Phase Shifting & Time Delay

What Is Phase Shifting?

Phase shifting is essentially at the core of every audio filter or equalizer. For example, all-pass filters are the heart of phase-shifting effects. They can create an artificial stereo from a mono sound source. The flanging effect is similar but uses a simple time delay instead of phase shifting.

Sine Wave Example

We can observe the phase-shifting effect through a sine wave. First, we will consider a lower sine wave, starting at the same time as the upper sine wave but was sent through an all-pass filter that delays this particular frequency by 90 degrees. Viewing both waves together, you can see the time delay added by the all-pass filter. The phase shift from the all-pass filter is similar to a simple time delay but not precisely the same. Time delay shifts all frequencies simultaneously, whereas phase shift delays some frequencies longer than others. An all-pass filter center frequency is the frequency at which the phase shift is 90 degrees.

Altering Frequency Response

To further understand this process, let's see what happens when music, which typically contains many frequencies at once, is passed through an all-pass filter or time delay, and the delayed audio is mixed with the original. When you combine the audio with a delayed version of itself, you alter the frequency response. As one cycle of the wave rises, the delayed version falls, or perhaps it hasn't yet risen as far. So, when combined, they only partially cancel at the same frequency. This is also the basis for all analog equalizers. They shift the phase for a range of frequencies and combine the phase-shifted audio with the original.

Fake Stereo

A different method for using an all-pass filter to create a fake stereo is to apply different amounts of phase shift to the left and right channels without mixing the original versions. In this case, you are not altering the frequency response, but the sound takes on an exaggerated sense of width and dimension. This technique can also make sounds seem to come from a point beyond the speaker's physical location. Further, if the amount of the phase shift is modulated to change over time, the audible result is similar to a Leslie sound speaker. This is pretty much what the Leslie speaker does. Creating a phase shift and constantly varying time delays via the motion of the speaker driver. With an actual Leslie speaker, the Doppler effect causes the pitch to rise and fall as the rotating horn driver moves toward and away from you. But this also happens when the phase shift is varied over time.