Phaser (effect)

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This article is about the audio effect. For other uses, see Phase.

A phaser is an audio signal processing technique used to filter a signal by attenuating a series of notches in the frequency spectrum. The position of the notches typically varies over time (modulation), and phasers usually include a low frequency oscillator for this purpose.

1 Process
2 Usage
3 See also
4 External links

[edit] Process

The phasing effect was originally produced by simply copying the sound onto two analogue tape decks and mixing them together. One tape was run slightly faster than the other, so that one copy of the sound would overtake the other, resulting in a rising then falling effect caused by wave interference. DJs can achieve the same effect by playing two copies of the same record from the same point at the same time.

However, as more practical solid-state electronics and latterly software were used to re-create an approximation of the unwieldy tape-flanging set-up, the term Phasing more specifically refers to a swept comb-filtering effect where there is no linear harmonic relationship between the teeth of the comb (compare this with flanging, where the teeth of the comb-filter are spaced along the frequency spectrum in a linear harmonic series).

A flanger is a subtype of phaser in which the notches are uniformly spaced. In this case, the notches are created by mixing the signal with a delayed version of a signal. Flangers tend to sound more natural, like the "jet plane whoosh" effect, whereas phasers tend to sound more unnatural.

The electronic phasing effect is created by splitting an audio signal into two, electronically shifting the phase of one signal (usually by passing it through an all-pass filter), and then recombining the two signals. The all-pass filter passes all frequencies unchanged in amplitude, but has a frequency-dependent, non-linear effect on the phase of each frequency. The result is a signal whose overall spectrum is shifted by various amounts at each frequency. For example, the phase of a frequency at the low end of the spectrum may be shifted by 1/4 of a wavelength, while a frequency at the high end of the spectrum may be shifted by 3/4 of a wavelength.

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 do not alter the sound themselves (human ears are not very responsive to phase differences), but they yield constructive and destructive interference when mixed back with the dry (unprocessed) signal. Additionally, the output can be fed back to the input to create a more intense effect. 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.

When the filtered and non-filtered signals are recombined, the phase differences between them now cause peaks and notches of reinforcement and cancellation along the frequency spectrum (the so-called comb filter pattern). The degree of phase shift is periodically modulated (usually using an LFO), causing the peaks and notches to 'sweep' up and down the frequency spectrum, producing the characteristic rolling timbral changes of the phasing effect.

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.

Phaser audio effect (file info) — play in browser (beta)
- A short sample of unprocessed organ, followed by a few different phasing effects on the same sample
- Problems listening to the file? See media help.

[edit] 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.

Many electronic keyboard instruments are commonly treated with a phaser to "sweeten" their sounds - the Rhodes and the Clavinet being popular choices. 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." Led Zeppelin also used a phaser on "No Quarter" to get a very mysterious and spooky sound with the Rhodes.

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. A phaser was used on C-3PO's voice in Star Wars, for example.^{[citation needed]}

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.