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Author Topic: Need help identifying noise  (Read 431 times)

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Offline heathen

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Need help identifying noise
« on: May 13, 2018, 11:38:20 PM »
I'm hearing a noise that sounds like static in a recent recording and I'd like to figure out what caused it.  Here was my setup, using a Zoom F8 recorder:

Tracks 1&2: AT4031 mics, direct into recorder (no pre)
Tracks 3&4: CA14 omnis into CA9200 (+10 gain) into recorder
Tracks 5&6: dual recording of 1&2 at lower levels
Tracks 7&8: dual recording of 3&4 at lower levels

None of the tracks (main or the dual tracks) went higher than -6 all night (I don't think they even got close...particularly the dual tracks which peaked no higher than -16).  I'm hearing the same noise on all tracks, though it seems quieter to me on the AT tracks.  I've attached clips of the dual tracks (because those certainly weren't peaking high enough to clip).  All I've done is amplify them using Audacity (and to be clear, the noise is heard even on the completely raw tracks).

I can't rule out the possibility that this noise came through the PA, but my paranoid side makes me think it's from my gear.  I'm hoping that those of you with more experience will hear this and be able to recognize the sound, like "oh yeah this sounds like ______."  Here's a download link for clips that contain the noise: https://we.tl/tc2qwjYvG2
Mics: Core Sound TetraMic | AT4031s | CA-14 omnis | AT853s (C/SC) | Line Audio CM3s | AKG 460/ck61 | Studio Projects CS5
Pre: CA9200
Decks: Zoom F8 | Roland R-05

Offline morst

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Re: Need help identifying noise
« Reply #1 on: May 14, 2018, 01:44:59 PM »
Is it present the whole time? I don't even notice any noise at my low listening level, but my computer has a fan running.

I did notice that the relative polarity of the two samples appears to be opposite. Upward peaks on CA14 track look like downward peaks on the AT.
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Offline heathen

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Re: Need help identifying noise
« Reply #2 on: May 14, 2018, 02:11:39 PM »
Is it present the whole time?

I haven't finished listening to the whole show yet but so far this is the only spot I've heard it.

Quote
I did notice that the relative polarity of the two samples appears to be opposite. Upward peaks on CA14 track look like downward peaks on the AT.

Shit, I didn't notice that.  What effect does that have?
Mics: Core Sound TetraMic | AT4031s | CA-14 omnis | AT853s (C/SC) | Line Audio CM3s | AKG 460/ck61 | Studio Projects CS5
Pre: CA9200
Decks: Zoom F8 | Roland R-05

Offline ycoop

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Re: Need help identifying noise
« Reply #3 on: May 15, 2018, 08:44:05 PM »
Is it present the whole time?

I haven't finished listening to the whole show yet but so far this is the only spot I've heard it.

Quote
I did notice that the relative polarity of the two samples appears to be opposite. Upward peaks on CA14 track look like downward peaks on the AT.

Shit, I didn't notice that.  What effect does that have?

Someone please correct me if I’m wrong, but two sources having inverted polarity can result in sound cancellation when the two sources are mixed. Inverting polarity is a simple process in post, but as an additional process it can degrade audio quality.

Offline Perry

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Re: Need help identifying noise
« Reply #4 on: May 16, 2018, 06:15:41 PM »
Is it present the whole time?

I haven't finished listening to the whole show yet but so far this is the only spot I've heard it.

Quote
I did notice that the relative polarity of the two samples appears to be opposite. Upward peaks on CA14 track look like downward peaks on the AT.

Shit, I didn't notice that.  What effect does that have?

Someone please correct me if I’m wrong, but two sources having inverted polarity can result in sound cancellation when the two sources are mixed. Inverting polarity is a simple process in post, but as an additional process it can degrade audio quality.

You are correct, it's all relative. By itself, an inverted source will sound fine, and there's nothing to gain by "inverting it back to normal" But if you're mixing sources, and one is inverted, there will definitely be some frequency cancellation. I have a pair of 4.7K mod AT853's that came to me inverted- the hot and cold are reversed. I just invert that track in post, before mixing with my AKG source. You'll notice the difference in the lower frequencies first- the bass will tighten up when the polarity of both sources match.
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Offline heathen

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Re: Need help identifying noise
« Reply #5 on: May 16, 2018, 06:31:41 PM »
Thanks for the info about polarity.  It's something I never even thought to look out for before this.

