Phase One-Contrary to popular belief, phase and polarity are not the same thing.

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

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Phase Out

FIG. 2: Sine waves a and b have the same amplitude and frequency, but they are 90 degrees out of phase. When they are added together, a single sine wave with greater amplitude is created.

Whereas polarity refers to the values of a signal voltage, phase concerns the time relationship between identical signals. The sine waves in Fig. 2a and Fig. 2b are exactly alike, but the second signal begins a quarter of a cycle later than the first. Whenever two identical signals begin their cycles at different points in time, they’re out of phase. The time difference between them is called the phase shift, and it is measured in degrees. In Fig. 2, the two signals are 90 degrees out of phase. If you add these signals together, they won’t cancel each other out. Instead, the voltages add up at each point in time to create an output waveform with a greater amplitude than the original’s.

Sine waves are great for illustrating these concepts, because they have only one frequency component: the fundamental. Musical tones are more complex and interesting, because they contain a fundamental frequency plus many overtones. These frequency components are called partials. Each partial is really a sine wave at a particular frequency, and the combination of the partials creates the timbre of the sound.

FIG. 3: When a complex signal is added to an out-of-phase copy of itself, comb filtering results. The term comb filtering stems from the appearance of the spectrum that is produced.

When you combine a complex signal, such as a trumpet recording, with a copy of it that has been shifted in phase, the partials add to and subtract from each other in several ways. Depending on the amount of phase shift, some partials cancel completely, some partials are only reduced a bit, and others reinforce each other and increase in amplitude. This phenomenon of cancellation and reinforcement at different frequencies is called comb filtering, because a graph of the areas of cancellation and reinforcement resembles a comb (see Fig. 3).

If the phase relationship between the two signals changes, the comb filtering changes as well. The comb filtering can change even more if the phase-shifted signal is reversed in polarity, changing all the cancellations and reinforcements accordingly. The comb filtering also changes if the phase-shifted signal is greater or lesser in amplitude than the original.

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