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Author Topic: dual mic recording phase issues?  (Read 13683 times)

iCombs

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Re: dual mic recording phase issues?
« Reply #15 on: October 19, 2006, 10:25:55 AM »

J.J. Blair wrote on Thu, 19 October 2006 01:04

jimmyjazz wrote on Wed, 18 October 2006 19:10

OK, that doesn't make any sense, JJ.  DAWs only show amplitude because (time-dependent) amplitude is all there is!



I didn't say the DAW was misrepresenting anything.  I said that it is only representing one thing (besides time): Amplitude.  My point being that you cant visually align the phase, as is being suggested, if the only visual cue is amplitude.

Yes.  It makes sense.


In all my math classes that dealt with periodic equations, I remember learning that phase IS represented on a graph of time vs. amplitude.  The swing from positive to negative amplitude (or, for a slightly better example, from zero to peak positive amplitude, from peak positive through zero to peak negative, and then back up to zero) represents 360 degrees of phase.  I think jimmyjazz has the right idea here, and it's also why I think nudging tracks around is a completely valid way of working.  I'm tired of hearing "don't use your eyes, use your ears only..."  that's like telling me not to use my thumb, only my fingers while I'm turning a knob.  I understand that the end result is that things sound "good," but why in the blue hell shouldn't I use all my senses to make sure that happens?  That's like saying "Don't use VU meters, or any other metering, for that matter..."  It gives you a reference that can help your aural comprehension of the situation.  I don't see how that's a problem.

Also, hasn't delaying tracks with a DDL of some sort been standard practice among engineers for a long time now?  I completely hear where you are coming from JJ, don't get me wrong, and I know that the FIRST order of business is to ensure that you are recording sounds the right way the first time so that you don't have to nudge them around, but that said, it doesn't work perfectly all the time...so why not use all the tools I've got to make the best sounds I can?  Why hamper that process with some arbitrary rule or some dogmatic semantics?  
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jimmyjazz

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Re: dual mic recording phase issues?
« Reply #16 on: October 19, 2006, 10:49:01 AM »

iCombs wrote on Thu, 19 October 2006 10:25

I completely hear where you are coming from JJ, don't get me wrong, and I know that the FIRST order of business is to ensure that you are recording sounds the right way the first time so that you don't have to nudge them around, but that said, it doesn't work perfectly all the time...so why not use all the tools I've got to make the best sounds I can? Why hamper that process with some arbitrary rule or some dogmatic semantics?



I'd go a step further, and suggest that in theory, one might be able to IGNORE phase issues between, say, close drum mics and overheads and just put the things where they sound the best solo-ed.  Then, during mix, one could slip the close tracks to align with the overheads and blend to taste in order to get a bit more definition.

Notice I said in theory.  Again, I'm pretty old-school.  I get my sounds the old-fashioned way and I don't manipulate them.  Still, I'm looking forward to some experimentation in the near future.  (I have no qualms re-inventing the wheel!  No doubt others have done these things many, many times, and discovered the good, the bad, and the ugly of what I'm suggesting.)
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resolectric

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Re: dual mic recording phase issues?
« Reply #17 on: October 19, 2006, 10:50:59 AM »

Microphone selection and positioning IS sound manipulation.
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jimmyjazz

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Re: dual mic recording phase issues?
« Reply #18 on: October 19, 2006, 10:53:21 AM »

resolectric wrote on Thu, 19 October 2006 10:50

Microphone selection and positioning IS sound manipulation.


Well, I don't agree, but regardless, what I was talking about was the fact that I don't slip tracks forward or backward in time.  

Yet.
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J.J. Blair

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Re: dual mic recording phase issues?
« Reply #19 on: October 19, 2006, 12:52:12 PM »

jimmyjazz wrote on Wed, 18 October 2006 23:18

Well, let's get specific.  A snare hit.  Are you saying that sliding a close-miced snare track in time such that the transient peaks at the same time it does in the overheads is ambiguous?  Is there some "mystery phase" which might be positive or might be negative, even if we make sure the sign of both peaks is the same?


What I'm saying is that a DAW does not show positive or negative phase.  You could be 180˚ out of phase and the DAW would not visually indicate that.  
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J.J. Blair

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Re: dual mic recording phase issues?
« Reply #20 on: October 19, 2006, 01:09:26 PM »

iCombs wrote on Thu, 19 October 2006 07:25

In all my math classes that dealt with periodic equations, I remember learning that phase IS represented on a graph of time vs. amplitude.


