A phaser 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. For this purpose, phasers usually include a low frequency oscillator.
The electronic phasing effect is created by splitting an audio signal 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%.
The definition of phaser typically excludes such devices where the all-pass section is a delay line; such a device is called a flanger. Using a delay line creates an unlimited series of equally spaced notches and peaks. It is possible to cascade a delay line with another type of all-pass filter as in, this combines the unlimited number of notches from the flanger with the uneven spacing of the phaser.
Phasing is a popular effect for electric guitar. The term was often used to refer the original tape flanging effect heard on many psychedelic records of the late 1960s, notably "Itchycoo Park" by the Small Faces, and "Pictures of Matchstick Men" by Status Quo. Eddie Van Halen often used a phaser as part of his signal chain, after his distortion effects, including the amplifier itself: Van Halen used a power attenuator to bring the amp's output down to line level so he could put effect boxes after it. Phasing is primarily responsible for the soaring and unique guitar sounds Brian May achieved with the band Queen in such songs as Bohemian Rhapsody.
Many electronic keyboard instruments like the Rhodes and the Clavinet are commonly treated with a phaser to "sweeten" their sounds. Examples can be heard in Billy Joel's "Just The Way You Are", Styx's "Babe" and Paul Simon's "Still Crazy After All These Years."
In motion picture or television production, the effect created by a phaser is often used to imply that the sound is synthetically generated, like turning a natural human voice into a computer or robot voice.
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