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What is phase?
Old 8th July 2005
  #1
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What is phase?

OK. sorry for the basic question...

I've tried to look on line but to no avail. Can anyone point me in the right direction?
Old 8th July 2005
  #2
Gear Maniac
 
Zeppelin4Life's Avatar
 

phase describes a relationship between two signals.

in phase would have a similar relationship between the waves (see pic)

out of phase would have shift in the waveform

the waves have to be identical or similar to be totally out of phase.


for example, the sound of a phase change can be heard in a guitar phaser...or a flanger. the idea is that the signal is two waves, one which is being phase shifted over the other, to make a weird phasy sound.

Phase creates stereo depth.

Phase problems occur on drum overheads that are poor distances apart, mics recording on opposites sides of sources, and reflective surfaces that create 'comb-filtering'. a weird effect where phase is created due to the wave rebounding off a surface.

IN PHASE


PHASE SHIFT
Old 8th July 2005
  #3
Quote:
Originally Posted by Zeppelin4Life
consider it done

Your out of phase picture displays more of a phase shift than 2 signals out of phase.

This is because some of the waves over their excursions(180 degrees) are starting at different times.

A phase shift sounds(comb filtering) different than 2 signals completely out of phase(absolute cancelation).
Old 8th July 2005
  #4
Gear Maniac
 
Zeppelin4Life's Avatar
 

yes you are right...heres a total phase cancellation (on the right)

Old 8th July 2005
  #5
Quote:
Originally Posted by Zeppelin4Life

for example, the sound of a phase change can be heard in a guitar phaser...or a flanger. the idea is that the signal is two waves, one which is being phase shifted over the other, to make a weird phasy sound.
The difference here is that a flanger needs a modulator to give it the "flanging effect".

On its own 2 signals can't flange or sound like they are flanging.

A Phaser on the other hand is different effect.

Shorter delay times than a flanger(more like comb filtering).

Quote:
Originally Posted by Zeppelin4Life

Phase creates stereo depth.
It can help in that department.

I would say having signals in phase of a mix can give you a tighter image in response to where things are placed.
Old 8th July 2005
  #6
Quote:
Originally Posted by Zeppelin4Life
yes you are right...heres a total phase cancellation (on the right)


Old 8th July 2005
  #7
Gear Maniac
 
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Quote:
Originally Posted by Zeppelin4Life

...one which is being phase shifted over the other, to make a weird phasy sound...
Quote:
Originally Posted by thethrillfactor
The difference here is that a flanger needs a modulator to give it the "flanging effect".



yeah, two signals with the phaser or flanger shifting the phase.

both require some sort of modulation device....

to my understanding a flanger is a phaser that mixes in a delayed dry signal...the change in delay time gives it the flanging effect. trust me on this one, i remember this class in audio elec.
Old 8th July 2005
  #8
Quote:
Originally Posted by Zeppelin4Life
..

to my understanding a flanger is a phaser that mixes in a delayed dry signal...the change in delay time gives it the flanging effect. trust me on this one, i remember this class in audio elec.
thumbsup
Old 8th July 2005
  #9
Gear Addict
 
RyanR's Avatar
 

I understand the above, but what does it mean when people say the whole mix is in/out of phase?

For example, for vinyl it's recommend you have no phase problems. So, I've just make sure it has a solid mono image (especially low freqs). But how do I actually check for phase problems on the entire mix?

Is it as simple as getting a phase checking plugin (e.g. Waves PAZ), slapping it on the mix, then if it goes outside the 90° boundaries, I need to move the tracks causing it more towards the center?
Old 8th July 2005
  #10
Old 8th July 2005
  #11
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Curve Dominant's Avatar
Quote:
What is phase?
From the Greek: Phasis to appear.

In digital audio, phase is the point on the xyz plot which defines the position above or below the zero crossing. The other two points represent amplitude, and frequency.

One could define phase as the "half-life" of a wave.

