Phase and Polarity

Tech talk about audio recording and live stage production.
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Postby Mathieu Benoit » Fri Nov 23, 2007 3:37 pm

I'm sure Cubase must then have a function... probably in the control room section. I will investigate.
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Postby sammyp » Fri Nov 30, 2007 11:02 pm

I'm a fan of the nudge or slide process' in the modern DAW - if i reamp a kick or bass say thru a guitar amp or mic a snare top and bottom i'll just zoom in on the two waves and nudge the new track back a few samples so things line up.
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Postby macrae11 » Mon Dec 03, 2007 3:09 pm

I'm just going to try to clear up the misconception around here that time and phase are two interchangeable parameters. In fact they are not.

In this first example you can see to sine waves that are completely out of phase with each other and cancel perfectly.

Image

Now I can slide one of these waveforms over to make them "in" phase.

Image

Now while these two waveforms have amplifying peaks and nulls they are still not perfectly in phase. Since the timing has changed, when panned hard left and right the stereo image of these two waveforms actually shifts to one side. This is caused by inter aural time difference which is the main way that animals localize sound. This is more noticeable at lower frequencies due their longer wavelengths. http://en.wikipedia.org/wiki/Interaural_time_difference

So yes it's true that there shouldn't be any "phasey" sounds, which is a good thing, your stereo image will have changed.


Now it actually gets even more complex than this when you start dealing with multiple frequencies. Here's example #2

Image

In this image, imagine the top two waveforms are from one microphone and the bottom two waveforms are from a second microphone. The frequencies are 98 and 126Hz, a slightly out of tune major third. (Ignore the titles on the waveforms in the pictures as they aren't accurate.)

As you can see, in the first microphone, both waveforms begin with a negative excursion. In the second microphone, 98 Hz has a positive excursion and the 128 Hz has a negative excursion. So the 98 Hz is out of phase, but the 128Hz is in phase.
Now look at what happens when we try to "correct" based on the 98 Hz

Image

Not only are we out of time now, but 128Hz is more out of phase than it was to begin with!

I'm not condemning the use of time aligning for phase correction, I just don't want people to think it's a silver bullet. I mean really it doesn't matter as long as it sounds better right? However if you can get it right with the microphones, even though it will take bit longer setting up, It will save you tons of time in editing and also will usually sound better in the end. I realize there's not always time to be had during rough and tumble recording sessions, but it's what you should be striving for.


P.S.
I also realize this is an extremely simplified example that's not terribly real world, but using Fourier analysis(http://en.wikipedia.org/wiki/Fourier_analysis), we realize that all sounds are just a combination of different sine waves so I think this test is still fairly valid at demonstrating the point.
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Postby Malcolm Boyce » Fri Dec 07, 2007 11:13 pm

Thanks Andrew. That's a great analysis of the differences between the two issues. It is indeed a common misconception, and not just around here.

Some people, myself included, think that the differences in arrival time in multi mic setups improves the denseness and quality of the blend when all rolled together. The idea mimicking the effect of different reflection arrival times that creates ambiance or reverb.

Since I know that in a common close mic, large drum setup, it is impossible to align every input to a single reference, I don't understand how you decide what zero is. I also know that differences of only 5-10ms can affect the "feel" of a drum track in relation to the song.
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Postby macrae11 » Sat Dec 08, 2007 12:35 am

Oh I completely agree Malcolm. Your always going to have differences in arrival times. If your room mics are hitting at the same time as your close snare mics, it will generally sound horrible.

I know some people set the "0" reference to either the kick or the snare. I think in anything other than maybe some types of electronic or extremely processed music, this will not yield an ideal result.

But you're right even slight timing adjustments can ruin a drum feel. All the excellent drummers on the forum can attest to that. So if you are doing editing you have to make sure to keep with the feel of the original track. Not to throw the baby out with the bath water so to speak.

Personally I only worry about aligning mics that are directly related. Snare top and snare bottom for example, or overheads. Kick in and Kick out I don't think there is any point in time aligning, because I think the delay from the outer kick generally sounds more natural. Especially when it's got a nice boom and decay. As long as you like the phase relationship between the two mics. Same rule between the snare close mic and overheads.

