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Old June 28th, 2009, 03:03 PM   #1
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Fixing a quirk in audio

I recently purchased the Senny G2 wireless lav sets. So far the audio sounds great except I have a slight "tunnel" effect when I have two talents talking side by side. When one person is talking the others mic is picking it up and vice versa creating audio that sounds like we are in a tunnel. Its not terribly bad, but is noticable. In post we are simply cutting out the audio line of the person not speaking at the time. However we do lots of two person interviews so this is kinda becoming allot of work. Is there anything esle we can do or am I approaching this the right way?
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Old June 28th, 2009, 04:52 PM   #2
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Reduce the sensitivity or move your talent farther away. You are getting a echo effect due to both mics mixing
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Old June 28th, 2009, 04:58 PM   #3
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Brandon, do you use omni or cardioid lavs? You should have cardioid and try pinning one hopefully both under their clothing with the appropriate protection like the Rycote pads.

http://www.bhphotovideo.com/c/produc...lier_Wind.html

In post, depending on your NLE you might be able to find a set of audio gates as add ons. You record the talent on separate tracks and once you set the audio threshhold they open and close with the speech.

They're also available as external rack mount rigs.

Cheers.

Last edited by Allan Black; June 28th, 2009 at 05:37 PM.
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Old June 28th, 2009, 06:12 PM   #4
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Quote:
Originally Posted by Brandon Carter View Post
I recently purchased the Senny G2 wireless lav sets. So far the audio sounds great except I have a slight "tunnel" effect when I have two talents talking side by side. When one person is talking the others mic is picking it up and vice versa creating audio that sounds like we are in a tunnel. Its not terribly bad, but is noticable. In post we are simply cutting out the audio line of the person not speaking at the time. However we do lots of two person interviews so this is kinda becoming allot of work. Is there anything esle we can do or am I approaching this the right way?
I'll have to disagree with both Mark and Allan. Bleed between the mics is a common problem but reducing the recording levels won't fix it - all that does is reduce the level of the desired sound along with the undesired bleed. Restoring normal levels in post will restore both the good and the bad in equal measure. Using cardioid mics isn't usually a good solution either. Cardioids are usually reserved for sound reinforcment applications where controlling feedback is a dominant issue. In film and video applications, normal head movements will cause the talent to go on and off mic with a cardioid. The best solution is to insure each mic is recorded to its own isolated track, never mixing them in the field. In post, both tracks are panned centre and when talent A is speaking you mute the track from talent B and vice versa. Sure it's a lot of work but doing things the right way usually is.
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Old June 28th, 2009, 06:23 PM   #5
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I learned way back that if two mics are used for close-quarters capture, they should be three times as far from each other, than they are to the source. Therefore, 6 inches from talent to capsule translates to a 1.5 foot distance between the two mic capsules. This might not always be possible, but it would have likely helped you with this phasing issue. Filtration won't fix this (to my knowledge). Needs to be done in production or in sweetening (if tracks were recorded separately).
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Old June 28th, 2009, 09:07 PM   #6
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Not sure if you can adjust the phase of one of the channels with you edit system you but you could be hearing phasing if the the individual tracks are clean but the they don't mix well. As Oren said the 3 to 1 rule is very useful in situations like this.
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Old June 28th, 2009, 09:27 PM   #7
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Thanks

Steve, that is what we have been doing. Recording the audio on its own line and then in post muting the non speaking mic. I had a cheap set of lavs we were using and this was not a problem. Perhaps its because the Senny's are just picking up more sound. I was just wondering if there was another fix, but it looks like post is where my solution will lie.
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Old June 29th, 2009, 03:39 AM   #8
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Somewhat surprised no one included using audio 'noise' gates in post, we used them all the time in these situations probably because we also ran a music recording complex which featured gates in the consoles.

The idea is the gate opens and closes, triggered by the audio passing through it. Once you set the gates audio threshold .. off you go.

We could fix any duration take in one pass, except for intentional off mic (low level) voices, those we'd fix manually (or once a year :)

The early gates used to pop and click as they opened up (if you set the opening time to fast) so we'd copy a track and advance that a couple of msecs and use that to trigger the online tracks gate to open up. You can adjust the opening and closing times of each gate.

But newer model rack mount gates are no problem and some are built into voice processors with eq, limiter/compressors, de-essers etc. All serious NLEs have them.

One important thing is to record 15-30secs of typical location room tone, lay that up and loop it to cover the pauses/silences in the take.

If you've got an ongoing business doing this type of work it's the only way to go to stay sane.

Cheers.
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Old June 29th, 2009, 11:17 AM   #9
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Fix for Audio Quirk

Guys: The point that seems to be mostly overlooked here is that the microphones are out of phase or to put it correctly this is a polarity reversal. It could be the microphones themselves or it might be a polarity reversal taking place in the electronics; either receiver or transmitters. The easiest way to prove this is the problem is to reverse the polarity of one of the microphones; where it is fed into the transmitter. You can then hold both mikes together in your hand & feed the output of the receivers into a small mixer with the levels on both channels being equal & both channels of the mixer panned either hard left or hard right.