Anyone have thoughts about the noise?
Mics: Core Sound TetraMic | AT4031s | CA-14 omnis | AT853s (C/SC) | Line Audio CM3s | AKG 460/ck61 | Studio Projects CS5
Pre: CA9200
Decks: Zoom F8 | Roland R-05

Offline Perry

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Re: Need help identifying noise
« Reply #6 on: May 16, 2018, 07:49:36 PM »
Just got home and listened to both samples (it's definitely more noticeable on the CA14 source). The noise sounds like what I hear when the capsule connections are dirty on my AKG's- it gets worse with higher SPL's and fast transients. But since you have it with both mics I'm going to guess there was a bad connection somewhere in the house's signal chain. It sounds drum-driven so the first thing I'd suspect is a bad mic on the kick or tom. Let's hope that's the case.
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Offline kuba e

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Re: Need help identifying noise
« Reply #7 on: May 17, 2018, 05:42:35 AM »
Thanks for the info about polarity.  It's something I never even thought to look out for before this.

It does not always matter. When you mix two very different tracks (aud + sbd, onstage+ spot mic), the opposite phase isn't harmful. The difference in the signals is so great that the comb filtering will not happen. Then you can try both versions - phase and reverse phase and choose the best.

If you use spaced omni with center mic and you flip phase, focus on bass region. There may be some canceling there.
Omni itself captures great low, so I don't use low from center mic. I am doing HPF on the center mic on Gutbucket's advice. It will come to me sometimes that the bass region is more clear. In this case, the opposite phase should not cause problems too.

Just got home and listened to both samples (it's definitely more noticeable on the CA14 source). The noise sounds like what I hear when the capsule connections are dirty on my AKG's- it gets worse with higher SPL's and fast transients.  But since you have it with both mics I'm going to guess there was a bad connection somewhere in the house's signal chain.

I agree with Perry. I had a similar noise when I had problems with cables and connectors.

Offline morst

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Re: Need help identifying noise
« Reply #8 on: May 18, 2018, 03:39:50 AM »
Someone please correct me if I’m wrong, but two sources having inverted polarity can result in sound cancellation when the two sources are mixed. Inverting polarity is a simple process in post, but as an additional process it can degrade audio quality.
You're correct on the first part, that mixing two similar signals with one having inverted polarity CAN result in sound cancellation.
You're also correct that inverting polarity in post is simple.

I would quibble, however, with the notion that the additional process would degrade signal quality in any way. If it's done in the digital domain, then it's just math, and rather simple math at that, to invert a waveform of a digital file. On the other hand, if it's done in the analog realm, it could be implemented by a flip of a switch or a cross-wired cable with no particular degradation. But yeah, if you did another pass from analog tape to analog tape just to invert a signal? Sure, that would cause degradation, but I don't think that applies to this situation.

Thanks for letting me nitpick, and while we're at it, please note that polarity, and not "phase" is the correct term for that which is upside down but not time-shifted. Phase would involve a time-shift, and not just a hot/cold switcheroo!  ^-^
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Offline Perry

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Re: Need help identifying noise
« Reply #9 on: May 18, 2018, 12:29:44 PM »
Someone please correct me if I’m wrong, but two sources having inverted polarity can result in sound cancellation when the two sources are mixed. Inverting polarity is a simple process in post, but as an additional process it can degrade audio quality.
You're correct on the first part, that mixing two similar signals with one having inverted polarity CAN result in sound cancellation.
You're also correct that inverting polarity in post is simple.

I would quibble, however, with the notion that the additional process would degrade signal quality in any way. If it's done in the digital domain, then it's just math, and rather simple math at that, to invert a waveform of a digital file. On the other hand, if it's done in the analog realm, it could be implemented by a flip of a switch or a cross-wired cable with no particular degradation. But yeah, if you did another pass from analog tape to analog tape just to invert a signal? Sure, that would cause degradation, but I don't think that applies to this situation.

Thanks for letting me nitpick, and while we're at it, please note that polarity, and not "phase" is the correct term for that which is upside down but not time-shifted. Phase would involve a time-shift, and not just a hot/cold switcheroo!  ^-^

Well said on all points and thanks for that correction, I often use the term "phase" incorrectly. All I'm doing is inverting the polarity of one source. Phase is a relational term. In our case, it describes the time relationship between two or more sources, usually measured on a scale of 0 to 180 degrees.
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