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Author Topic: Why does this wave form look like this?  (Read 4812 times)

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Offline Twenty8

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Why does this wave form look like this?
« on: May 11, 2023, 04:22:45 PM »
This is 15 seconds or so of a soundboard I took on Sat.  Reaper, left channel on top, right below, pay no attention to the pan as I was messing around.

Why does the wave form appear spiked so much rather than its more regular appearance?

First time I have taken a board feed in the venue.  Rest of the show does have a higher 'top end' to the wave form, but this screenshot is a drastic image of what I am seeing in smaller doses.
Open/Closed ~ AT U853 (subc, h, c, o):
Decks ~ MixPre 6ii, A10, M10
Pres ~ 2x Oade mod UA-5 (W+, ACM+)
Also ~ Line Audio CM3/CM4/OM1 : AT3031 : AT831 : DR2D

Offline Derp1

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Re: Why does this wave form look like this?
« Reply #1 on: May 11, 2023, 05:08:10 PM »
I have this happen to me every once in a while on certain boards. I believe its either a DC offset issue or a phase thing. Id love to know more about what causes this.
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Offline morst

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Re: Why does this wave form look like this?
« Reply #2 on: May 11, 2023, 05:25:54 PM »
It could have to do with drum mikes.
Remember that aiming a mic at a drum's top head is the opposite polarity from putting a mic inside a kick drum aimed at the beater.


Outside a drum, the impact of stick upon head causes a rarefaction of the surrounding air.
Inside a drum, the impact of the beater moving the head towards the mic causes a compression of surrounding air.
Neither is a right or wrong way to mic a drum, but the careful mix engineer might flip polarity of some of the mics, and thus cause addition of signal, which could potentially explain this.


On the other hand, if the peaks are bass guitar, maybe the battery in the phantom DI box is getting low?
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Offline EmRR

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Re: Why does this wave form look like this?
« Reply #3 on: May 11, 2023, 09:51:10 PM »
Asymmetrical signal. Could be the source, the mic placement, the input overloading.  Horns tend to look like this naturally, as does raw vocals.
Mics: DPA 4060 w/MPS 6030 PSU/DAD6001/DAD4099, Neumann KM 131, Oktava MK 012, Sennheiser MKH 105, MKH 20, MKH 30, MKH 40, MKH 800 TWIN
Recorders: Zoom F8n, Sony MZ-R50


Offline morst

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Re: Why does this wave form look like this?
« Reply #5 on: May 12, 2023, 01:28:14 PM »
https://www.producenewmedia.com/asymmetric-waveforms-should-you-be-concerned/

https://www.soundonsound.com/sound-advice/q-why-do-waveforms-sometimes-look-lop-sided

Ooh, thanks for the cool links! The first one contains this nugget:
Quote
High-pass filters, and aggressive low-end processing are common causes of asymmetric waveforms.

and the second one has this:
Quote
In contrast, natural waveform asymmetry cannot be 'corrected' with a high-pass filter, and a rather more complicated solution is required called a 'phase rotator'. Generally, there is no need to 'correct' a naturally asymmetrical signal, but occasionally the asymmetry can restrict how much the signal can be amplified because the stronger half of the waveform will reach the clip level before the weaker side. By using a phase rotator process to alter the harmonic phase relationships, a more balanced symmetry can be established, allowing slightly more gain to be applied before both sides reach the clipping level at the same amplitude. Asymmetrical waveforms can also sometimes confuse the side-chain level-detection circuitry (or algorithms) of some compressors, resulting in less effective compression than might be expected.

and so then I looked up Phase Rotator and someone said this over on Gear...SPACE (new name)
https://gearspace.com/board/mastering-forum/814899-phase-rotator.html
Quote from: FabienTDR;9408825
In case someone's interested, a "phase rotator" is in most cases just a very steep phase distortion around the (speaker's) fundamental. That is, a 4-8th order allpass around 200Hz.

There's nothing complicated about it, it's just that this approach works, sometimes (!). But imho, it's no a good idea for mastering. It radically changes the sound of the whole low end and tends to blur the important low mids. Especially the latter makes it pretty useless on full mixes. Phase rotator is a marketing term, better think of it as a phase scrambler. A standard steep Peaking/Shelving filter has a very similar side-effect.
« Last Edit: May 12, 2023, 01:46:19 PM by morst »
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Offline Twenty8

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Re: Why does this wave form look like this?
« Reply #6 on: May 12, 2023, 02:59:45 PM »
All interesting stuff.  Thanks, peoples.
Open/Closed ~ AT U853 (subc, h, c, o):
Decks ~ MixPre 6ii, A10, M10
Pres ~ 2x Oade mod UA-5 (W+, ACM+)
Also ~ Line Audio CM3/CM4/OM1 : AT3031 : AT831 : DR2D

Offline Gordon

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Re: Why does this wave form look like this?
« Reply #7 on: May 12, 2023, 03:08:33 PM »
Horns tend to look like this naturally

yep!
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Offline Derp1

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Re: Why does this wave form look like this?
« Reply #8 on: May 13, 2023, 12:58:48 PM »
Horns tend to look like this naturally

yep!
This makes total sense. Usually a sax with pedals creates these funky waves.
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