Old 07-29-2008, 04:21 PM   #1
1dell
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Default Invert Phase

Ummm, I'm about to show my newbieness, but what is Phasing and why would I want to invert it? When would I use it? and how would I recognize when something is out of phase?


Thanks yall
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Old 07-29-2008, 04:29 PM   #2
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You can think about the mixer as adding together the various signals in each track.

If you need to subtract instead of add, your mixer does it with help of the phase invert button by adding the inverted version of a signal, and that's where it's useful.
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Old 07-29-2008, 05:17 PM   #3
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I appreciate that response bro, but I didn't understand a word you said. Thats how new I am.

Quote:
Originally Posted by the all new rob View Post
You can think about the mixer as adding together the various signals in each track.

If you need to subtract instead of add, your mixer does it with help of the phase invert button by adding the inverted version of a signal, and that's where it's useful.
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Old 07-29-2008, 05:24 PM   #4
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Imagine a sound wave with the wave going up first and down next.

Invert that and it goes down first and up next.

Add the two together and you get silence.

That's handy for some testing purposes to prove that two audio files or streams are identical - invert the phase of one against the other and they "null" or produce silence.

Now sometimes when you've got a lot of mics in use, you can get a similar effect, whereby the output from one mic nulls or partly nulls another. Or one might be wired the wrong way round. Reversing the phase of one can help.
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Old 07-30-2008, 05:17 AM   #5
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Originally Posted by 1dell View Post
I appreciate that response bro, but I didn't understand a word you said. Thats how new I am.
http://en.wikipedia.org/wiki/Phase_c...e_interference
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Old 07-30-2008, 05:32 AM   #6
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think of 2 speakers sitting besiside each other. if they go in and out the same then they are in phase and the sounds are added. if one goes in while the other goes out the they are out of phase and the sound cancels.

to fix this you would swap over the two wires on the back of one speaker, the phase button in reaper acheaves the same thing

the technical term is actually polarity but the term phase gets used more

this is a very simplified example
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Old 07-30-2008, 05:38 AM   #7
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Hey 1dell!
The others wrote almost everything about it. I'd like to add one more thing, qoute from RME QuadMic manual.It is useful information imo.
//
PHASE changes the polarity. Phase cancellations and sound changes can be caused by using
multiple microphones at different places, or wrongly soldered cables. In such cases PHASE can
eliminate the error by adding an additional phase inversion.
//

cheers,
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Old 07-30-2008, 06:49 AM   #8
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Default Slight OT, sorry

Just to add my 2p (UK) worth; I actually have some issues with phase when using my guitar rig. I take a send from my mini-mixer out to my Boss RC20XL pedal, but when I plug the return back into the mixer, there is some phase shift of the direct signal. Not completely 180 degs out of phase, but just slightly phase shifted, (maybe a few msec of delay), so that it adds a 'hollowness' to the tone. Does anyone have any ideas how I can prevent this (unfortunately, the RC20 does not allow a 100% wet mix) ?

Apologies if this is too off topic, it's just that the topic of 'phase' reminded me.
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Old 07-30-2008, 08:42 AM   #9
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Quote:
Originally Posted by stevoj View Post
Just to add my 2p (UK) worth; I actually have some issues with phase when using my guitar rig. I take a send from my mini-mixer out to my Boss RC20XL pedal, but when I plug the return back into the mixer, there is some phase shift of the direct signal. Not completely 180 degs out of phase, but just slightly phase shifted, (maybe a few msec of delay), so that it adds a 'hollowness' to the tone. Does anyone have any ideas how I can prevent this (unfortunately, the RC20 does not allow a 100% wet mix) ?
I have the same exact issue and I think it's just the fact that there are 2 electronic paths which puts em only a fraction out of phase.
The RC20XL was intended to be run through only (my guess), not used as an effect loop in the mixer, which is what I do.

