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studiostuff

Identify interference...?

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Can anyone give me some information as to what this sort of interference might be?

(Attached are two screenshots from a brief iZotope RX-5 sample of a session I recorded last night.

One shows the interference as a somewhat continuous (there is some amplitude modulation) frequency at 1kHz, with overtones at 2, 3, and 4 kHz. There are additional harmonics that get quieter as the frequency goes up and so are difficult to see on this example.

The second screenshot is the same sample, using a feature of RX-5 that allows one to select the fundamental frequency, and then select for the overtones too. Here , it is easy to see additional harmonics at 5, 6, 7, 8, 9, and 10kHz. The frequencies are very specific.

A little background, the rig is an Apogee Ensemble Thunderbolt into a MacBook Pro into ProTools. I was running eight channels of audio, using a Neumann KM-184 pair and AKG C414 pair and C451 pair condenser mics, a Sennheiser M-88, and an Electrovoice RE-20. Mics and cables were all on a medium size stage. The mic and cable that were bringing the interference to the recorder was off on the left side of the stage, 5-10 feet from other cables.

I use the rig all the time without seeing this sort of interference. On Friday, I used the rig in one location, and Saturday in another (same everything; mics, cables, etc.). There was no interference on Friday and on Saturday, it there was.

I also believe I have seen the interference at the same venue in the past.

iZotope RX-5 fixes the issue without killing a lot of time, and in this situation, the result is unnoticeable.

My question is; what does this look like to you?

Screen Shot 2016-03-13 at 4.14.08 PM.png

Screen Shot 2016-03-13 at 4.14.36 PM.png

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You haven't told us anything about the recording environment, which in this case is much more important than what mics you used.  Was this a location dialog job?  A concert recording?  What sort of venue was it?  There are many possible sources for noise like this, from internal system issues on your rig to noise generated in the recording space that was heard by your mics.

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Yes. I guess you're right. This was a concert recording... a 40 voice choir. Did you read that there were seven other mics on the same stage that didn't "hear" the interference that was recorded on one track only? The venue was an old church. The stage (altar) was built in the last 50 years, and my suspicion is, may have some ethernet or some other leaky, in terms of electromagnetic radiation, wiring going on under the stage. I don't want to seem overly aggressive in my question to you, but if you look at the screenshots, this should probably suggest something other than the trees you are barking up... I think it's safe to assume there are not "many possible sources for noise like this". I think it's some sort of induced contamination. Anyone else?

 

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Perhaps...  On just one mic? The interference was on one of the KM184 pair, both of which were up 15' or so in the air. The lights were quite a bit higher. The interference was continuous in terms of frequency content and amplitude for two hours. Level of the interference was very low compared to any acoustic sound the artists were making.

All the mic lines were balanced XLR cables in good shape. So common mode rejection should keep everything quiet, but one mic was getting something from somewhere. 

I use this rig all the time, and the signature of the noise is pretty odd. I guess it could be the rig, but the issue seems to link to this venue.

Definitely not an acoustic sort of overtone series. Ruler level at each frequency and in terms of amplitude. The amplitude modulation that can be seen in the diagrams and heard sounds a little like high speed Morris. Fast, but not so fast that one is not able to determine the tiny pauses occasionally between the frequency bursts, which are predominantly continuous. 

C'mon. This seems like the sort of thing that someone should recognize. The odd clues are the overtone relationships to the fundamental, and the continuous nature to the contamination. It's some sort of radio... Wireless mic? It's a church, and if there is a wireless system, it ain't mine, nor something I've encountered before. 

 

 

 

 

 

 

 

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Seems unlikely, but maybe some kind of motion-activated sensor? AFAIK those are mostly ultrasonic, but the nature of the frequencies points towards something intentional. Something like that would be pretty directional also.

Fun puzzle, doesn't seem like a horrible problem, but it is curious! 

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2 hours ago, studiostuff said:

Yes. I guess you're right. This was a concert recording... a 40 voice choir. Did you read that there were seven other mics on the same stage that didn't "hear" the interference that was recorded on one track only? The venue was an old church. The stage (altar) was built in the last 50 years, and my suspicion is, may have some ethernet or some other leaky, in terms of electromagnetic radiation, wiring going on under the stage. I don't want to seem overly aggressive in my question to you, but if you look at the screenshots, this should probably suggest something other than the trees you are barking up... I think it's safe to assume there are not "many possible sources for noise like this". I think it's some sort of induced contamination. Anyone else?

