Phase vs Polarity

Bob Capotosto · Post by **Bob Capotosto** » Friday Sep 19, 2003

Just want to get a thread started. Can a pair of like signals be in polarity yet out of phase ? Any takers ? Let the debate begin.

facingwest · Post by **facingwest** » Sunday Sep 21, 2003

I'd say yes because you can still have something out of phase even if the polarity was reversed. All polarity is going to do is change from a push in the cone of a speaker to a pull. Say if you're mixing something and something is out of phase within a mix, I don't think that reversing the polarity will fix the problem. I might be totally off the wall with this one. It's been a good while and I'm a little rusty with remembering. heh

tonefight · Post by **tonefight** » Sunday Sep 21, 2003

I would say yes also, speaking as an electrician I know Phase deals with the ac waves, in that aspect and you can have 2 hot wires of different phase but the same polarity that if touched together go "boom" . In audio I would make a guess that the 2 signals although hooked up + to + and - to - would not be perfectly in synch and cause an audible problem instead of the "boom" as in an electrical circuit.
What 2 like signal are we talking about also? 2 mikes? 2 pickups? 2 effect processors?

onetooloud · Post by **onetooloud** » Monday Sep 22, 2003

I am no expert but, feel this can be looked at two ways, electrically, and acousticly.
A waveshape can be positive at a marked point in time with a certain shape. If waveshape two is also postitive at the same point (or marked point) in time with a similer shape then I feel they are in phase or sync. This could happen at marked intervals such as the case with a/c power each phase compliments the other.
If waveshape two doesn't have a similer shape at a certain point in time they are out of phase. So I feel to be in phase is dependent on time, waveshape and desired effect not necessarily polarity.
Speakers can be of the correct polarity and in mechanical phase but out of acoustic phase due to physical location. Time alignement generally helps this.
So I feel polarity can be correct but out of phase.
Just my 10 cents
Nice thought provoking thread!

tonefight · Post by **tonefight** » Monday Sep 22, 2003

That sounds like a good explanation and makes sense, especially the "time" part puts some visual/ mental picture to it.

I'm still curious what signals and situation exactly we are talking about, or is it meant to be in general?

Post by **Ron** » Tuesday Sep 23, 2003

The main difference between polarity and phase is:
Polarity is usually used to describe a DC voltage, and phase is usually used to describe the time difference between two AC voltages.

Onetooloud was entirely correct about the time thing.

An AC signal basically has no definable polarity since it is changing polarity at the frequency of the AC waveform, but two signals can be out of phase from 0° to 360° relative to time.
If two AC signals are 180° out of phase at the same frequency and volume, they will basically cancel each other out. One is zigging while the other is zagging at the same time.

Music signals are so complex that you will always have some acoustical and electrical phase cancellation at different times. The opposite happens also, where two signals are in phase and the amplitude doubles.

AC power is usally described as hot and ground, but this doesn't mean the same thing as polarity in a DC sense. It's just that one side of the AC signal is tied to ground for safety reasons.

Here's a good link about AC phase:
http://www.doctronics.co.uk/signals.htm#sine

Next?

lonewolf · Post by **lonewolf** » Wednesday Oct 01, 2003

Polarity and phase are electronically unrelated, although two side-by-side speakers that have reverse polarity relative to each other will theoretically produce acoustic waves that are 180 degrees out of phase.

If you want to see two identical signals out of phase with each other, just put a mono signal into a stereo delay processor and set different delay times for the left and right delay (no feedback or echo, just delay). If your signal is a sine wave, you can calculate the phase differential by Phase=sin(1/(4xfreq) - DelayTime), but since phase is dependent on frequency, you can't really calculate the phase differential with dynamic music. It is simply "out of phase".

Rockin and a rollin is only howling at the moon.

lonewolf · Post by **lonewolf** » Thursday Oct 02, 2003

Sorry, that is the ARCsin of all that.

If you want two dynamic signals to be perfectly 180 degrees out of phase with each other so that they would cancel if combined, all you have to do is INVERT the signal on one channel. The easiest way to do that is to run it thru a single inverting transistor stage with unity gain. There will be a slight propagation delay, so you would have to compensate for the delay on the other channel with a non-inverting unity gain stage with the same propagation delay.

Some multiFX units have an algorithm to invert the signal.

tonefight · Post by **tonefight** » Saturday Oct 11, 2003

I came across an article that helped me undestand this more, for us local bar guys where it effects us is that putting 2 mikes on your guitar cab or miking a snare top and bottom will actually give you a weaker signal than a single mike. This is due to the slight time difference in the signal hitting the mikes making a phase problem.
So if we must mike the same signal twice we need to reverse polarity on one of the mikes.
I suppose, you could run them stereo and not have a problem though. Is this correct?

Post by **Ron** » Sunday Oct 12, 2003

Right on tonefight.

You definitely need a phase reversing cable when dealing with the top and bottom micing of a snare, since the mics are mechanically 180° out of phase ( facing each other ).

I've only had problems when micing 2 guitar cabs when the rig is running in stereo and there was some stereo modulation or phase shifting effects. That can be a real nightmare if the PA is mono, and putting the mics out of phase really doesn't help, since the phase of the guitar signal between cabs can vary from 0° to 180° out on a wide stereo modulation sweep. (Like at the beginning of Van Halen's "Atomic Punk").

If the two cabs are in mono and receiving the same signal, I've never had any problems, but who needs two mics then?

tonefight · Post by **tonefight** » Sunday Oct 12, 2003

Well the best bet for me will be to avoid this situation, but this thread helped me out because the drummer mentioned that eventually she wanted to get another snare mike and go top and bottom....... I would have had a problem and not had a clue why!!! Now at least I can give an explanation as to why we're not going to mike her snare twice.