Introduction to Sound Synthesis Effects
Phaser
A phaser passes a signal through a chain of allpass filters and mixes it with the original. The filters shift some frequencies further out of step than others, so the mix has a few notches where the two cancel, and an LFO moves the notches up and down in frequency.
An allpass filter passes every frequency at the same level, so on its own it makes no audible change. It changes phase instead: frequencies below its centre come through almost unchanged and frequencies above it are shifted further and further out of step. Mixed back with the original, any frequency that has been shifted by exactly half a cycle cancels, and a notch appears. A phaser chains several allpass filters so that there are several such frequencies, and sweeps the filters' centre frequency with an LFO so that the notches move.
The tool below phases a chord or a drum loop. The plot shows the phaser's response, the level of each frequency after the mix, with the notches moving as the LFO sweeps.
A phaser
A chord or drums pass through a chain of one to four allpass filters swept by an LFO and are mixed with the original. Each filter adds one notch to the response.
Listen for With one filter there is a single notch moving up and down, and the change in tone is slight. Each added filter adds another notch, so more of the spectrum is cut at once and the sweep is easier to hear. At a slow rate the notches take several seconds to move up and back, the setting often used on electric piano. At a fast rate they move several times a second and the sound wobbles. Turning the mix to zero removes the effect completely, because the allpass filters on their own change only phase.
Stages and notches
Analogue phasers count their allpass filters in stages, each a first-order filter that can shift the phase by up to half a cycle. Two stages together reach a full half-cycle shift at one frequency, so every two stages make one notch: a four-stage phaser, such as the MXR Phase 90 of 1974, has two notches. The filters in this tool are second-order, each equal to two first-order stages, so each one adds a notch. More notches give a deeper, more obvious sweep, and some phasers add feedback from the output to the input to sharpen the peaks between the notches.
Because the notches come from allpass filters, they are few and unevenly spaced. A flanger makes its notches with a short delay, which gives many notches evenly spaced in hertz, with peaks between them at whole multiples of one frequency, and the ear hears that series as a pitch. A phaser's notches form no such series, so its sweep has less sense of pitch. The sweep is set in octaves because the LFO moves the filters' frequencies exponentially: a given change in the LFO moves the notches by the same number of octaves at any point in the sweep.
Common questions
How does a phaser work?
It passes a signal through a chain of allpass filters, which leave every frequency at the same level but delay some more than others, and mixes the result with the original. At frequencies where the two are half a cycle out of step they cancel, making notches, and an LFO sweeps the filters' centre frequency so the notches move up and down.
What is the difference between a phaser and a flanger?
A flanger makes its notches with a short delay, so they are evenly spaced in hertz and there are many of them. A phaser makes them with allpass filters, so there are only a few, as many as the filters allow, and they are spaced unevenly. A flanger's peaks fall at whole multiples of one frequency, which is heard as a pitch, and a phaser's notches do not form such a series, so a phaser's sweep has less sense of pitch.
What does the number of stages on a phaser mean?
It is the number of first-order allpass filters in the chain. Every two stages make one notch, so a four-stage phaser has two notches and an eight-stage phaser has four. More stages cut more of the spectrum at once, so the sweep is easier to hear.
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