Phase. I just can't understand!

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wjgypsy

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Ok. So i know what happens when you combine 2 signals, both picking up the same thing, and one 180 degrees out of phase. And when you combine them, they cancel. Ok. I understand that, but how does one mic get out of phase? Mic placement or something?

thanks,

Zeke
 
has to do with the time the sound source arrives at each mic. If the peaks of the source at one mic align with the troughs of the source at the other mic you have complete phase cancellation. (to keep things simple) It is about time. Also if you double mic a snare for example (top and bottom) you have positive pressure at one mic and negative pressure at another which results in phasing cancellation. It mostly it is a time based problem though.
 
it can also be an electrical problem which results from "backwards" wiring.
 
Also watch those walls young zeke. Refelctions can combine with the direct sound and cause "phasing", even with one mic. still a result of time delay though.
 
Right. There are two different things going on here: phase and polarity are only the same thing at one specific special-case point, really. Phase is a much more complex topic.

When two signals have exactly opposite voltages at each and every point in time, they are said to be "opposite polarity". They also happen to be 180deg out of phase with each other: that is the most gross phase error possible, and it happens to apply equally at all frequencies for this one special case. Add 'em together, you get zero: everybody knows that.

However, when you introduce *time* delays by spacing mics apart, the simple "opposite polarity" special case doesn't occur: because the sound takes time to go from here to there, and the wavelengths differ according to frequency, you have differing amounts of phase shift at different frequencies. This "different-phase-shift-per-frequency" thing is what gives you the "phasey" sound with a pair of mics at different distances from a source: you are creating a comb filter, where some frequencies are 180deg out of phase and cancel, and other frequencies aren't quite out and don't quite cancel, and others still are exactly in phase and *add* instead of cancel. What you really work with in mic placement is this parasitic comb filtering: it comes for free, because of the time delay.

Let's work an example, just for shits and grins. Sound moves about 1100 feet per second (close enough for this example). So let's stick a pair of mics on a guitar amp: one right up against the cone, and one back a few feet. The picker graunches a chord, and the resulting signal has all sorts of components at different frequencies, from the lowest string's fundamental at 100Hz, say, all the way up to high harmonics up at 10kHz or so.

The wavelength of a 100Hz sine wave is 1100/100: 11 feet (nice, round number, eh?). So theoretically, to perfectly cancel that 100Hz out between the two mics, you'd put the second mic back 5.5 feet: one half wavelength. Ignoring reflections and all that unpleasant real-world dreck, when the pressure was just hitting its max positive value at the close-in mic, it'd just be hitting its max _negative_ value at the mic 5.5 feet away: one half wavelength, 180deg phase shift. With me so far?

Now let's look at some 1kHz stuff in the same signal. That 1kHz stuff has a much shorter wavelength: 1100/1000, or 1.1 feet (start to see the relationship?). So to completely cancel that 1kHz, you'd only have to move back .55 foot: 6.6 inches. The perfect cancellation distance for any given frequency is _not_ the perfect concellation distance for any other frequency, because the wavelengths differ. Make sense?

If we move the second mic back that 6.6 inches behind the first, we have 180deg phase shift (cancellation) at 1kHz, and only 18deg phase shift at 100Hz: not much at all, and very little cancellation. The 1kHz stuff covers 10 wavelengths in 1 single wavelength of 100Hz. That's the key!

So phase shift is a function of frequency, when you're talking about spacing between mics. In the first example there, that perfect cancellation of 100hz at 5.5 feet would give you a perfect _doubling_ at 200hz (360deg phase shift, or perfectly in phase), and a perfect cancellation at 300hz (540deg phase shift, or effectively 180deg out of phase) and so on: so you end up with a frequency response that has a series of peaks and notches in it. Thus the term "comb filter".

Changing the spacing moves the notches around, and that's what you are doing when you play with mic placement. It also changes the contributions of reflections and all sort of that real-world stuff sweetnubs mentioned that is really a damned sight more important than this nerdy theoretical shit. So you never get the precise cancellation or reinforcement that this contrived and oversimplified example might seem to predict: the comb filtering can be pretty subtle. But that's how it works.

Hope that helps, anyway.
 
Ok. So how can i prevent it? I have a pre-amp (ART TBS) with a phase switch.

Thanks for your help! :)

Zeke
 
I promise this post has useful information, even if it is obscenely long. If you can wade through it all, it should help to clear up the Phase/Polarity confusion.






First of all, you are confused on the difference between phase and polarity. They are two VERY different things, though they are frequently taught in a way as to make the difference less than obvious. 180 degrees out of phase is really, mostly, a theoretical concept. It is only possible at one frequency at a time, and as all the sounds we record (sine waves are boring) contain more than one frequency, the theory is pretty much useless.

At any rate, phase is an issue of time, polarity is an issue of electrical direction. The following post contains a graphic of this.

https://homerecording.com/bbs/showthread.php?threadid=62002

First, a couple of terms you should know.

