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Author Topic: Phase Demystified  (Read 319 times)

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Offline heathen

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Phase Demystified
« on: October 26, 2017, 09:57:00 AM »
I found this article about phase interesting as I know virtually nothing about the more technical side of sound and recording: https://www.soundonsound.com/techniques/phase-demystified

Some of the article is geared more towards studio recording, and I'm sure the more technically inclined will find nothing new here.  For the neophytes like myself, however, it may be useful.
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Offline morst

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Re: Phase Demystified
« Reply #1 on: October 26, 2017, 05:44:03 PM »
That is a cool article. It's really clear and simple. For the more advanced, it now becomes a teaching tool. I appreciate the way they explain the difference between 180 degrees out of phase and reversed polarity:

Quote
What's confusing about the '180 degrees out of phase' term is that it is sometimes used to refer to a situation where the second channel has not been delayed, but has had its waveform flipped upside down, so that the peaks become troughs and vice versa — a process more unambiguously referred to as polarity reversal. This scenario also results in silence at the combined output, hence the common confusion in terminology, but it's very important to realise that the total phase cancellation here is brought about by inverting one of the waveforms, not by delaying it. In this example, it might seem like we're splitting hairs, but in practice the distinction between time delays and polarity becomes much more important.
(italics mine)
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Offline Gutbucket

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Re: Phase Demystified
« Reply #2 on: October 26, 2017, 07:06:44 PM »
Haven't checked the article yet but will.

I appreciate the way they explain the difference between 180 degrees out of phase and reversed polarity:

Quote
What's confusing about the '180 degrees out of phase' term is that it is sometimes used to refer to a situation where the second channel has not been delayed, but has had its waveform flipped upside down, so that the peaks become troughs and vice versa — a process more unambiguously referred to as polarity reversal. This scenario also results in silence at the combined output, hence the common confusion in terminology, but it's very important to realise that the total phase cancellation here is brought about by inverting one of the waveforms, not by delaying it. In this example, it might seem like we're splitting hairs, but in practice the distinction between time delays and polarity becomes much more important.
(italics mine)

Yeah, this is a pet peeve of mine.  I will always say polarity when I mean signal inversion. That button on a mixing board or preamp is not a phase switch its a polarity switch! Label it correctly manufacturers, you know better!

And except in special cases, total cancellation only happens when summing a polarity inverted copy with the original.  A delay will produce total cancellation only at frequency multiples which correspond to the delay time. That's a comb-filter.  The minimal delay required to produce a 180 phase shift that nulls a high frequency sinewave is short.  The delay required to do the same with a low frequency sine is far far longer.  And the minimal delay required to produce a 180 degree shift for any frequency in-between requires a different delay time.
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Offline Moke

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Re: Phase Demystified
« Reply #3 on: November 10, 2017, 04:51:58 PM »
This is a great compositional study of phase cancellation and comb filtering.
It will do nothing to demystify the topic though [chuckles]

Alvin Lucier: An Homeage to James Tenney for Double Contrabass and Pure Sine Wav Oscillator
https://www.youtube.com/watch?v=ufMRGTUJvJs
Do yourself a favor, and at least speed forward to 9min.45sec, and listen from there before dismissing it.


I got to record a performance of this one once. Really cool stuff.
sample of mine: https://soundcloud.com/user-271082364/live-phase-cancellation-comb-filtering
Lots of peripheral crap in this one - sorry.
Scene: Art Museum, abstract art exhibit, and this performance. The audience was quiet and respectful; but the kids in the day care next door, and the street noise,.... [rolls eyes smilie here][slaps forehead smilie here]
Hey,.. At least it gets you an idea of what went down.
« Last Edit: November 10, 2017, 05:48:10 PM by Moke »
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