Yes.  But that is the phase of a wave.  A DAW does not render a sound wave.  

Quote:

The swing from positive to negative amplitude (or, for a slightly better example, from zero to peak positive amplitude, from peak positive through zero to peak negative, and then back up to zero) represents 360 degrees of phase.  I think jimmyjazz has the right idea here, and it's also why I think nudging tracks around is a completely valid way of working.


Here's the problem with this logic: A DAW rendering of a sound file is a crude representation of signal amplitude and time.  You are not actually looking at a wave form, as you would see it on an oscilloscope, which is showing you waveshape, frequency and amplitude.  There is nothing on a DAW to represent freqency.  This is the point you keep missing.  The only thing you are aligning is time.  There is NO indication of phase of the frequncies.  You could visually time align something and still be 180˚ out of phase.

Quote:

I'm tired of hearing "don't use your eyes, use your ears only..."  that's like telling me not to use my thumb, only my fingers while I'm turning a knob.  I understand that the end result is that things sound "good," but why in the blue hell shouldn't I use all my senses to make sure that happens?  That's like saying "Don't use VU meters, or any other metering, for that matter..."  It gives you a reference that can help your aural comprehension of the situation.  I don't see how that's a problem.

Also, hasn't delaying tracks with a DDL of some sort been standard practice among engineers for a long time now?  I completely hear where you are coming from JJ, don't get me wrong, and I know that the FIRST order of business is to ensure that you are recording sounds the right way the first time so that you don't have to nudge them around, but that said, it doesn't work perfectly all the time...so why not use all the tools I've got to make the best sounds I can?  Why hamper that process with some arbitrary rule or some dogmatic semantics?  



OK, two things: As I just said a couple of times, you could rely on your eyes and then still be out of phase.  Secondly, it's not about semantics.  It's only about semantics to you because you still think that a DAW is showing you actual phase, which I keep trying to explain that it's not.  Maybe if I have a spare hour over the weekend, I'll do a post with sound files and pics to illustrate the concept.

BTW, I have never nudged files around for these purposes or used a DDL for anything other than as a delay.
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They say the heart of Rock & Roll is still beating, which is amazing if you consider all the blow it's done over the years.

"The Internet enables pompous blowhards to interact with other pompous blowhards in a big circle jerk of pomposity." - Bill Maher

"The negative aspects of this business, not only will continue to prevail, but will continue to accelerate in madness. Conditions aren't going to get better, because the economics of rock and roll are getting closer and closer to the economics of Big Business America." - Bill Graham

iCombs

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Re: dual mic recording phase issues?
« Reply #21 on: October 19, 2006, 01:14:09 PM »

jimmyjazz wrote on Thu, 19 October 2006 09:49


I'd go a step further, and suggest that in theory, one might be able to IGNORE phase issues between, say, close drum mics and overheads and just put the things where they sound the best solo-ed.  Then, during mix, one could slip the close tracks to align with the overheads and blend to taste in order to get a bit more definition.

Notice I said in theory.  Again, I'm pretty old-school.  I get my sounds the old-fashioned way and I don't manipulate them.  Still, I'm looking forward to some experimentation in the near future.  (I have no qualms re-inventing the wheel!  No doubt others have done these things many, many times, and discovered the good, the bad, and the ugly of what I'm suggesting.)


The only issue with that, especially in the drum micing scenario, is that you are not dealing with a single source (as you are when micing one speaker of a guitar amp).  Re-aligning close mics with OH tracks will most probably create some pretty ridiculous phase anomalies between each other...which is why I'm very much anti-track-shifting in the drum world.  I really use kit bleed when I mix, and I don't want that to sound phasey or chewy or just plain bad.
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Ian Combs
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minister

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Re: dual mic recording phase issues?
« Reply #22 on: October 19, 2006, 01:34:01 PM »

J.J. Blair wrote on Tue, 17 October 2006 23:57


When mics are spaced apart, you are capturing a different phase of that sound wave.  If you make it earlier or later, it does not change whether the wave is compressing or rarefacting.  The only way to change that is by actually adjusting the phase of the signal, or aligning the diaphragms of the two mics, so that the phase is coincidental.  

