negotiations, to minimize noise problems in a given situation. I have negotiated moving entire setups into other parts of buildings to avoid noise.
3. Wireless can save the day when choked up to the thorax on an actor. Not a lovely track, but a useable one. I have gotten usable tracks on airport tarmacs this way.
Of course, some noises are justifiable in the background if the shot reveals them!
4. Headphones accentuate noise problems, and many mixers listen at too loud a level.
Experience lets you judge what is acceptable and what is not.
5. This "phase trick" is basically an electrical analogy of a cardiod microphone. At best, you get the same results as.... using a cardiod microphone!
6. DSP noise reduction software can dramatically reduce many kinds of noise. I use it, it's fantastic if you don't overdo it.

I have a phase device for listening to shortwave radio. Using two antennas, one primarly tunes the desired signal + local noise and the other is for receiving mostly the noise. By careful phasing of the two signals, I can eliminate the noise. It works because the antennas, the noise source and the station are all physically fixed. But it takes lots of careful tweaking to make the noise drop out.
Pete Verrando C.A.S.