Phase shifters are cool; my favorite is the ElectroHarmonix Small Stone.
Old 8th July 2005
  #12
Quote:
Originally Posted by Curve Dominant
One could define phase as the "half-life" of a wave.
No, phase is a real constant in an imaginary exponential:

z=exp(wt + iø), ø real, ø=phase.

Half life is the value of T such that exp(-kT)=1/2 i.e T=ln2/k where k is real.
Old 8th July 2005
  #13
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Quote:
Originally Posted by thethrillfactor
Your out of phase picture displays more of a phase shift than 2 signals out of phase.

This is because some of the waves over their excursions(180 degrees) are starting at different times.

A phase shift sounds(comb filtering) different than 2 signals completely out of phase(absolute cancelation).

A phase shift could be perceived as something that doesn't sound tight?

When you can "flip phase" on a mic pre, what does this do? Does it delay the signal by a half a cycle?

Is this a black/white issue? ie, if someone says the mix is in phase, would that mean that the signal of each instrument/vocal is in phase with each other to the sample?

grats...

Old 8th July 2005
  #14
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Quote:
Originally Posted by thethrillfactor
I would say having signals in phase of a mix can give you a tighter image in response to where things are placed.

hmm.. vs. taking something out of phase intentionally? if so, would that mean taking L and R of the mix and shifting that?

how would the mix be tighter this way?
Old 8th July 2005
  #15


More specifically, phase is the particular part of the waveform you are at. Usually, there is 360 degrees of phase in a periodic signal (like a sine wave). Sometimes it is called 2*pi radians.

The thing to remember about "phase" in recordings is that most of the problems are caused my a time delay. That means that some frequencies are "in-phase" (where the two signals are the same at that frequency) and some frequencies are "out-of-phase" (where the phase difference is 180, 540, 900, etc degrees) and the signals cancel each other out. Most frequencies are somewhere in-between.



-tINY

Old 8th July 2005
  #16
Gear Maniac
 

I don't think that anyone mentioned that phase issues are mostly directed at stereo miking on one sound source. He seems a bit lost. Simple terms: Think of a guitar cabinet with two speakers in it. Alot of people mic both of the speakers w/ two mics, one on each speaker. If one of the mics is closer to the cabinet than the other, the sound will hit one mic before the other, causing a slight delay of a few milliseconds. When played back, the sound is therefore out of phase with one another which makes it sound odd. Correct me if i'm wrong at all on this.
Old 8th July 2005
  #17
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nosebleedaudio's Avatar
 

Quote:
Originally Posted by djavid15
I don't think that anyone mentioned that phase issues are mostly directed at stereo miking on one sound source. He seems a bit lost. Simple terms: Think of a guitar cabinet with two speakers in it. Alot of people mic both of the speakers w/ two mics, one on each speaker. If one of the mics is closer to the cabinet than the other, the sound will hit one mic before the other, causing a slight delay of a few milliseconds. When played back, the sound is therefore out of phase with one another which makes it sound odd. Correct me if i'm wrong at all on this.
That is correct. And its out of phase with repect to the same source. If your console has a mono switch this makes it easy to check, and good engineers do this quite a bit during tracking. The Polarity switch will only correct those that are 180 degrees out. Alot of people have a problem with phase. By the way the polarity switch will not change a phase problem that is say 90 degrees out.
Old 8th July 2005
  #18
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DrFrankencopter's Avatar
To understand phase first you need to understand that all signals can be decomposed into a series of summed sine waves of various frequencies. Each of these sine waves has a magnitude, which gives the spectrum magnitude plot that you see on most specrum analyzers. But, each sine wave also has a phase angle that decribes it's timing relative to a pure sine which starts at 0 magnitude for time=0.

These phase delays are caused by signal processing, filtering, inherent responses of amplifiers, etc, and also by pure time delays. These phase manipulations that I just described are functions of frequency...ie. the phase shift for a given filter varies as you consider different frequencies. For example, a 1 pole low pass filter has 0 degrees phase shift relative to the unfiltered signal at frequencies about 1/10th the filter breakpoint, and is 90 degrees behind (lagged) at frequencies 10X the breakpoint.