At the end of the day you've really got to just listen to your set up and if it sounds good do it.
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Postby Malcolm Boyce » Mon Dec 10, 2007 11:48 pm

I was just reminded of another common misconception regarding phase, when I was reading another question on another forum.

The "direction" in which the microphone is pointing does not affect the phase response in relation to the acoustic sound. The common mistake I hear is that the reason why the "bottom" or resonant head mic needs to be flipped is because it is pointed in the opposite direction of the "top" or batter head mic. This displays a fundamental lack of understanding how microphones, and transducers in general work.

If you take two identical microphones, point the capsules at one another with a space of 8", and snap your fingers centered between them, is the signal captured by the two microphones inherently out of phase? Of course not. This is the equivalent of saying that if you face two speakers at one another, the area centered between the two would cancel one another out... which we know is false.

The basic reason why a lot of the time top and bottom mics need to be "flipped" in relation to one another is because the stick striking the batter head is creating an impact impression in the skin, which immediately creates a negative pressure area. The force inside the shell pushes the resonant head in relation to the batter head, this time out of the drum, immediately creating a positive pressure area. Assuming the top and bottom mics are the same, and in the same relative position to the top and bottom heads, the chance is that you will have to flip one in relation to the other to avoid a cancellation of low frequencies.
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Postby Alain Benoit » Tue Dec 11, 2007 4:07 am

Malcolm Boyce wrote: The common mistake I hear is that the reason why the "bottom" or resonant head mic needs to be flipped is because it is pointed in the opposite direction of the "top" or batter head mic.


Really? There are people out there who think that?
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Postby Malcolm Boyce » Tue Dec 11, 2007 1:59 pm

Alain Benoit wrote:Really? There are people out there who think that?
I have heard this, and read this time and again.
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Postby Mathieu Benoit » Wed Jan 27, 2010 5:12 pm

This thread needs a good bumping... and while this isn't exactly about phase, it is kind of in line with the concept of how sound gets to the microphone from the source.

I recently recorded a drum kit to a reasonable amount of success. I thought I may have had a strange phase issue, but it turns out the phase relations were good. I noticed that the cymbal movement created a strage doppler effect in the OHs that to me sounds really off. Alain says that there isn't much that can be done about that since the cymbal moves quite a bit, and thus its relation to the OHs changes with unusual acoustic results.

Any thoughts on this phenomena?
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Postby macrae11 » Wed Jan 27, 2010 6:06 pm

Cutting a little mids out of the overheads can make this effect less noticeable. Mono overheads can sometimes help too. What micing technique did you use?
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Postby Mathieu Benoit » Wed Jan 27, 2010 6:23 pm

Just a regular X/Y position. Would raising the height of the mics make any difference? Is this something that just can't be avoided in tracking or...?
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Postby Malcolm Boyce » Thu Jan 28, 2010 2:30 am

Drumwaiter wrote:Just a regular X/Y position. Would raising the height of the mics make any difference? Is this something that just can't be avoided in tracking or...?
Without seeing the setup it's difficult to say for sure but yes, raising the microphones could avoid what you're hearing as Doppler. I typically mic higher up over the kit, especially in good, larger rooms.

When closer to cymbals, specifically spaced pairs, or on individual cymbals in small setups, I remember learning to keep the capsules centered over the bell of the cymbal, especially larger cymbals to avoid effects caused by the swing toward and away from the mic. This works when OHs are mostly for "cymbal" mics.
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Postby Mathieu Benoit » Thu Jan 28, 2010 10:05 am

We desperately need a pair of C-Stands at the studio. Otherwise It's going to be nearly impossible to get this right...
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Postby Malcolm Boyce » Wed Sep 08, 2010 12:11 am

Sorry for the "bump", but I had to do it again...

http://www.gearslutz.com/board/5765343-post25.html
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Postby Mathieu Benoit » Wed Sep 08, 2010 12:52 am

