What does "Phase Reverse" do?

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drummerboy_04AP

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Ive heard about it before, but I dont know what it does. How would it be helpfull?
 
lol, but what does that do? Like, whats the point?
 
More accurately, it reverses the polarity of a signal. Some microphones and some electronic equipment reverses the polarity of a signal and you need a switch to restore it to normal. A reversed polarity signal simply means that all the positive peaks are now at the bottom of the signal, showing as negative peaks. You flip the polarity with a phase switch.

A 180° "phase" shift is not exactly the same thing, since that slides the signal in time..
 
Harvey Gerst said:
A 180° "phase" shift is not exactly the same thing, since that slides the signal in time..
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well, it is...and it isn't. I've had several discussions on here with some people, and without getting too deep into the physics of it all ('cause I hardly understand it myself), the switch does do a phase shift as well. It's just not in the same "time" domain that we're so used to when talking about two distant mics. But in the rest of the physics world it does. I can probably hunt down the link of the convo we had awhile back if you want.

but drummerboy for your questions, Harvey's right. It takes the parts of the wave form that goes up and makes them go down instead. And vice versa for the part of the wave that goes downwards. It can help in situations where gear may have been improperly wired (although very few people may actually realize this just by hearing it), but more importantly it may help correct where you have phase problems with your signal. This can happen anytime you use two microphones on a source. For example, using a top and bottom microphone on a snare. Using the phase/polarity button helps you listen to a quick fix of how the two sounds appear when they are played back together. Sometimes it helps, sometimes it doesn't.
 
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You see. You should have stuck with my answer. Now both our brains hurt. :eek:
 
bennychico11 said:
well, it is...and it isn't. I've had several discussions on here with some people, and without getting too deep into the physics of it all ('cause I hardly understand it myself),
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Well, seeing as how Harvey (and I, for that matter) DO understand the physics of it very well, take our word for it that they are NOT the same thing. They can have similar EFFECTS, but even there, not really. Using a polarity switch to correct a phase problem will ALWAYS cause other phase problems (admittedly, they are not always audible, but they are there), because it does nothing to the TIME issue which is the problem.


Ah hell, just look at THIS - CHECK OUT MY POST AT THE END. And bear in mind that even there I am simplifying things significantly.


Light

"Cowards can never be moral."
M.K. Gandhi
 
Light said:
Well, seeing as how Harvey (and I, for that matter) DO understand the physics of it very well, take our word for it that they are NOT the same thing. They can have similar EFFECTS, but even there, not really. Using a polarity switch to correct a phase problem will ALWAYS cause other phase problems (admittedly, they are not always audible, but they are there), because it does nothing to the TIME issue which is the problem.


Ah hell, just look at THIS - CHECK OUT MY POST AT THE END. And bear in mind that even there I am simplifying things significantly.


Light

"Cowards can never be moral."
M.K. Gandhi
Click to expand...


Hey Light-

I appreciate your response. I totally understand what you are saying and I used to preach the same thing around the forum (and elsewhere) for quite awhile. Basically saying that phase is a component of time. And that it's a polarity switch not a phase switch because the audio doesn't physically move 180 degrees in time. Exactly what you put in your above link.

But then I joined in on this conversation. Down near the middle cpl_crud, Farview and I start discussing this all and how it relates to wave theory. Basically I got that cpl_crud was saying that you can't forget that we are dealing with complex wave forms where phase is depenant on angle and not time. It's just manifested as a time difference on the X/T graph. But the important part he was getting at was:
Polatiry is a static vector term
Phase is a dynamic vector term.
If you have a look at at a wave on the complex plane, you will see it's vector spinning around in a circle. It is this behaviour that excites nerdy types, as expressing a wave as a circle makes things so much easier than trying to draw sin waves ad infinitium.
If you express a swith in polatiy on the complex plane, you have a straight line passing through the origin, with equal lengths both sides of the origin.

a "Physical" phase shift is one that you can "do" with your hands- like switching pins 2 and 3, pressing a phase button... whatever.
What this does to the electronics is a poliarty reverse, as it switches two bits of metal, which are static.
However, what it does to the wave (as seen by the next downstream instrument) is switch it's phase.
Once again I reffer to the the cis(omega*t + phi) part of the above equation- whilst a change in time or phase can manifest itself with the same effect as a change in the other, that isn't what's happeneing.
Click to expand...

