A phaser is an electronic sound processor 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.
A short, unprocessed string sample, followed by a few different phasing effects on the same sample
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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.
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, 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:
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.
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.
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. By the early 1970s, phasing was available as a portable guitar effect, one of the most notable early examples being the MXR Phase 90. From 1974, Steve Hackett in Selling England by the Pound studio album and tour used this one for his Les Pauls, and from The Lamb Lies Down On Broadway album and tour used the "phase filter" section in his EMS Synthi Hi-Fli. In the late 1970s, Brian May used large amounts of phasing, in such songs as "Sheer Heart Attack." In the 1980s, Eddie Van Halen, for instance, often used the MXR Phase 90 as part of his signal chain, for example in the instrumental "Eruption" and on the song "Atomic Punk".
Keyboard players also used phasing: in the 1970s, keyboard instruments like the Rhodes, the Eminent 310, and the Clavinet were commonly treated with a phaser, especially in avant-garde jazz. Bill Evans, for instance, used a Maestro phaser on Intuition. The phaser is also used to "sweeten" their sounds. Examples can be heard in Billy Joel's "Just The Way You Are", Styx's "Babe", and Jean Michel Jarre's Oxygène on which he used an EHX Small Stone phaser extensively. Tony Banks (Genesis) used an MXR Phase 100 on his RMI 368x Electra piano from 1974 (later he will insert this effect, as well as the fuzzbox, into the Electra's panel); from late 1977 he will also use the phaser (with a Boss CE-1 too) on his Hammond organ, for replacing the Leslie's rotating effect. Daft Punk helped to re-popularize the effect in the 21st century, utilizing it on a number of tracks on their Discovery album in 2001.
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 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.
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 pronounced and natural, like the "jet plane whoosh" effect, whereas phasers tend to sound more subtle and otherworldly. For comparison of the two effects, check flanging.
See also 
- Hunter, Dave (2004). Guitar effects pedals: the practical handbook. Hal Leonard. p. 15. ISBN 978-0-87930-806-3.
- Hunter, Dave (2004). Guitar effects pedals: the practical handbook. Hal Leonard. p. 81. ISBN 978-0-87930-806-3.
- The Boss Book: The Ultimate Guide to the World's Most Popular Compact Effects for Guitar. Hal Leonard. 2002. p. 104. ISBN 978-0-634-04480-9.
- Brewster, David M. (2003). Introduction to Guitar Tone and Effects: A Manual for Getting the Sounds from Electric Guitars, Amplifiers, Effects Pedals and Processors. Hal Leonard. p. 28. ISBN 978-0-634-06046-5.
- Pettinger, Peter (2002). Bill Evans: How My Heart Sings. Yale UP. pp. 227–28. ISBN 978-0-300-09727-6.
- Photos of New & Vintage Phaser-Pedals
- The technology of phase shifters and flangers
- How does a phaser work?, video by Empress Effects