I don't think we have any recourse short of delaying our original signal a few ms.
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Old 07-30-2008, 08:45 AM   #10
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Quote:
Originally Posted by martygras View Post
I have the same exact issue and I think it's just the fact that there are 2 electronic paths which puts em only a fraction out of phase.
The RC20XL was intended to be run through only (my guess), not used as an effect loop in the mixer, which is what I do.

I don't think we have any recourse short of delaying our original signal a few ms.
It's annoying, that's for sure. Thanks for replying.

Stephen
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Old 07-30-2008, 09:07 AM   #11
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Due to ad/da conversion in your rc20xl the signal is delayed by a small amount what leads to the phase issues you described.
I have noticed the same thing when pressing the bypass key on my lexicon mpx100. If bypassed, there is still ad/da conversion -> phase issues. Workaround in my case: instead of pressing bypass i turn the wet/dry knob (which works analog) from the wet to the dry side.
Maybe you can close the aux send or mute the return when the rc20xl is not in use.

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Old 07-30-2008, 10:03 AM   #12
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Quote:
Originally Posted by Blechi View Post
Maybe you can close the aux send or mute the return when the rc20xl is not in use.
Yeah, that's what I do. I was hoping there might be some sort of mod I could make to the RC20XL to give a wet only out. Stephen
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Old 07-30-2008, 10:27 AM   #13
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Behringer has a new dual A/B switch that I'm trying out. It should let me pop away from the loop, but then I'll have 2 pedals to tap to add to the loop.

Yeah wet/dry knob should be a standard feature.
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Old 07-30-2008, 01:58 PM   #14
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Quote:
Originally Posted by boka View Post
Hey 1dell!

PHASE changes the polarity. Phase cancellations and sound changes can be caused by using
multiple microphones at different places, or wrongly soldered cables. In such cases PHASE can
eliminate the error by adding an additional phase inversion.
Err, kinda.... Phase is a shifting in time of a wave form measured in radians. So a reverse in polarity results in a phase shift of 180 degrees. However, a phase shift of say 90 degrees is not a reverse in polarity.

For practical purposes though, we are looking at the phase button on the mixer. This button shifts the phase 180 degrees which as I stated is the equivalent to a polarity reversal. How and why is this handy? One main reason is to compensate for mics placed at opposite directions of a sound source. One very common instance is a mic on the top and the bottom of a snare drum. For simplicity sake, we are going to imagine our snare puts out a pure sine wave.

Look at the graph below. The blue line we will say is "in phase" and what the mic on the top of a snare will hear and the green line is "out of phase" and what a mic on the bottom of a snare will hear.



If we remember back to trig class, two waves like that cancel each other out. Where blue = 1 on the axis, green = -1.

1 + (-1) = 0 which means zero sound.

That said, if we reverse the polarity ( that is to cause a 180 degree phase shift) where blue is 1, green is also 1. 1+1 = 2 so we have DOUBLE the amplitude rather than zero amplitude. Note that where in the above image the blue wave was at 1, it is now at 2!



Make sense?

So we have a case here where you have a top and bottom snare mic. You may also do this if you have over heads out of phase with a kick mic. Reverse the polarity of the kick and tada, it sounds a whole lot bigger.

Now reversing the polarity does not fix fractional phase shift issues. For instance, here are two waves 90 degrees out of phase.



Where blue = 1, green = 0. Where blue = .25, green = .75... This creates a weird wave form. This is a whole other flavor of bad. The result would be something that looks like this and isn't even in the ball park of the original sound. The red graph is the result of this phase issue. Note where blue and green intersect, red peaks...

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Old 07-30-2008, 02:44 PM   #15
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If we were to switch the phase 180 degrees using our handy button, this is the result.



Note that green is opposite of the first graph and all we have really done is shifted the resultant red wave 180 degrees. Essentially we did nothing but move the resultant red wave 180 degrees.

To prevent this sort of problem we use the 3:1 rule. This states that the distance between two mics is 3 time the distance from the original sound source. So say you have 2 mics on two guitar cabs. Each mic is 1 foot from the speaker of the guitar cab. Therefore, the mics must be at least 3 feet from each other. This causes a drop in bleed such that the bleed becomes low enough to be insignificant in relation to phase.

