A phaser is an electronic sound processor, that creates a sweeping effect by modulating a narrow notch signal filter. This hardware or software device is also known as a phase shifter. Its sound is similar to that of flanging . 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.
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.