Phase One
Contrary to Popular Belief, Phase and Polarity Are Not the Same Thing

By Jeff Baust

  In the audio world, you often see the words polarity and phase used interchangeably, which might lead you to believe that they mean the same thing. However, these terms refer to two different concepts. I’ll look at each concept, discuss how they relate to one another, and give you some examples that illustrate them in terms of music production.

Polar Regions
Polarity refers to the positive and negative values of a signal voltage. Consider the sine wave in Fig. 1a. This waveform, like all analog audio signals, is an alternating-current (AC) signal, which means that the signal voltage repeatedly alternates from positive to negative. The voltage starts at 0V, goes positive, then negative, then back to 0. This completes 360 degrees of the waveform, or one cycle.

FIG. 1: Sine Waves a and b have the same amplitude and frequency, but their polarities are reversed. When they are added together, they cancel each other out.

Now look at the sine wave in Fig. 1b, which is a copy of the first. The voltage starts at 0, goes negative, then positive, and ends up back at 0. This waveform is like a mirror image of the original, because its polarity is reversed, or inverted, with respect to the original. Whenever the first sine wave’s voltage values are positive, the second wave’s voltages are negative, and vice versa.

If you add these signals together, perhaps by summing them in a mixer, the two signals cancel each other out, as illustrated in Fig. 1. At every point in time, you are adding equivalent positive and negative voltages, and the result always equals 0V. If the signals have different amplitudes, they will only partially cancel each other. This makes sense, because at every point in time, you are subtracting the lesser voltage of one sine wave from the greater voltage of the other. The signal that remains is the difference in voltage between them.

The situation is the same with all signals, not just sine waves. For example, if you mix a trumpet recording with an inverted copy of itself, the two signals cancel out, just as with the sine waves. Again, at every point in time, equivalent positive and negative voltages are added together, regardless of the complexity of the signal. The result is always 0V.

Obviously, the result is different when you sum two different signals. If you are mixing a recording of a flute with a recording of a trumpet, inverting the polarity of the flute track might give the mix a slightly different timbre or tone. However, the signals are not identical, so they won’t cancel each other out.

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Reprinted with permission from Magazine, November, 2000
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