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Mono channels 'cancelling' each other out when I record with an external mic

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Greg Cunningham
Mono channels 'cancelling' each other out when I record with an external mic
on Jun 30, 2009 at 3:33:32 pm

Hello, and thank you in advance...

I'm recording audio from a rode ntg2 to a canon gl2. I use an xlr to mini jack to plug it directly into the camera, and record on L/R channels. When I'm listening to the sound on headphones as I record, I can adjust the levels, tweak it, and everything sounds great. However, when I put it into final cut I get a very faint hiss, and the speech or sound of whatever I'm recording is hardly audible. However, when I delete one of the channels, it gives the mono sound that is similar to the one I heard while I was recording (it sounds a little less sharp than when I when I was recording). This result happens regardless of using the mic attenuator or not. Do you know what's going wrong when I record? Since the ntg2 is mono should I should be recording on one channel? should I be using an xlr switch?



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Peter Groom
Re: Mono channels 'cancelling' each other out when I record with an external mic
on Jun 30, 2009 at 9:14:43 pm

Hi
Youve got an out of phase cable or link in there somewhere, or FCP may have a phase switch (although I doubt it) which is depressed on 1 ch.

Basically, when 1 channel has a positive rise in amplitude, the other channel is negative. When you mono both up, they phase cancel each other out as theyre 180 degrees out of relative phase to eachother.

So
either find your phase inversion and fix it....
or take one channel in fcp only and pan center.

Cheers

Peter

Dubbing Mixer



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Ty Ford
Re: Mono channels 'cancelling' each other out when I record with an external mic
on Jul 1, 2009 at 12:32:25 pm

Well sort of, Peter.

But properly recorded channels of audio from a single mic are phase coherent. That means the rises and falls of the waveform happen at exactly the same time on both channels.

More likely, what's going on is that the connectors are not wired properly to bridge a mono mic to two inputs. To do that, the #2 pin of the female XLR needs to be connected to both the tip and the ring of the 1/8" TRS plug that goes into the camera. That way, both inputs get fed the same signal properly.

You may also have another issue. Some camcorders mic input circuitry generates a small voltage that is used to power some mics. This bias voltage can work against you with some mics, causing distorted audio. Trew makes cables for these inputs that block the DC bias voltage. They make a two channel rig for when you want to use two mics, each to a separate track,
http://www.trewaudio.com/store/product.php?productid=147&cat=21&page=1

and a mono rig for when you want to send the audio from one mic to both channels.
http://www.trewaudio.com/store/product.php?productid=146&cat=21&page=1

Regards,

Ty Ford




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






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Greg Cunningham
Re: Mono channels 'cancelling' each other out when I record with an external mic
on Jul 1, 2009 at 4:14:05 pm


Thanks for for helping me out guys-

So I had a mini breakthrough when I reflected on the last thing you said Peter, to take one channel and pan it in the center...

I had heard that before, and was about to say how it's weird that each audio signal I get comes in panned in the center already, so when I delete one everything sounds fine. DUH! I panned each from -1 to 1, and now the signal is a lot clearer and sounds like when I recorded it.

However, that does not solve everything. Some of the audio I get sounds fine the way it's imported on both tracks, while at other points of the recording it suddenly goes back into the 'signal cancellation' sound, where everything is fuzzy and distant sounding. Also, the audio still sounds better with one track panned in the middle than two at each end.

Ty, the GL2 offers no phantom power to microphones. On the other hand, I think this is on the right track:

"More likely, what's going on is that the connectors are not wired properly to bridge a mono mic to two inputs."

However, I don't quite understand what this means:

"To do that, the #2 pin of the female XLR needs to be connected to both the tip and the ring of the 1/8" TRS plug that goes into the camera. That way, both inputs get fed the same signal properly."

Are you basically saying I have a faulty xlr to mini cable? That could definitely be the problem, since this has only been happening since I started using that cable.

Also, is FCP importing my audio tracks correctly? When I record with a mono mic on L/R, should it come in as 2 tracks, each panned to the center?


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Ty Ford
Re: Mono channels 'cancelling' each other out when I record with an external mic
on Jul 1, 2009 at 6:16:13 pm

Greg,

Phantom Power and bias voltage are not the same.

Your cable isn't faulty, you choice of cables was.

Stop and think about it. You're plugging a MONO source into a STEREO input. Why should that work?

Just because the plug fits into the hole doesn't mean they are wired correctly for the job you'd like to do.

Regards,

Ty Ford

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






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Greg Cunningham
Re: Mono channels 'cancelling' each other out when I record with an external mic
on Jul 1, 2009 at 6:31:09 pm

Ty, you are absolutely right; I just checked the specs on the cable and it's for stereo recording. I had no idea the cable itself had anything to do with it. I'll look into getting something else...

In the mean time, does the stereo cable distort the sound I am getting? ie, should the one channel of sound I get be of the same quality as what I'm hearing when I record?

Thanks again,

Greg


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Greg Cunningham
Re: Mono channels 'cancelling' each other out when I record with an external mic
on Jul 1, 2009 at 7:48:59 pm

Now I am confused again...

I contacted the guy I bought it from and he told me this:

"But just want to let you know that the cable you bought from me is a mono cable. 3pin XLR Mic's are all mono mics.

It's the camcorder itself that takes the mono signal from the mic and splits it into left and right channels.

However, it doesn't seem to me that you are running the cable directly into the camera mic input."

The only relevant part of that is "the cable you bought from me is a mono cable." When I said that It was indeed stereo, that was off something I misread on the auction. Are there such things as mono and stereo cables? Now I'm not sure if I've messed up the whole concept.

I bought it from an auction on ebay, you can look at it here and tell me what you think:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220446230677&ssPageName=...

Thank you so much,

Greg




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Ty Ford
Re: Mono channels 'cancelling' each other out when I record with an external mic
on Jul 1, 2009 at 8:40:14 pm

[Greg Cunningham] "I bought it from an auction on ebay, you can look at it here and tell me what you think:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220446230677&ssPageName=..... "


I have solved your problem with two previous posts. Please refer to them for your answer.

Ty Ford

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






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Ty Ford
Re: Mono channels 'cancelling' each other out when I record with an external mic
on Jul 1, 2009 at 8:33:40 pm

Greg,

Not so much the cable as the connectors. Look earlier in this string for cable with connectors that will work.

All bets are off until you get the right gear.

Regards,

Ty Ford



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






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Peter Groom
Re: Mono channels 'cancelling' each other out when I record with an external mic
on Jul 4, 2009 at 4:53:07 pm

Hi

Your man is right. All 3 pin XLR mics are mono. The confusion here arises from the fact that mono signal sources with 3 pin XLR to 3 connector jacks (screen, hot, cold or tip ring sleeve) are not stereo they are "balanced". Those with tip and ring only are "un-balanced"

Nothing to do with stereo or mono. But you need to put a balanced plug into a balanced socket, or else the connections inside will short out causing no end of problem. (This can often be aidd by half pulling out the jack to undo one of the mis matched terminals inside the socket.

NB RE your FCP import. You have to tell FCP if audio is to be paired stereo or not. This affects its left right panning arrangement default. Its a button on the timeline. Hovver over them to find out which one it is as i cant remember. I dont like fcp so rarely touch it.

peter



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