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Old March 13th, 2013, 06:24 PM   #16
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Re: Zoom H2n Sound

Yeah, I don't use mono anymore, so I'm not sure why I had it turned on.
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Old March 13th, 2013, 09:53 PM   #17
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Re: Zoom H2n Sound

Rick,

90% of the noises I heard were almost exactly 50 Hz. There was a lot of higher-harmonic buzz, but if you looked at it closly, the spikes were 960 samples apart. 960/48000 = .02 seconds = 50 Hz. That ruled out USA power line as a source. When Dana said the loudest noise happened when the talent was using/touching his cellphone, that pretty much clinched it. Apparently his cellphone has something that's scanning at exactly 50 Hz (probably the refresh rate for the screen).

There was a bit of predominantly low frequency noise, here and there; that, too, was exactly 50 Hz. Same source.

Then there were occasional bursts of noise that do sound very much like a cellphone when it's communicating with the cellular tower. So I'd say those were pretty unambiguous.

As a lot of us know, cellphones are really bad juju in an audio environment. Turn it off, and move it ten feet away, before you start recording!

And yes, Rick, you are absoluely right: If you recorded from the mic jacks, and had only one mic plugged in, the other (unterminated) input could be very susceptible to noise pickup. And if you recorded in "mono mix" mode, that noise would be permanently mixed with your good mic channel. IMHO, that's a very good reason to never use the "mono mix" mode in the field.


Dana,

I'm saying it's dumb of ZOOM to design their recorder that way. (NOT that it's dumb for you to use the mono mode.)

There may sometimes be a valid reason to produce a file that's a mix of the two channels. But once you make a mix in the field, there's no good reason to record exactly the same information on two channels of a stereo file. Zoom should have designed the firmware so the "mono mix" mode records a single-track mono file. That way, the file size will be half as big, and you can get twice as much time on a given card.

Anyway, I'm 99% certain that the cellphone was the source of all your problems. (And the file from your Zoom, which was two feet farther from the talent, had no noise at all, right?) Of course if the recorder can pick that up, it might also pick up other noise from something like an old CRT monitor, or... who knows what?

But for sure, before you start recording, be sure all cellphones are completely off not just in "silent" or "airplane" mode. And then put them on a table that's at least ten feet away from all your audio gear!

--

BTW, I've attached two files to show what I looked at. The first one is a short section of noise. You can see the time scale at the bottom, and find the exact same thing on the original file.

The second one shows the results after I ran it through a HPF to get rid of the LF component, and make it easier to locate and mark the spikes. I used the negative spikes, they're obviously much more apparent.

Note that I selected an area that's exactly 100 cycles long. Note that the total length of this file is 96,012 samples. So 96,012/100 = 960.12 samples per cycle. 960.12/48,000 = 0.0200025 seconds/cycle. That's the same as 49.993751 Hz. As I said, almost exactly 50 Hz. (If I hadn't checked that, I might have mistakenly thought it was harmonics from the AC power line; but since I know our power line is 60 Hz, obviously the power line was NOT the source of the problem, which leads us to look further... then we find the cellphone as the culprit.)
Attached Thumbnails
Zoom H2n Sound-dana-noise-01.gif   Zoom H2n Sound-dana-noise-02.gif  

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Old March 13th, 2013, 10:36 PM   #18
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Re: Zoom H2n Sound

Wow. I learn so much from you guys. Do you think having the two 20" 60 Hz monitors 3 feet away from my H2n would cause problems in the future. They're both fairly new, and I use them while recording.
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Old March 14th, 2013, 12:20 AM   #19
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Re: Zoom H2n Sound

If they were old CRT monitors, there's a fair chance they might cause problems.

If they are LCD monitors, I think you're probably safe.

Different mics will have different amounts of noise immunity, too. In general, if you're using an external mic, you should use one that has balanced wiring (so it will definitely need an XLR connector). Unbalanced wiring is much more susceptible to electromagnetic noise pickup.

And there's a sad little footnote to that. Almost all modern clip/lav mics have electret capsules, and the wiring from the capsule back to the power pack is almost always unbalanced, which makes them much more susceptible to noise. Usually the wiring from the power pack back to the recorder is balanced, which is much safer. A cellphone in the pocket, a few inches from the wiring of an electret clip mic, can be a real disaster.

My best advice is: test in advance. Pick the mic you're going to use with those new monitors, and record some test tracks while moving the mics around at different distances and locations in relation to the monitors. Do your best to keep the mics away from any power supplies, power wiring, motors, etc... and cell phones! If your test environment is exactly the same as your actual recording situation, and the tests are 100% clean, then your final tracks should be clean, too. And if you catch a problem during the tests, at least you have some time to work it out before your actual recording session.
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Old March 14th, 2013, 10:20 AM   #20
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Re: Zoom H2n Sound

I agree with your suspect cellphone Greg.
FWIW, Way back when... I recall getting frequency spikes around 15kHz when close to (60Hz) CRT monitors. As well as electromagnetic interference when in close proximity to transformers and A/C lines, depending on certain (questionable) cables. At one point I picked-up the Imus show's IFBs when working across the street from 2 Penn Plaza, but that was the hotel's mic cables.
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Old March 14th, 2013, 10:50 AM   #21
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Re: Zoom H2n Sound

Rick, that would have been the horizontal sweep, at 15,750 Hz. (30 x 525 lines)

I remember hearing the horizontal sweep on some early computer monitors, years ago, especially in POS terminals in stores. Either the deflection yokes on the CRTs, or the flyback transformers, were pretty badly made. I doubt that I could hear that today, and anyway they're all LCD now.
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Old March 15th, 2013, 10:53 AM   #22
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Re: Zoom H2n Sound

Yeah Greg, 15.7kHz, it was always clearly visible on spectral graph, it was not very audible though, at least to me, but I always used a LP filter on dialog anyway.
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Old March 15th, 2013, 08:41 PM   #23
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Re: Zoom H2n Sound

I don't mean that I heard it when I played back an audio file.

I mean I heard it audibly coming from the CRT displays! Apparently some of the flyback transformers or horizontal deflection yokes (both of which operated at 15,750) were not well made, and they actually vibrated mechanically at that frequency... so I'd hear this very HF whistle/buzz actually coming out of the monitors when I got close to them. Some were almost painfully loud. I'm sure glad that's a thing of the past.
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