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Old February 9th, 2012, 09:55 AM   #1
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Interference? What is that?

I'm running a Sennheiser MKH416P48 off the Zoom H4N and am getting some CRAZY interference, or something. I've noticed this in multiple locations, and have tried all that I know to get it to stop. I've changed the XLR, I've changed the batteries, and it still comes in and out, with not real consistency. Does this sound like an issue with the mic? My only other thought is to power the Zoom directly off of AC, to see if it's just a fluctuation in battery power, but I'm not sure if that's the problem. The mic has never been dropped, and has no damage that I know of to it. It's worked fine for years, and then, a few weeks ago, this. It's not location specific, as I've had it come in and out at different shoots. Any thoughts would be greatly appreciated.
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File Type: wav MONO-005.wav (2.16 MB, 92 views)
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Old February 9th, 2012, 10:12 AM   #2
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Re: Interference? What is that?

Audacity shows this as a flat line, no sound.
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Old February 9th, 2012, 10:22 AM   #3
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Re: Interference? What is that?

Jim, I'm not sure what the problem is. I've opened the file with both Media Player Classic, and WinAmp, and both are showing wave forms and the sound that I'm talking about. Course, I open it with Media Player, and it doesn't play anything. Not sure what might be the problem. Anything I can change? Since it's working here, I'm not sure what to suggest.
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Old February 9th, 2012, 10:26 AM   #4
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Re: Interference? What is that?

I've installed Audacity, and also show a flat line, but there certainly is sound to it. I can hear both the hum of my laptop, and the high pitched interference. I'm not sure why there wouldn't be any wave form though, as the Zoom was set to 85 on the gain.
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Old February 9th, 2012, 10:37 AM   #5
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Re: Interference? What is that?

I don't hear anything on Widows Media Player either.
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Old February 9th, 2012, 10:49 AM   #6
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Re: Interference? What is that?

Not sure if this will help, but I've attached a blank screen to it and made it a video. If you still can't hear it, I'm just not sure what to do. It's working just fine here.
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File Type: zip MONO.zip (178.0 KB, 52 views)
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Old February 9th, 2012, 10:58 AM   #7
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Re: Interference? What is that?

Only other thought as to why it might not be showing up for you is installed codec packs (K-lite) but I can't imagine why that would matter, as this file is straight from the Zoom.
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Old February 9th, 2012, 10:59 AM   #8
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Re: Interference? What is that?

Nope, no audio there either.
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Old February 9th, 2012, 11:10 AM   #9
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Re: Interference? What is that?

Well now, looks like I have two mysteries to figure out...

./sadface
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Old February 9th, 2012, 11:18 AM   #10
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Re: Interference? What is that?

I downloaded it and opened it in Cool Edit Pro. It is clearly an audio file, two channels, 48kHz sample rate, average peaks around -54dBFS, occasional peaks ~ 3dB higher than that.

I hear the room, and I hear a HF "birdie" wandering around the range from ~ 5kHz to ~ 6kHz, and it seems to have a harmonic an octave higher. You can even see a bit of the birdie here, on CEP's Frequency Analysis tool.

I feel pretty sure it's some sort of RF problem, but can't speculate as to the source. RF getting back into the audio, perhaps? It would be very hard to filter out of the received audio, because the frequency is very unstable. Is there anything else you can isolate? Unplug? Turn on/off?

(BTW, I can play the downloaded file just fine on Windoze Media Player... no problem whatsoever.)

Good luck!
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Old February 9th, 2012, 11:34 AM   #11
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Re: Interference? What is that?

Yeah, I shut down the computer and made it as quiet as possible in my office. The thing is, this has happened all over town, once in a diner, a couple of times in a residential area, and of course, here in my office on the university. As I have it turned on right now and am just listening to the audio in live preview mode, I barely hear it, at a very consistent, HF level. It comes and goes, which may be due to RF interference when something passes nearby, or in this case, my computer. It's just hard to tell because I can't seem to find what causes it, or when it starts, I don't know how to stop it. I'm mostly concerned with it being a problem with my microphone, as I hope it's not damaged.

Here's some room tone, very, very quiet obviously. I can't really tell if it's there right now, but I don't have Soundbooth in front of me here to do a frequency analysis.
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File Type: wav MONO-006.wav (2.82 MB, 44 views)
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Old February 9th, 2012, 11:48 AM   #12
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Re: Interference? What is that?

Brian,

I had to turn my volume way up but I hear what sounds like a continuous whistling (for lack of a better word) almost like a teapot way far away.

It seems if you have someone talking over that it would be hardly noticeable. You want to hear line noise, you should hear some of my audio ;-} I'm getting better at it though.

Listen to a room with my eyes closed to see what may interfere with recording audio, and then try to remedy that.

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Old February 9th, 2012, 11:52 AM   #13
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Re: Interference? What is that?

Sometimes, it's not that bad, and voices do cover it up sometimes, but sometimes it's so loud that there's no way to actually avoid hearing it. In the past, I've gone in and isolated the frequency that it was buzzing in, and removed it, and it was alright. But it's when it fluctuates so wildly, that I just don't know what to do. The project I'm working on right now has a lot of quiet dialogue, and so, it doesn't hide anywhere. I'd just rather know if it's something local that I can seek out and turn off, or if something's gone wrong with the mic or the recorder. It sounds like, though, that it's interference from something other than those two, and that it's not something that I'm going to be able to avoid, which seems crazy to me, but if that's the case, that's the case. I'm not sure if a different sound capturing device, like say... Sound Devices 722 would somehow fix this, but I'm guessing it'd just make it 10x louder. Thanks!
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Old February 9th, 2012, 03:02 PM   #14
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Re: Interference? What is that?

Well, I think your best bet is process of elimination.

Do you have anything else plugged into the other input of the recorder? If so, unplug it.

Are you sure the mic cable is OK? Shielding OK?

Try the mic with a different recorder/mixer.

Try a different phantom mic with the existing recorder. Turn off the phantom power and try a dynamic mic. (Remember, this mic is a bit of an oddball, it uses RF demodulation to get the audio from the capsule, rather than DC bias voltage and a hi-z preamp.)

Try the recorder on AC power. (Maybe the phantom power supply in the recorder is acting up.)

Is the recorder's monitor level set extremely high? Possibly some coupling from the output to the input, from headphone cable to mic cable, etc.?

IMHO there's no good way to eliminate this noise in post, because there are at least two harmonics, and the frequency moves around quite a lot. Having voice "cover it" is not a solution. Downward expansion would drastically effect room tone, so that's not a good choice either.

Let us know how you make out with some substitution tests. Good luck!
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Old February 9th, 2012, 05:00 PM   #15
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Re: Interference? What is that?

I was finally able to hear it. I normalized the wav you posted in Soundtrack Pro and played on the good speakers. Kind of sounds like some of the noise you can get from a computer system or something with a bad capacitor. Doesn't sound to me so much like RF noise.
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