I'd love to hear your thoughts on this Light. I like discussing phase. :)
I just was never that great at all the math, so I just stopped commenting when it started getting into talks about the Fourier Transform and such. But please, fire away with what you think about his post.

-B
 
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Just set your phaser to "rock" and you'll be good to go. End of story.

jk

Think of it as the bizzaro world of the wave......the exact opposite in theory. Peaks are now valleys, valleys now peaks...pretty much. I'm a newb too, but I felt like putting my 1.5 cents in LOL
 
bennychico11 said:
Basically I got that cpl_crud was saying that you can't forget that we are dealing with complex wave forms where phase is depenant on angle and not time.
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He's wrong. Phase is an issue of time, period. It's just basic physics. And the truth of the matter is that complex waves are simply made up of a whole bunch of sine waves. A polarity change which makes one sine wave IN phase will, by definition, make another one OUT of phase (`cause different frequencies have different wave lengths, so the phase issues with two mics will be different at different frequencies). A polarity switch will never get around that, as it is simply basic physics. One of those "I ca'nae change the laws of physics, Jim," kinds of things.

Go buy a book on the physics of acoustics, if you really want to look it up.


Light

"Cowards can never be moral."
M.K. Gandhi
 
Phase, time alignment, and polarity are three different things.

A phase switch on your average console switches the polarity, the only thing wrong here is the labelling of the button.

You can make a out-of-phase-sound sound better with switching polarity, or moving it in time in your DAW. But it won't make it in phase. When it sounds good, and you think it's in phase, you have gotten a result where the dominant frequencies are in phase. To get 100% in phase, buy a Little Labs IBP, or re-record with different mic setup. Preferably less microphones.
 
Light said:
And the truth of the matter is that complex waves are simply made up of a whole bunch of sine waves. A polarity change which makes one sine wave IN phase will, by definition, make another one OUT of phase (`cause different frequencies have different wave lengths, so the phase issues with two mics will be different at different frequencies).
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wait, so you're saying the polarity switch does not affect phase (in the world of time).
But just above you are saying that a polarity change of a complex wave will make one of the sine waves that make up that complex wave IN phase and also make another one OUT of phase. So the phase relationships between the two sine waves have been reversed? Does that not mean the switch can now be called a phase reverse switch?


So nothing happens in the complex plane with regards to the phase angle?

The wave equation, in it's most correct form, is:
d^2u/dt^2 = c^2(delta u) - or, in other words, the acceleration of a particle is relevant to it's displacement. This, of course, will usually result in the sinusoidal movement of the particle, however when we integrate the double derivative, you end up with a few constants that cannot be forgotten about.
One of these is the phase shift.

Performing the double integration with the Fourier Transform, and over simplify, you end up with:
u = cis(omega*t + phi), where u is the displacement as a function of time, cis is the complex sinusoidal term ( cos(X) + i Sin(X)), omega is the angular velocity (complex word for frequency, represented by the "velocity" of the phase angle on the complex plane- ie f/2pi), t is the time and phi is the constant of integration.
This is where the major "Phase = time!" fallacy comes into play, because a change in phase will have the same effect on u as a change in time would have (as they are both in the same set of brackets)
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I can't believe I'm sticking my head in here, I generally like to keep the super technical stuff far away from music, but........
The concept of real importance here is phase relationship. If you have two signals that are out of phase with each other and therefore causing summing problems, and you invert the polarity of one of them, you have effectively changed the phase relationship of the two signals. Yes?
 
Robert D said:
If you have two signals that are out of phase with each other and therefore causing summing problems, and you invert the polarity of one of them, you have effectively changed the phase relationship of the two signals. Yes?
Click to expand...

correct.

