A phaser is an audio signal processing technique used to audio filter a signal by creating a series of peaks and troughs in the frequency spectrum. The position of the peaks and troughs is typically modulation so that they vary over time, creating a sweeping effect. For this purpose, phasers usually include a low frequency oscillator

Process

Image:Phaser spectrogram.jpg of an 8-stage phaser modulated by a sine Low-frequency oscillation applied to white noise.]] 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 phasers 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 lt;ref>http://ccrma.stanford.edu/~jos/pasp/Phasing.html]. 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,http://home.debitel.net/user/jhaible/jh_storm_tide_flanger.html] 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 filter phase-shifter 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 Interference (wave propagation) when mixed back with the dry (unprocessed) signal, creating notches. The simplified structure of a mono phaser is shown below: Image:Phaser.svg The number of all-pass filter (usually called stages varies with different models, some analog phasers offer 4, 6, 8 or 12 stages. Digital phasers 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 n stages 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: Image:Phaser feedback.svg Image:Phaser response.png Image:Phaser response with feedback.png The frequency response of an 8-stage phaser with or without feedback is shown. Note that the peaks between the notches are sharper when theres 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 plugin for sound editing software, as a part of a monolithic rackmount sound effect unit, or as "stompbox guitar effects.

Usage

Phasing is a popular effect for electric guitar The term was often used to refer the original tape flanging effect heard on many Psychedelia records of the late 1960s, notably "Itchycoo Park by the Small Faces "Pictures of Matchstick Men by Status Quo Eddie Van Halen often used a phaser as part of his signal chain, after his distortion effects. Phasing is primarily responsible for the soaring and unique guitar sounds Brian May achieved with the band Queen (band) in such songs as Bohemian Rhapsody His signature opening guitar riff from "Keep yourself alive" is an easily discernable example. Many electronic keyboard instrument like the Rhodes Piano the Eminent 310 and the Clavinet are commonly treated with a phaserto "sweeten" their sounds. Examples can be heardin Billy Joel s "Just The Way You Are", Styx (band) s "Babe", and Jean Michel Jarre s "Oxygène . In film 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 commonly associated 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. Tony Levin often uses a phaser on a Chapman Stick for bass lines. Two distinctive examples are Peter Gabriel s "Shock the Monkey" and King Crimson s "Indiscipline".

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 Delay (audio effect) d 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#Comparison with phasing

See also

* Flanging * Wave interference

References

External links

* http://www.harmony-central.com/Effects/Articles/Phase_Shifting/ Phase shifting] article on Harmony Central * http://www.ehx.com/products/small-stone Electro-Harmonix: Small Stone] * http://www.ehx.com/browse/chorus-phase-shifters-flangers Electro-Harmonix: Phase Shifters] * http://www.rocknroll-pedals.com/vintage-pedal-photo-gallery.htm Photos of Vintage Pedals & Effects] * http://www.pedalheaven.com/categories?c5 Photos of New & Vintage Phaser-Pedals] * http://www.geofex.com/Article_Folders/phasers/phase.html The technology of phase shifters and flangers] Category:Audio effects de:Phaser (Musik) es:Phaser eo:Fasero fr:Phaser it:Phaser (musica) nl:Phaser ja:フェイザー (音響機器) ru:Фэйзер tr:Phaser (efekt) uk:Фейзер