The cycle of a wave is the time it takes for a signal to start to repeat. During this time, a typical signal will start at zero, reach its peak positive amplitude, pass through zero, reach its peak negative amplitude, and reach zero again. The number of cycles in a second equals the frequency (pitch) of the signal.

The period of a signal is one half of the cycle. It is defined more precisely as the time it takes a signal to go from zero to peak amplitude (positive or negative) and back to zero. You will note it only goes to one peak, not both.

Phase is the time difference in two waves which are the same. For our purposes, it is the time difference between two electrical representations of the same sound.

The degrees of phase shift is how much of a cycle difference there is between two signals. There are 360 degrees in a cycle, or 180 degrees in a period.

A phase switch is a myth. It is also the name marketing people with no knowledge of audio give to the polarity switch on many pieces of gear.

This is the image within the above post.
https://homerecording.com/bbs/attachment.php?postid=495077

In the top pair of (poorly drawn) sine waves, you have exactly one complete cycle of two signals, which have reversed polarity. They are NOT out of phase, as they begin and end at the same point in time. The first and second half of the period is the same in both signals.

In the second pair of sine waves, you have one complete cycle of two signals, 180 degrees out of phase. You will note, that, during the second period of the first wave (which is the first period of the second wave) it looks exactly like the second period of the two signals which have their polarity reversed. However, in the first period of the first signal, the second signal has not yet started.

When using sine waves, the importance of this difference is not obvious, because with two sine waves, reversed polarity and a 180 degree phase shift sound the same. This is not true, however, with more complex wave shapes.

The third pair of signals, which are triangle waves, have their polarity inverted. As you can see, these two signals have a zero sum. You will also note that they are more complex signals. The amplitude is different in each cycle. This is a more natural acting signal (though it is still a manufactured signal).

If you look at the last set of signals, of which there are three, you will see the real problem with using sine waves as a teaching aid for phase/polarity. The first of the three is the same as the pair of signals above. The second is the same signal with its polarity shifted 180 degrees. Because of the complex nature of the amplitude, these signals do not create a zero sum signal.

None the less, if you had a perfect 180- degree phase shift, the polarity switch will "fix" the issue. Any two signals, when summed, will sound (marginally) different if the polarity is reversed on one of them, and it is good to try both ways on every signal whenever you are mixing. The difference will, of course, be most pronounced with two signals which are similar.

The polarity switch will NEVER actually fix a phase issue, because while it will make the peaks of one frequency line up, it will have the opposite effect on another frequency in the same pitch. It may sound better with the switch in a reversed position, but you have only created another issue. (This may not make sense if you do not understand the Harmonic/Overtone Series. Even as long winded as I am, I don't have the energy to explain this right now. You can probably find everything you need to know by doing a web search for "Overtone Series.")

I hope this helps.

(You will note I am not calling them phase problems. Some phase issues will sound good, some bad. You will never know for sure until you have tried it both ways.)

Here are a couple of other posts on related topics.

https://homerecording.com/bbs/showthread.php?threadid=59380

https://homerecording.com/bbs/showthread.php?threadid=57636


Light

"Cowards can never be moral."
M.K. Gandhi
 
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By the way, this is not easy stuff. Just remember that, once you understand this, you are well on your way to having one of the three fundamentals of audio down.

By the way, they are, time manipulation, signal path, and amplitude manipulation.


Light

"Cowards can never be moral."
M.K. Gandhi
 
yes polarity and phase are different things. best to think of it that way too so everyone is on the same page. I just tend to think of it as phasing because the most common situation i run into involving polarity problems are "backwards" wired monitors, i.e. one monitor "pushes" while the other "pulls" which sounds like a phasing problem.
 
well, polarity and phase, despite their different causes, can have exactly the same effect on the waveform, especially if that waveform happens to be a theoretically perfect sine wave, or close to it.

it IS best to think of the CAUSES "polarity" and "phase" as two different things, but their EFFECTS can be the same. for all intents and purposes, 180 degrees of phase shift has the same effect as reversed polarity.
 
Light said:
The polarity switch will NEVER actually fix a phase issue, because while it will make the peaks of one frequency line up, it will have the opposite effect on another frequency in the same pitch. It may sound better with the switch in a reversed position, but you have only created another issue. (This may not make sense if you do not understand the Harmonic/Overtone Series. Even as long winded as I am, I don't have the energy to explain this right now. You can probably find everything you need to know by doing a web search for "Overtone Series.")
Click to expand...


I know what you're saying here, but isn't this sometimes not true? For example, when dealing with a bottom snare mic in conjuction with a top snare mic. We're not dealing with timing issues here - the sound arrives at the same time to the two different mics, but one's positive pressure, the other negative pressure. Doesn't the polarity switch FIX the issue here, which you say is never the case?
 
bleyrad said:
...their EFFECTS can be the same. for all intents and purposes, 180 degrees of phase shift has the same effect as reversed polarity.
Click to expand...