The question at hand that I've been trying to figure out for awhile is can a signal be out of phase with itself? Is phase only represented on the time/amplitude graph or can the phase angle be manifested elsewhere in the form of a circle like cpl_crud was after. And if so, is it called a phase shift in the complex plane if the conjugate of the original is created. Because if the circle swings the opposite direction, has it not just shifted a certain amount that can be measured in degrees as well?
 
Polarity reversal is the only method to shift a complex waveform by exactly 180°. A single sinewave can be slowly shifted in time till the resultant waveform is 180° out of phase with the original signal, but the start time of the second wave must be delayed, relative to the original signal. Throw in a second signal of a different frequency and the game is over.

With polarity reversal, there's no delay, and the phase is reversed for any complex asymmetrical signal, not just a simple, single sinewave.
 
Robert D said:
I can't believe I'm sticking my head in here, I generally like to keep the super technical stuff far away from music, but........
The concept of real importance here is phase relationship. If you have two signals that are out of phase with each other and therefore causing summing problems, and you invert the polarity of one of them, you have effectively changed the phase relationship of the two signals. Yes?
Click to expand...
I think how phase and polarity got intertwined is because in the lab with perfect waves (which are often used in science to demonstrate and test) polarity reversals can duplicate one of many states of phase shifting. But only one of many states. Well, actually two if you count the orignal polarity state AND the flip.

Here's a phase relationship...

phase.jpg


With that graphic in mind (and a perfect sine wave) you can slide one of those waveforms horizontally in time to a place that would result in 100% "phase cancellation", identical to a polarity reversal of a wave that was in perfect phase with the other. You could put one wave in any time based position, flip the polarity and then duplicate that perfectly (with two other perfect waves) with a phase shift of one to a certain single postion in time.

With those perfect waves a horizontal shift would eventually reach an identical result as any polarity flip from any position. With perfect waves, NOT with complex waveforms... i.e. music.

This is why people refer to polarity as phase, in a circumstance with perfect tones - in the lab - they can do exactly the same thing. Although polarity is limited to two states phase shifting can duplicate any polarity state between those two using perfect tones. We don't work with perfect waves.

With music though...

As far as the voltages of the analog waves and the combined result goes, yes, flipping the polarity does change the relationship between the two audibly but different from what would happen with a horizontal phase shift (with even identical complex waveforms) . I would suggest you could not horizontally shift complex waveforms and get the same result you get from reversing the polarity. Why? Because unlike the perfect wave they constanly change over time. Phase shifting would give virtually unlimted variations, none of which would be the same as the polarity reversal with those complex waves static at the same place in time.

Two different things, impossible to get the same result with complex waveforms.
 
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bennychico11 said:
well, it is...and it isn't. I've had several discussions on here with some people, and without getting too deep into the physics of it all ('cause I hardly understand it myself), the switch does do a phase shift as well. It's just not in the same "time" domain that we're so used to when talking about two distant mics. But in the rest of the physics world it does. I can probably hunt down the link of the convo we had awhile back if you want.

but drummerboy for your questions, Harvey's right. It takes the parts of the wave form that goes up and makes them go down instead. And vice versa for the part of the wave that goes downwards. It can help in situations where gear may have been improperly wired (although very few people may actually realize this just by hearing it), but more importantly it may help correct where you have phase problems with your signal. This can happen anytime you use two microphones on a source. For example, using a top and bottom microphone on a snare. Using the phase/polarity button helps you listen to a quick fix of how the two sounds appear when they are played back together. Sometimes it helps, sometimes it doesn't.
Click to expand...

bennyboi trying to school Harvey! CLASSIC!!! :rolleyes:
 
Ford Van said:
bennyboi trying to school Harvey! CLASSIC!!! :rolleyes:
Click to expand...

not trying to school anyone Bored Fan (hence the "Harvey's right comment"). I was simply bringing up a different discussion some of us had awhile back which related to his post...in case he was interested. And then Light jumped in with some of his own comments on the matter. So I responded with some more questions for the purpose of a discussion...it's how these threads work. :rolleyes:
I respect Harvey and Light's thoughts on these matters and their posts...yours are getting tiresome.
If you have something to bring to the conversation about my questions, do so. Otherwise stop posting mindless crap.
 
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