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.
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Process
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 [1]. 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 [2], this combines the unlimited number of notches from the flanger with the uneven spacing of the phaser.
Structure
Traditional electronic phasers use a series of variable all-pass phase-shift networks which alter the phases of the different frequency components in the signal. These networks pass all frequencies at equal volume, introducing only phase change to the signal. Human ears are not very responsive to phase differences, but this creates audible interferences when mixed back with the dry (unprocessed) signal, creating notches. The simplified structure of a mono phaser is shown below:
The number of all-pass filters (usually called stages) varies with different models, some analog phasers offer 4, 8 or 12 stages. Digital phases may offer up to 32 or even more. This determines the number of notches/peaks in the sound, affecting the general sound character. A phaser with a stages of n generally has n/2 notches in the spectrum, so a 4-stage phaser will have two notches. Additionally, the output can be fed back to the input for a more intense effect, creating a resonant effect by emphasizing frequencies between notches. This involves feeding the output of the all-pass filter chain back to the input, as shown here:
The frequency response of an 8-stage phaser with or without feedback is shown. Note that the peaks between the notches are sharper when there's feedback, giving a distinct sound. A stereo phaser is usually two identical phasers modulated by a quadrature signal; the output of the oscillators for the left and right channels are a quarter-wave out of phase. Most modern phasers are a part of a digital signal processor, often trying to emulate analog phasers. Phasers are mostly found as plugins for sound editing software, as a part of a monolithic rackmount sound effect unit, or as "stompbox" guitar effects.
Phaser audio effect Image:Phasing effect.ogg
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Usage
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. The technique works because the frequency filtering produces sound we associate with mechanical sources, which only generate specific frequencies, rather than natural sources, which produce a range of frequencies. A vocoder is a different effect used for similar purposes.
Similar effects
A specific type of phasing, flanging is a similar effect, in which the notches are linearly spaced. In a flanger effect, the notches are created by mixing the signal with a delayed version of itself. Flangers tend to sound more natural, like the "jet plane whoosh" effect, whereas phasers tend to sound more otherworldy. For comparison of the two effects, check flanging.