Let me illustrate it this way: A mic one inch from the source might capture the 1kHz portion of a wave's phase at 30˚ above zero, but one nine inches away might cappture it at 70˚.  But the 100Hz portion of the wave might be at 20˚ above zero at one inch and 18˚ below zero at nine inches.  The sum of those two is going to give you constructive interference at 1Khz, making that louder, but cancellation at 100Hz, making that quieter.  Delaying one or the other never aligns the phase.  You just may be negating the amount of cancellation is all.


i thought this explained it very well, and might be worth re-reading.  it explains why the time nudge method in a DAW is not sufficient.


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jimmyjazz

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Re: dual mic recording phase issues?
« Reply #23 on: October 19, 2006, 01:40:56 PM »

J.J. Blair wrote on Thu, 19 October 2006 13:09

Here's the problem with this logic: A DAW rendering of a sound file is a crude representation of signal amplitude and time.  You are not actually looking at a wave form, as you would see it on an oscilloscope, which is showing you waveshape, frequency and amplitude.


What?  A 'scope would show you EXACTLY amplitude and time!  Only when you tranform the signal into the frequency domain (via a Fourier transform) do you get amplitude and phase.


Quote:

There is nothing on a DAW to represent freqency.  This is the point you keep missing.  The only thing you are aligning is time.  There is NO indication of phase of the frequncies.  You could visually time align something and still be 180˚ out of phase.


I'm sorry, J.J., but I completely disagree.  If you take two sine waves of identical frequency and slip one such that the zero crossings coincide, then they're either in phase (0 degrees) or out of phase (180 degrees).  It's easy to determine which is which -- if both signals have the same "sign", they're in phase.  If one is the inverse of the other, they're out of phase.

If you want to explain how that's a poor analog of a transient like a drum hit, then be my guest, but I'm not buying it.


Quote:

Maybe if I have a spare hour over the weekend, I'll do a post with sound files and pics to illustrate the concept.


I think that's a great idea!  This is a topic that rears its ugly head quite often, so maybe we need some sounds and pictures to get some clarity.  I'll agree to capitulate if you're right, as long as you agree to post the information anyway if you're wrong.  Deal?
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iCombs

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Re: dual mic recording phase issues?
« Reply #24 on: October 19, 2006, 03:00:06 PM »

minister wrote on Thu, 19 October 2006 12:34

J.J. Blair wrote on Tue, 17 October 2006 23:57


When mics are spaced apart, you are capturing a different phase of that sound wave.  If you make it earlier or later, it does not change whether the wave is compressing or rarefacting.  The only way to change that is by actually adjusting the phase of the signal, or aligning the diaphragms of the two mics, so that the phase is coincidental.  

Let me illustrate it this way: A mic one inch from the source might capture the 1kHz portion of a wave's phase at 30˚ above zero, but one nine inches away might cappture it at 70˚.  But the 100Hz portion of the wave might be at 20˚ above zero at one inch and 18˚ below zero at nine inches.  The sum of those two is going to give you constructive interference at 1Khz, making that louder, but cancellation at 100Hz, making that quieter.  Delaying one or the other never aligns the phase.  You just may be negating the amount of cancellation is all.


i thought this explained it very well, and might be worth re-reading.  it explains why the time nudge method in a DAW is not sufficient.





Not necessarily.  Here's something that is lost in that explanation...that 100HZ portion of the sound HAS to have a longer period than the 1kHz portion because it is a lower frequency, so the difference in phase vs. distance (and in this equation, since velocity remains constant, the change in distance = the change in time) over that longer period will equal fewer degrees of shift.  Periodic functions work like that.  All phase is, by definition, is the amplitude of a waveform at a given point in time, given in degrees.  So by definition, a waveform that shows amplitude and time shows phase.

Also we're not talking about a distance difference of 9 inches...we're dealing in distances that are well within a centimeter...more likely less than an eigth of an inch, which also means that the difference in the sound the mics are capturing in terms of direct vs. reflected signal won't be significantly different.  

Putting waves up on a scope can show COMBINED phase relationships, but as far as one track interacting with another track is concerned, phase relationships, assuming similar source material (i.e., the 2 mic guitar amp setup as described), visually aligning tracks by amplitude and time IS aligning them in phase.