Flipping the polarity of a signal (inverting) results in a 180 degree phase shift that does not vary with frequency.

Hope this helps...

Cheers,

Kris
Old 8th July 2005
  #19
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audioez's Avatar
 

electrical out of phase!!!
accoustical out of phase!!!
Old 8th July 2005
  #20
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wallace's Avatar
 

How does EQ change the waveform?
Old 8th July 2005
  #21
Gear Head
 
tnelson494's Avatar
 

I was curious about the EQ thing too. I recorded some overheads and when I pulled some of the low end out of them the kick seemed to be out of phase more. Also can anyone explain the 3 to 1 rule? I've seen it mentioned a couple of times but I don't understand the reson behind it and when it would be used? Thanks
Old 8th July 2005
  #22
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DrFrankencopter's Avatar
Quote:
How does EQ change the waveform?
The long answer is "it depends on the EQ, and how you set it up". But the short answer is (on an analog, or analog style EQ) boosting EQ generates phase lead (i.e. the boosted signal is 'ahead' of the original signal), and cutting generates phase lag (i.e. where the cut signal is behind the original signal). The amount of phase lead/lag generated depends on the amount of cut/boost, and frequencies over which the phase is shifted depends on the Q of the filter, and filter type (shelving, lo pass, hi pass, bandpass, etc).

In the digital world its possible to create linear phase EQ's, which don't generate the same alteration of the phase response, instead, they appear (in the phase domain) as pure time delays (which have a phase shift versus frequency, but is easy to compensate for).

It's worth noting that phase shift alone (i.e. without any associated boosts or cuts to the magnitude of the frequency response) is pretty much impossible to hear. In other words, iIf you had a knob on your mixer that just affected the phase response of a signal, I can bet that if you had the solo button on you wouldn't hear it do anything! You need cancellation, or combination with other signals in order to hear the effects of phase. Then, what we hear is the deep cancellation (a magnitude effect) associated with 'out of phase' summations.

You can experiment with pure phase shift in any DAW editor. Take an EQ, and do a boost at some frequency....boost the crap out of it. Then, reverse the waveform, and do an exactly opposite EQ cut. The end result has the same magnitude response as the original signal, but the phase has been shifted in spots. Try and hear a difference......I couldn't.

Cheers,

Kris
Old 8th July 2005
  #23
Gear Guru
 
lucey's Avatar
Quote:
Originally Posted by tnelson494
I was curious about the EQ thing too. I recorded some overheads and when I pulled some of the low end out of them the kick seemed to be out of phase more. Also can anyone explain the 3 to 1 rule? I've seen it mentioned a couple of times but I don't understand the reson behind it and when it would be used? Thanks
3 times further from the other mic, than each mic is to the source.



phase is the position in degrees of a sine wave in time ... 0, 90, 180, etc.

distance is time ... so different distances makes different time/phase relationships

so do frequency changes from different mics or eqs



if you recorded everything in mono, one track at a time, A+ phase. if you record looped stereo drums, and mono track everything else, A+ phase.

if you record 4 or 8 mics on a drum kit, or 2 mics on a guitar cab, or a bass mic and DI, you need to check phase carefully.

a X-Y scope, or a phase scope plug in works ... but the by-ear method is best IMO

the scientific methods cannot account for eq variations hi to low in the signal, and complex waveforms, your ear can hear phase at all the eqs if you train it.

to check by ear: mono everything by panning hard L or hard R, then bring up each fader from -inf to 0. as more signal is added there should be more volume and the tone should be as clear as possible, with no eq shifts.

phase shift will make the sound either drop in level, in extreme cases, as the fader comes up, or it will vary the eq response as the fader is raised ... because certain freqs are out of phase with the previous tracks, and other freq's are not.

perfect phase on 2, 3, 4 mics is impossible, even with the exact distance and perfect capsule matching (also impossible), plus the room changes the freq response in the mic ....