This graph shows the result of the 3:1 rule. The green represents the bleed which has been reduced by 9db due to the mic spacing. (If you follow the 3:1 rule, the sound that bleeds will be reduced by at least 9db, so this illustrates a worst care scenario assuming we are only doing 3:1 and not a greater ratio.) Note how the resultant red line is nearly identical to the blue line (blue being the original sound).



Be warned though, some phase issues can be a good thing. Two spaced microphones over a drum kit then panned L and R use this phase interaction to create the stereo image. Without the cancellation and amplification caused by phase, you would simply have a mono signal. Mono signals we absolutely want in phase or you have cancellation. This is why it is so imperative to use a technique such as M/S or Recorerman for drum overheads so that the snare and kick are in phase.

And there is entirely more than you ever cared to know... and I am having entirely too much fun with this software:

http://www.mathgv.com/
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Old 07-31-2008, 04:50 AM   #16
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Here you can find a .pdf that describes the difference between reversed polarity and shifted phase: http://www.community.chester.pa.us/f...e/polphase.pdf
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Old 07-31-2008, 05:04 AM   #17
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As others have said in various replies, your "phase invert" button gives you the ability to change the phase of a signal by exactly one amount -- 180 degrees. There are exactly an infinite number of other possibilities that you don't get to choose from.

However, in a DAW like Reaper you DO have those options, because you can slide one track relative to all the others in time.

By far, the most common application of a phase flip button is to minimize the negative effects one gets when recording an instrement or ensemble with multiple microphones. Imagine you are recording a drum kit, and you are using spaced overheads, a close mic on the snare, and a close mic out in front of the kick drum. Either by chance or by choice, you have placed the overheads such that each is an equal distance from the snare drum. (This is often a good starting point, because it helps "center" the snare in the image, which in most cases is a desired result.) Everything sounds great in the overheads. Now, let's say you've fiddled with the snare mic such that by itself, things sound great, but you bring up the overheads, and the overall snare sound goes to hell, getting thin and lifeless. Why? Well, more than likely, the snare signal in the close mic is out of phase with the snare signal in the overheads, simply because sound takes longer to travel (say) several feet than it does a few inches. That time delay represents a phase shift. One quick option which might help things (if not cure them completely) is your snare track's phase invert switch. Push it and see (hear) what happens. Does the snare get more bottom end? Yeah? Leave the phase flip button pushed in! Maybe you can now move on to the kick drum, maybe not, it's your call, but at least you probably helped things a bit, and you still have the further option to slip tracks at the mix stage if you so desire. When you get to the kick, go through the same process, although now you're trying to get it phased correctly relative to THREE microphones, and things can get even trickier. (I find it's usually not as much of a problem with kick drums, though.)

I would say that (a) I tend to flip phase on my preamps instead of the DAW mixer so my tracks recorded with my intentions in mind and (b) I prefer to get everything recorded as well as possible and not assume I can "fix it in the mix", but that may just be me.

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Old 07-31-2008, 05:08 AM   #18
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Quote:
Originally Posted by Bubbagump View Post
One main reason is to compensate for mics placed at opposite directions of s sound source.
Very good explanation with the images,I like it,thanks!
but I guess the qoute i pasted was about something like this...wasn't it? or did i misunderstand something?

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Old 07-31-2008, 02:01 PM   #19
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In practice the quote you posted was right in some cases... but I wanted to be sure it was known that phase and polarity are different animals... however improperly used as interchangeable terms. Polarity is a a very simple black or white situation where as phase is not. The switch on mixers is really a polarity change, not a phase change. All it does in digital is simply multiply everything by -1 and an on analog boards reverse hot and neutral.

The more important issue for a noob to understand is that the polarity switch will fix phase issues in some cases. Do not look to is as a replacement for proper mic technique or the like as it won't always fix things.
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