Look at it like this:
http://www.kwantlen.ca/science/physics/faculty/mcoombes/P2421_Notes/Phasors/doublesine.gif

http://en.wikipedia.org/wiki/Phase_%28waves%29
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Ian Combs
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minister

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Re: dual mic recording phase issues?
« Reply #25 on: October 19, 2006, 04:12:42 PM »

you are still not moving the mic in space relative to how the frequency period comes off the drums by sliding it in time in a DAW.  and that is the crux of it.  jj's point is that when you are capturing with a mic, especially as it captures reltive to its pair, you are capuring things at a certain point as they come off the source.  sliding a track further back in DAW is not the same thing as moving the mic.  by moving the mic, you are capturing a different phase of the sound.

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jimmyjazz

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Re: dual mic recording phase issues?
« Reply #26 on: October 19, 2006, 04:52:23 PM »

By moving the mic, you are changing the relative phase between the mic and all other mics on the kit.
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iCombs

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Re: dual mic recording phase issues?
« Reply #27 on: October 19, 2006, 05:19:46 PM »

we weren't talking about drums.  as I said before, in relation to drums, I'm NOT for sliding those tracks because of the loss of phase coherence between the 8-12 microphones I use for that. we're talking about guitar amps, and in that application, assuming you are trying to combine the signals of two mics that are supposed to be coincident (as bryan k was shooting for in his original question).  we aren't talking about combining a distance mic and a close mic (or at least, I'm not).  

We all have to keep in mind that when a microphone picks up a sound, it picks up all 360 degrees of every cycle.  The mic that is further back picks up the exact same sound (assuming it isn't so far back as to significantly change the ratio of direct sound to reflected sound) a fraction of a second later.  THEREFORE, if you were to move that track back in time in relation to the reference track, they would HAVE to be in phase.

The phase of a sound does not change in any significant way over the distances we are using.  Look at my other posts.

jimmyjazz...yeah...that's what I'm getting at with drums.  I'd just like to stick to the original topic because I think it's a valid discussion, and as far as I can tell, I've got a pretty good handle on the situation, however, I'm really interested to see what JJ brings back in terms of illustrations.

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Ian Combs
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Peter Weihe

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Re: dual mic recording phase issues?
« Reply #28 on: October 19, 2006, 06:02:55 PM »

There are no two spots in front of a git.cab speaker that sound the same.
Every zone produces a different mix of the complex mixture of frequencies coming from the amp.
The bright frequencies coming from the cone will arrive later at the edge.
There are frequency dependent phase issues in front of each speaker.
Consequently there is no chance to put two microphones at two positions in front of one speaker where they could capture identical waveforms even if you put them at equal distance on both sides of the cone.
There is no chance to perfectly time align them later in the DAW because there is no perfect position. Sure you can do it but then you will be surprised by your sound after the recording, which might be fun but I  prefer to decide by ear and taste when I record.

But there are lots of positions for two mics, which 's combination results in great sounds!

I have lots of different mics in front of my guitar cabs, because I don't have the time to search the perfect  positions for each take on my sessions as guitar player and I can quickly choose from "presets" that have proved to sound great for certain sounds.

One perfectly positioned mic only for each individual take would probably be even better!

I aligned most of them in phase as good as possible and none of their combinations sounds ugly or phased .
Simply aligning them to the positions where you get the highest level on the meters out of two or three mics mostly results in too much bass.

I aligned them first by knowing where the diaphragm sits but then  as  JJ proposed, I fine tuned them by ear that way, that mic two doesn't steel something that I like from mic one but adds it's special quality to the sound.

To create a great sound by moving mics is fun, new positions can be very inspiring, you learn a lot about guitar sounds and built your own taste.

I don't understand why somebody would want to record waveformes without knowing how the music sounds that he is recording???????
I want to hear and feel my sound, my tone as guitar player because my hands will react to what I hear.

Peter



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jimmyjazz

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Re: dual mic recording phase issues?
« Reply #29 on: October 19, 2006, 06:14:40 PM »

Yeah, sorry about that.  I wandered into drum-land without thinking about the original topic.  But, as you say, if we're talking about the SAME signal just being captured a bit later, then yes, it shouldn't be a big deal to slip one relative to the other and have no problems.

Of course, if they're the SAME signal, then why do it?

The answer is that they aren't the same, so there won't be perfect additive effects, and interesting things are going to happen (particularly if the mics used are different).  Some of those things are going to be "interesting GOOD", and some will be "interesting BAD".  But it's not likely to be because of phase, it's going to be because of acoustic effects (room reflections, etc.) and microphone differences and things like that.
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