Malcolm Boyce wrote:Sorry for the "bump", but I had to do it again...

http://www.gearslutz.com/board/5765343-post25.html


It's funny as I was reading through this whole thread, I see how much I've learned in the past 3 years. Most of the learning has been from trying out things for my own, however a lot of it has been from threads like this, and from many conversations with Andrew that I'm sure have frustrated him. So I for one appreciate people like yourself that are trying to help clear up some misconceptions.
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Postby Malcolm Boyce » Wed Sep 08, 2010 12:56 am

Sometimes people go reaching for a reason when the don't really understand the "why". It's when things get repeated as fact on these boards that things get scary.
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Postby Malcolm Boyce » Wed Sep 08, 2010 2:33 pm

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Postby Mathieu Benoit » Wed Sep 08, 2010 3:10 pm

"Volume automation takes time. You don't got that kinda time. You could be getting naked with somebody somewhere." -Slipperman

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Re: Phase and Polarity

Postby Mathieu Benoit » Mon Oct 15, 2012 10:52 am

This whole thread hurts my head. A few people are trying to shed some actual light on this, but from the onset of the thread it's mostly people trying to give advice of something they don't understand. The OP is particularily dense about it.

http://www.gearslutz.com/board/so-much-gear-so-little-time/776465-drum-recording-phase-alignment-time-alignment.html
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Re: Phase and Polarity

Postby Malcolm Boyce » Mon Oct 15, 2012 12:26 pm

Yeah... I don't even think I can digest any of that right now.
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Re: Phase and Polarity

Postby dylanger » Thu Oct 18, 2012 12:11 am

I lightbulb just went off! I just watched the featured video on the homepage of recordingmag. Is this pretty much the meat of this concept?

http://www.recordingmag.com/
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Re: Phase and Polarity

Postby sean.boyer » Thu Oct 25, 2012 2:19 pm

One interesting experiment I like to do to demonstrate phase in a DAW, is to take a simple track (like a synth or bass part), and duplicate the track exactly.
Play the two tracks together and you get a BOOST at (close enough to) ALL frequencies, giving a perceived loudness increase.

Flip the phase 180, and you get NO signal. That gets a stare. Then, take one of the tracks and move it ahead or behind an ms or so in time and listen to where the filtering happens.
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Re: Phase and Polarity

Postby macrae11 » Thu Oct 25, 2012 3:39 pm

sean.boyer wrote:Flip the phase 180, and you get NO signal. That gets a stare. Then, take one of the tracks and move it ahead or behind an ms or so in time and listen to where the filtering happens.

Another fun trick you can do with this is to add a narrow EQ boost to one of the tracks. Creates an effect somewhere in the wah family. Great for just a filtered effect or automate the boost frequency to make it funky.
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Re: Phase and Polarity

Postby Malcolm Boyce » Thu Oct 25, 2012 7:23 pm

macrae11 wrote:
sean.boyer wrote:Flip the phase 180, and you get NO signal. That gets a stare. Then, take one of the tracks and move it ahead or behind an ms or so in time and listen to where the filtering happens.

Another fun trick you can do with this is to add a narrow EQ boost to one of the tracks. Creates an effect somewhere in the wah family. Great for just a filtered effect or automate the boost frequency to make it funky.
How 'bout actually double tracking something, and flipping one. That can be really fun too.
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Re: Phase and Polarity

Postby macrae11 » Thu Oct 25, 2012 8:05 pm

Malcolm Boyce wrote:
macrae11 wrote:
sean.boyer wrote:Flip the phase 180, and you get NO signal. That gets a stare. Then, take one of the tracks and move it ahead or behind an ms or so in time and listen to where the filtering happens.

Another fun trick you can do with this is to add a narrow EQ boost to one of the tracks. Creates an effect somewhere in the wah family. Great for just a filtered effect or automate the boost frequency to make it funky.
How 'bout actually double tracking something, and flipping one. That can be really fun too.

See I'm not crazy about that unless its something that's really in the background.
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