but you can get close enough by ear



eq at mixing changes phase ... in most eq's, that phase shift is part of the sound of the eq. so as this happens phase shifts in the dry tracks will occur at freq's in the boost/cut, making it important to be AWARE of phase once again, but not obsessed about it

if the basics are cool, you would haveto work hard to really whack the phase of a mix. triple tracked or double tracked guitars for example (if recorded one at a time) will be out of phase to each other each other in a way that can be too big to notice, or slight enough to create the 'depth' someone mentioned ... but this is the efffect. compressors in parallel will alter time and eq, thus phase.

phase shift in tracking is a hurt, in most cases

phase shift in general is part of the game
Old 8th July 2005
  #24
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Larrchild's Avatar
 

Just a Phase

A Phase is when otherwise intelligent people take older tube and discrete designs and toss them for IC and Digital based recording devices. Then realizing their mistake .. clammor to re-acquire the same gear 20 years later, thereby completing the sine wave.

*Looks down at post under me*

Phase is the timing between two signals, electrical or accoustic. Your ears use phase to locate sound. accoustic phase

when you set a tape head azimuth..electrical phase.
polarity is absolute reversal ..... phase is varying degrees. 0-360
Old 8th July 2005
  #25
Quote:
Originally Posted by audioez
electrical out of phase!!!
accoustical out of phase!!!

I'm no expert in the subject but when refering to electrical signals shouldn't the term polarity be used instead of phase ?

meaning acoustical uses the term phase
electrical using polarity ?

thx
Old 8th July 2005
  #26
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w000000000000t
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Old 8th July 2005
  #27
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Most common phase problem I found

Most common phase problem I found is in mixings drums that the Top Snare is slightly out of phase with Overheads. Easy fix flip the polarity on the Top Snare.
Snare fattens up right away(check btm snare). Even found this mistake made in the most experienced.

www.bluethumbproductions.com
Old 8th July 2005
  #28
Gear Head
 

Quote:
Originally Posted by Mario-C.
I'm no expert in the subject but when refering to electrical signals shouldn't the term polarity be used instead of phase ?

meaning acoustical uses the term phase
electrical using polarity ?

thx
I'm not an expert either, but I think I can help. The simplest way to think of it is this:

Phase is in reference to time. Any phase difference can be correlated to a time difference for a given frequency. ITD, or Interaural Time Difference, is the way that humans detect direction above about 1kHz, and is essentially dependent upon the 'phase shift' introduced by sound hitting one ear before the other. Phase shifts can take place in either acoustic or electronic environments.

Polarity is MUCH simpler than phase. A poliarty shift mirrors the signal and essentially makes what was positive into a negative, and vice-versa. In math terms (skip this if its not your thing ) you multiply the function by -1, or flip it about the X axis. A +.5 volt charge would be made into a -.5volt charge. It is an electronic function stike .

For another explanation: If you think of a mic capsule being pushed in and out by the compressions and rarefactions of a sound wave, poliarty is essentially (and forgive me for possibly over-simplifying this) whether a 'push' on the capsule causes a positive or negative voltage change to the signal.

To sum it up: Phase is dependent upon time, Polarity merely 'flips' the signal upside down.


I hope that helps, and please correct me if I've mispoken.
Old 8th July 2005
  #29
Gear Maniac
 
Larrchild's Avatar
 

Quote:
Originally Posted by [email protected]
Most common phase problem I found is in mixings drums that the Top Snare is slightly out of phase with Overheads. Easy fix flip the polarity on the Top Snare.
Snare fattens up right away(check btm snare). Even found this mistake made in the most experienced.

www.bluethumbproductions.com
here's the pisser, bro..some mics are pin 2 hot (pos) and some pin 3.
phase buttons, yeah. scopes are good too.
Old 8th July 2005
  #30
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lucey's Avatar
phase ranges from 0 to 360 degrees

polarity is -/+
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