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 of the waveform being affected is typically modulated so
that they vary over time, creating a sweeping effect. For this purpose,
phasers usually include a low-frequency oscillator.
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. 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. 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 signal, creating notches. The
simplified structure of a mono phaser is shown below:
The number of all-pass filters 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.
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.
Usage Phasing is a popular effect for electric
guitar. The term was often used to refer to 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 the 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". In the 1990s, Jonny Greenwood of
Radiohead used an EHX Small Stone phaser extensively, particularly on tracks such
as "Paranoid Android" and "The Tourist" from OK Computer.
Keyboard players also used phasing: in the 1970s, keyboard instruments like the
Fender Rhodes electric piano, the Eminent 310 electronic organ, 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 used an MXR Phase 100 on his
RMI 368x Electra piano from 1974; from late 1977 he will also use the phaser 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
Wave interference Shepard tone
Chorus effect References
External links Photos of New & Vintage Phaser-Pedals
The technology of phase shifters and flangers
How does a phaser work?, video by Empress Effects