Phasing
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This article is about the scientific meaning. For phasing in music see Phase shifting
Phasing describes relative phase shift in superposing waves. Waves may be of electromagnetic (light, RF), acoustic (sound) or other nature. By superposing waves using different phase shifts the waves can add (0° shift = "in phase") or cancel out each other (180°). A modulation of the relative phaseshift while superposing waves thus causes an amplitude modulatuion.
Phasing can be used as an audio effect. 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. 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.)
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 allpass filter), and then recombining the two signals. The allpass 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.
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 phaser is an electronic device used to produce this effect. It was originally produced by simply copying the sound onto two analogue tape decks and mixing them together with one tape running 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.
In motion picture or television production, the effect created by a phaser is often used to imply that the sound is synthetically generated (e.g. 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.
See also flanging, wave interference.