 

Barking along here, yes I read your post and all of the things I mentioned are not only possibilities, they have happened to me doing this kind of work!  Feel free to disregard them if you like.  You could have a mic pre going bad at the component level, ditto the mic head itself (have heard that freq range re that condition).  That channel's mic could have ended up near a motion detector or a lighting instrument that could very well emit that sort of sound (have had these in this range).   You could have a clocking issue in your interface.   The mic powering for that channel may be going.  Is that cable's shield really good?  Connected at only one end?  Yes, you could have some kind of hash being inducted into your mic line at that freq.--I had a series of shoots in a place where mics would pick up that hash but only in certain parts of the room and only at certain angles--the culprit was some kind of lighting motion control sensor.  I've also had that freq of hash when working in a place under some digital data repeaters--again very very "nodey", as in you could go in and out of the hash by moving just a few inches or making a small adjustment in the angle the mic was held at.  You may have just been unlucky about exactly where that mic ended up that night and at what precise angle.  You might try waving your mic around in that area to see if you pick the hash up again--in my cases it was pretty repeatable.   The only cure for when the mic HAD to be in one of those nodes was to go to a Sennheiser 416.  Schoeps, Neumann and Sanken mics all were affected, but not the Sennheiser.

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You certainly have a funny way of asking for help. It almost seems like you already know the answer. Anyway, there are many possibilties for this, Philip has listed them. My bet would be on an internal problem. Maybe a new graphics card? Or whatever. Without acess to your gear and without actually listening to the recording itself rather than looking at it, I don't think this will be solvable for us.

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Constantin: This place has a well earned rep for folks attempting to be aggressive with new posters. Since I don't feel like a newbie after doing this for 30-35 years, I found Philip's tone a too smug. I thought his original answers sounded like he had not read my post and was just trying to be the 7,000 post boss. To speak to your post, I'm not really asking for help. I'm asking if anyone can say specifically what this interference might be. So far, in my opinion, the only answers that are starting to get there have to do with motion detectors. 

 

Philip suggested it might be a bad cable. That's could be a possibility, too. I didn't have time to move the mic around, but did move the cable two or three times, and couldn't really tell under the circumstance whether it was making a difference or not. 

 

I think suggestions like bad clocking, bad preamp, bad mic capsule are pretty thin possibilities due to the facts that I use the rig all the time and it's this venue that offers the interference. 

 

I think the frequency bands and the consistency are the big clues, and was hoping someone in this oh so knowledgeable group would simply be able to say what it is and they see it all the time. 

 

The "fix" is easy enough with RX-5. 

 

So thanks all for any other ideas you may have and for your charming welcome to this group. 

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So thanks all for any other ideas you may have and for your charming welcome to this group. 

To roughly translate a German saying: "how you shout into the forest is how the forest will shout back at you".

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Two last thoughts if you are still listening: 1: take seriously the notion of some kind of RF type interference like I mentioned, really, because in the cases I talked about the interference was pretty much exactly on the freq in your situation; 2: because you use a rig all the time doesn't mean it can't screw up, or have a component going bad.  If it was me I'd tear at all apart and test everything unless I found a smoking gun in the recording environment first.

This is a great forum, the best there is (in English anyhow) re location recording.   What you find aggressive is me wanting to know enough about your issue to offer relevant advice.   Sorry if you find my free advice smug.  I will refrain from offering it to you in the future.

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Phil knows what he's talking about, and my advice to the poster is to listen what he has to say. Be aware that when you ask for free advice on the internet, sometimes the advice may not be what you want, what you expect, nor is it delivered in the way you anticipate. At least it gives you some possible clues, and all it cost was your time; it's up to you to decide what advice to take or reject.

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18 hours ago, studiostuff said:

This place certainly lives up to its reputation. Thank you for the interesting lesson about forests and Germans.

Seems to me you got some great and helpful replies.

One thing that helps around here is using your real name rather than remain anonymous. So perhaps change your membership or at least add your name to the signature of your posts.

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about 10 days ago, we were in a church and had some unexplained rf grief. turned out it was an assistive listening system that worked on 2.4ghz. 

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