In audio, the term phase refers to the relationship in time between two tones of the same frequency.
To understand what phase it, first, let’s take a closer look at a sine wave.
A sine wave has a frequency (the number of cycles per second of the vibration) and an amplitude or level (the strength or power of the vibration). Sine waves are typically represented on an XY graph, with time on the X axis and level on the Y axis.
The period of a sine wave is the length in time of one cycle; one cycle is 360 degrees.
The cycle of a sine wave begins at zero degrees at a level of zero. The wave rises to its positive maximum at 90°, one-quarter of its period. At one-half period (or 180°), it has fallen back to a level of zero. The wave continues to its negative maximum at 270°, three-quarters of its period. Finally, it rises back to zero level. This point can be described as the end of a cycle, at 360°, or as the beginning of the next cycle, back to 0°.
Two sine waves of the same frequency are said to be “in phase” if their cycles begin at the same time. If one wave begins at a different time, the signals are said to be “out of phase.”
Phase shift is often not audible, unless the two (or more) channels in question are combined. In that case, there will be addition or subtraction of signal level, varying with frequency.
For a single sine wave at a particular frequency, the result will only be audible as a level change. For complex waveforms, some frequencies will be amplified and some will be attenuated or even cancelled completely, affecting the frequency response of the combined signals.
What can cause phase shift?
Delay of one channel in a circuit will shift the phase of the audio, relative to an undelayed channel. Filtering in a channel, which in analog audio is a consequence of reactance (inductance or capacitance) and in digital audio can be a consequence of DSP filter processing; will shift the phase relative to an unfiltered channel. A polarity fault in a channel, whether caused by reversed wiring, an inverting amplifier or poor design, will shift the phase of all frequencies by 180° relative to the reference channel.
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