The phaser, or phase shifter, is closely related to the flanger in that it also works by sweeping notches through the spectrum of the input signal. While the term phasing is sometimes used synonymously with flanging ,6.4 typical commercial phase shifters have been observed to implement nonuniformly spaced notches.6.5 Furthermore, phasers such as the Univibe (1960s) were intended to simulate the Leslie rotating speaker effect (§5.9) as opposed to being a low-cost analog flanging approximation. In other words, the conceptual unification of phasing and flanging seems to be technical in nature; i.e., based on the fact that both effects operate by sweeping notches through the spectrum. Flangers are constrained to have an ``infinite series'' of harmonically spaced notches (§2.6.3), while phasers have a limited number of nonuniformly spaced notches. In both cases motion of the notches over time is essential to the effect, and this motion is classically periodic. We will therefore define a phaser as any linear filter which modulates the frequencies of a set of non-uniformly spaced notches, while a flanger will remain any device which modulates uniformly spaced notches.
Digital implementations of phase shifters are discussed in §8.9. A Faust phase-shifter may be found in the file effect.lib within the Faust software distribution [156,172]. The Faust programming example phaser_flanger.dsp demonstrates the phasing effect.