Principles of Phaser

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Peter 9Sep12: This is a Work In Progress - but merciless edits are welcomed.

Bill 10Sep12: The fourth definition is the most correct IMO, but leaves out modulation. #1 seems to describe a wah-wah. #2 includes modulation but does not describe how the peaks and troughs are created. In #3 a notch instead of an all-pass is used, which is I believe is incorrect; it does mention regeneration/feedback. #5 is correct.
Perhaps we should just reference this wikipedia page

  • Peter 10Sep12: where do you think I "sourced" most of this material in the first place Bill , Wikipedia is my "friend", ;-)
  1. A phaser (also known as a phase shifter) is an electronic sound processor, that creates a sweeping effect by modulating a narrow notch signal filter. Its sound is similar to that of flanging.
  2. It is an audio signal processing technique used to filter a signal by creating a series of peaks and troughs in the frequency spectrum. The position of the peaks and troughs is typically modulated so that they vary over time, creating a sweeping effect.
  3. A phase shifter works by applying a narrow signal filter to a sound source. This causes various amounts of canceling of frequency components in the original signal. The filter's output is mixed back in with the original source audio signal. To get the sweeping effect, a low frequency oscillator (LFO) is used to slowly move the frequency center of the notch filter. Blending the altered signal back into the signal path (called "regeneration" or "feedback") intensifies the effect.
  4. The audio signal is split into two paths; one path treats the signal with an all-pass filter, which preserves the amplitude of the original signal and alters the phase. The amount of change in phase depends on the frequency. When signals from the two paths are mixed, the frequencies that are out of phase will cancel each other out, creating the phaser's characteristic notches. Changing the mix ratio changes the depth of the notches; the deepest notches occur when the mix ratio is 50%.
  5. The definition of phaser typically excludes such devices where the all-pass section is a delay line; such a device is called a flanger.
  6. Phase shifting is a popular effect for keyboards and guitars. It is included in the special effects processors of a number of popular keyboards and digital effect devices and in most audio processing software packages as an effect that can be applied.