Phase?

[wyr2hs]

What is phase (in reference to mics being out of phase and such) and how do you switch the phase of a mic. Also how do you know if you have phase canncelation and such?

[sjjohnston]

Originally posted by wyr2hs
What is phase (in reference to mics being out of phase and such)

It's a word that's frequently misunderstood and confused with "polarity," such as when people talk about things like:

how do you switch the phase of a mic.

Picture a sine wave (you know, a sort of curvy line on a graph that goes up and down, varying from -1, at the bottom, to +1, at the top). If you invert the polarity, you flip it upside down, so the values that were -1 (or +0.2367) become +1 (or -0.2367). If you combine the original signal with the polarity-inverted version of the same signal, you get ... nothing.

Now picture two sine waves. Slide one a little to the left or the right. Now the two waves are "out of phase." If you make one exactly half a cycle (sometimes called 180 degrees) out of phase, it's the same thing as inverting the polarity! Hence the confusion.

In the real world, you don't record sine waves. You record much more complex waveforms. If you take two identical waves, slide one a little to the left or right, and combine them, various funny things happen.

A quick explanation of a way to think about them is: Think about a complex waves as a combination of a bunch of different components, which are sine waves with different frequencies. When combine the wave with a shifted version, the component with a frequency equal to double the shift will be cancelled out altogether. Various other frequencies will be affected in different ways.

Preamps and mixers often have a switch that inverts the polarity of a mic signal. Since mics (decent ones, anyway) put out a balanced signal, this is fairly easy -- you just switch the + and the - signal lines. You can use an XLR cable (or adapter) that crosses the lines too (i.e. that connects pin 2 at one end to pin 3 at the other end).

Also how do you know if you have phase canncelation and such?

If it sounds funny or different.

Some "phase cancellations" sound good: such as when the "shift" changes over time. That's more-or-less what chorus pedals and flangers do.