The result should be an increase in level when both mikes are live as opposed to the level from one mike. This is how we used to check microphone polarity in recording studios we serviced. It should work just as well with wireless mike systems.

The rule of having two mikes separated 3 times the distance from each other than the distance to the source is a very valid rule & that will minimize comb filter & phasing effects but what we seem to have here is a straight polarity reversal.
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Old June 29th, 2009, 11:39 AM   #10
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In addition to all the things already mentioned, you can get some benefit by slightly reducing the input sensitivity of the transmitters. While that will reduce the input level of the lavs and both of their voices on their own mic, it will reduce the bleed slightly more due to the greater distance. When you compensate for this lower level in post, there isn't as much bleed originally recorded to get boosted along with the voice. It could be only 2 or 3 db improvement, but combined with the 3 to 1 rule and good placement of the lavs on each talent, it can make a noticeable difference. Like any audio setting you have to experiment and not go too far one way or the other. It can be helpful if you monitor in mono while recording. That way you hear the combined effect of the two separate recording tracks. Keep the two tracks separate when recording, but listen to them mixed together in mono on the headphones. That's not always easy to do depending on your mixer, but it will demonstrate in your own ears what your settings are doing while you record.
Checkerboard editing like you've already been doing is the cleanest, but gates can help too if they are set properly and don't cause noticeable unintended effects. That can often take a lot of experimentation, but if you hit good settings for that situation it can speed up the process with long programs.
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Old June 29th, 2009, 03:34 PM   #11
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The tunnel effect is really called comb filtering and is a common problem in musical theatre where people have a tendency to sing close together, using omni-directional microphones.

No adjustment to levels in the tx packs will solve the problem, because if you reduce gain at source, you have to put it back somewhere else, so it makes no difference.

As has been said, the polarity switch can sometimes help a great deal. the same thing happens when miking up big bands or orchestra where you have lots of mics open at once. The trouble is that the polarity switch will just be better one way or the other, it doesn't have an intermediate position. The nasty phasing sound is an artefact of time. The distance between the sound sources and the mics is different, and so the sound arrives at both mics at slightly different times, producing the odd cancellation effect that varies with frequency, boosting some, and cutting others.

In live theatre, the usual trick is to use more of the microphone on the quieter person - the louder one's voice will carry the small extra difference - but, as soon as they move apart, the level has to come back up again - really tricky. In video, you have two tracks to play with and you can do some subtle fader movements with automation - it works, but takes time and relies on them not both talking at the same time. With patience you can often go through lime by line and sort it - but it really takes time.

Gates are simply horrible, the opening and closing is very distracting and I've heard them used like this, but the resultant sound is off-putting.

Comb filtering can only be really solved by moving the subjects apart, or swapping their omni microphones for cardioids, or the simplest way, moving their microphones as close to their mouths as possible, and on opposite sides - away from each other by the greatest amount.

Repairing an already badly recorded sequence means painstaking audio post - manually going through the piece. There is no gadget available off-the-shelf that will automate this. Sorry

If you google radio microphone comb filtering, you'll see the sort of issues people are having.

When mixing live, I often mark the script to remind me to remove a mic completely, just relying on the other at some points
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Old June 30th, 2009, 08:23 AM   #12
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Polarity

How do you fix a polarity issue?
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Old June 30th, 2009, 09:03 AM   #13
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How do you fix a polarity issue?
You reverse the polarity of ONE of the two channels.
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Old June 30th, 2009, 10:57 AM   #14
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The tunnel effect is really called comb filtering and is a common problem in musical theatre where people have a tendency to sing close together, using omni-directional microphones.

No adjustment to levels in the tx packs will solve the problem, because if you reduce gain at source, you have to put it back somewhere else, so it makes no difference.
The original poster was talking about two people standing near each other and talking. In that case, the reduction in transmitter input gain will have a benefit because the relationship in reduction between the on-mic talent and the bleed from the off-mic talent isn't linear. The reduction of the bleed is greater than the reduction of the on-mic signal in the originally recorded track. When the gain is raised in post there is much less of the bleed to be reamplified compared to the slightly reduced on-mic signal.
I've done it before and it does gain a small benefit due to the greater difference in level and distance between the on-mic signal and the off-mic signal. It's the same effect that occurs when using a low-sensitivity handheld mic in a noisy environment, although the benefit isn't nearly as dramatic.
I know that when two people on-stage are singing very close to each other with omni mics, there isn't enough difference in volume level between the on-mic and off-mic signals for this to work.
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Old June 30th, 2009, 11:17 PM   #15
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sorry

Quote:
Originally Posted by Steve House View Post
You reverse the polarity of ONE of the two channels.
Sorry for being so ignorant....I'm not an audio pro...obviously HAHA but how do you do that?
Thanks.
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