No, actually, this is not the case, because you can only ever be 180 degrees out of phase at one frequency. If you are 180 degrees out of phase at the first overtone (for instance) you will be 90 degrees out of phase at the fundamental, 90 degrees at the second overtone, and in phase at the 3rd overtone. It may or may not sound better that way, buy you will never know until you try the polarity switch both ways. They only sound the same with a sine wave, or other constant sound. There are very few sounds which are constant, outside of electronic keyboards.

Guitar amp backs and under snare mics are not phase issues. They are polarity issues. On one side, you will get positive pressure, and on the other you will get negative pressure. Phase is ONLY a time issue, and is not what you are dealing with in that situation.


sweetnubs said:
yes polarity and phase are different things. best to think of it that way too so everyone is on the same page. I just tend to think of it as phasing because the most common situation i run into involving polarity problems are "backwards" wired monitors, i.e. one monitor "pushes" while the other "pulls" which sounds like a phasing problem.
Click to expand...

It sounds similar, but "phasing" tends to imply Phaser to my mind, which is a time effect with a slow modulation. I like to be specific about this stuff, particularly when I am explaining it to people who are new to this. It is important to understand the difference, and so many people use the terms interchangeably that it leads to a great deal of confusion for many people. You must treat the two issues differently, so even if you are going to use the terms interchangeably, you need to know what you are actually talking about. It is easier when you are starting out to use the corect terms for the different issues.
 
Light said:
Guitar amp backs and under snare mics are not phase issues. They are polarity issues. On one side, you will get positive pressure, and on the other you will get negative pressure. Phase is ONLY a time issue, and is not what you are dealing with in that situation.
Click to expand...


Since you're being so specific about wording, I'm going to have to disagree with you here. "Polarity" implies something electrical or magnetic, not sound pressure. Polarity deals with POLES - positive or negative. Positive and negative sound pressure will cause the same effect as having reversed polarity by creating the a likewise positive or negative electrical waveform output, but it is not in itself a polarity issue. It is a pressure issue.
 
I printed out the sine wave chart and i think that it will help. So when one wave is in-time (in phase) and the other is 180 degrees out of phase the are both pushing each other out of there way, so they can get to the peak. But because they are both positive, they push each other out back, there for creating a cancel. Just like when you try to put 2 magnits with the same side (N & N or S & S) together, they will push each other away.

A'm i way off here?

ZeKe
 
zeke just think of it as simple addition. think of the amplitude of the wave in db. amplitude meaning "the height" of the wave. (to keep things simple for this example) Let's say at a certain point in time wave one's amplitude is at 6db. (its peak) sine wave two's amplitude is at -6db, i.e. it's trough. when you sum them (add) you get 0db. That's why you don't hear anything. this is two waveforms 180 degrees out of phase relative to one another. complete destructive phasing.
 
Uh-oh. Sorry, sweetnubs, using dB in that way is wrong, and is going to lead to more confusion: dB implies an amplitude ratio and not an absolute magnitude at any point in time. A quantity described in dB doesn't have a sign as such (in terms of "polarity", anyway), nor will quantities in dB necessarily combine linearly the way you describe. You have to use actual instantaneous voltages here, not magnitude ratios, to make this make sense.

If you have a sine wave with a peak-to-peak voltage of 1V, it's most-positive excursion will be 500mV (1/2V), and its most-negative excursion will be -500mV (-1/2V)- so the total swing is 1V.

Here's the problem with trying to use dB units for this example: it conveys no time-domain information. Let's rewrite your example: say you have a 1V peak-to-peak signal at 100Hz, just for grins. That signal measured with respect to some completely arbitrary, pulled-it-outta-my-butt reference level is, say, +6dB (just for discussion). Now, let's take another 1V peak-to-peak signal at 100Hz. However, let's say we reverse its polarity *with respect to the first signal*. It's amplitude is still +6dB! But at the instant that the first signal hits +500mV, the second signal is just hitting -500mV: they are exactly reversed in polarity (they are also 180deg out of phase, but let's let that ride for the moment). If you sum the first signal and the second signal, you get 0 _volts_ at all times: they cancel each other out precisely at every point in the waveform. And that resulting signal level is -infinity dB, with respect to our outta-the-butt reference.

You can't tell anything about the AC behavior of a signal expressed in dB: it's just a ratio of that signal's magnitude to some other reference signal's magnitude.

Anyhoo, two +12dB (with respect to our outta-the-butt reference level) sine waves summed together can give you anything from 0V, or -infinity dB (if they are exactly 180deg out of phase) to +18dB (if they are exactly in-phase), or any value in between if they are a more or less out of phase. The behavior of these relative-measured AC quantities are just flat counterintuitive for most folks, so let's leave dB units on the shelf for the nonce. It just makes an ugly concept even more ugly...
 
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What Skippy said. But yeah, it is just simple addition, aside from the fact that the numbers you are adding are constantly changing. Remember, the waves I drew are just graphical representaions of the audio signals, so you want to be carful not to be confused by the arbitrary and coincidental apperence of the drawings.


Light

"Cowards can never be moral."
M.K. Gandhi
 
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