What is a phaser?

The short answer

A phaser is a modulation effect that sweeps a small number of notches, thin gaps in the spectrum, slowly up and down through your sound. It builds those notches out of phase rather than volume: it passes the signal through a chain of allpass filters, mixes that phase-shifted result back with the untouched signal, and lets the two cancel wherever they fall out of step. A slow LFO glides the notches through the frequency range, and that gliding is the swirl you hear.

That description sounds a lot like a flanger, and the two really are cousins, but the difference is easy to hear once you know it. A flanger stamps many evenly spaced notches across the spectrum and sounds metallic and jet-like. A phaser carves only a few notches, spaced unevenly, and sounds smoother and more vocal. This page is about where that handful of notches comes from and why it swirls.

Allpass filters: phase without volume

The building block of a phaser is the allpass filter, and its name is the whole idea. It lets all frequencies pass through at full volume, changing nothing about how loud each one is. Feed music through one on its own and you would barely notice it was there.

What it changes instead is phase: the timing of each frequency, where it sits in its own cycle. An allpass filter delays some frequencies more than others, nudging them forward or back by an amount that depends on pitch. Low frequencies might come through almost untouched while higher ones get shifted nearly a full half-cycle. Because nothing is removed, nothing is audible yet. The allpass filter is a setup move, and the effect only appears in the next step.

Where the notches come from

The notches appear the moment you mix the phase-shifted copy back together with the original, dry signal. Two copies of the same sound, added together, either reinforce or cancel depending on how their phases line up. Where they are in step, they add and stay loud. Where the allpass has pushed a frequency to sit half a cycle out of step, the two copies are equal and opposite, so they cancel and that frequency drops out. A cancelled frequency is a notch.

Stack more allpass stages and you create more frequencies where the phase reaches that half-cycle mark, so you get more notches. The relationship is simple: each pair of stages produces one notch.

That is why a 4-stage phaser, the classic voicing and the default in patchd, gives you exactly two notches. It is a deliberately small number. The notches also land at frequencies set by the allpass math, which are not whole-number multiples of one another, so they sit unevenly across the spectrum instead of in a neat repeating pattern.

The sweep is what you actually hear

A pair of fixed notches on their own would just sound like a mild change in tone. The life of a phaser comes from moving them. The same LFO that any modulation effect uses, a slow control wave running well below the range you can hear, sweeps the allpass frequencies up and down. As they move, the notches glide across the spectrum, and your ear follows the shifting pattern of what is present and what is missing. That smooth, gliding motion is the phaser's signature swirl.

Two controls shape the motion. The Rate sets how fast the LFO sweeps, from a slow ocean-like drift to a fast wobble, and the Depth sets how far the notches travel on each pass. A Feedback control can route some of the output back into the allpass chain, which sharpens the notches and makes the swirl more resonant and pronounced.

Phaser versus flanger

Phasers and flangers are the two classic sweeping effects, and they get mixed up constantly because both move notches with an LFO and both add a sense of motion. The difference is in how the notches are made, and it changes everything about the sound.

• A flanger uses a short delay. Mixing a signal with a copy delayed by a millisecond or two creates a comb filter: a long row of notches spaced evenly all the way up the spectrum, at whole-number multiples of one frequency. Many notches, harmonically related, give the flanger its sharp, metallic, jet-plane whoosh.
• A phaser uses allpass filters. It makes only a few notches, and because they come from phase shifts rather than a delay, they are not evenly spaced. Fewer notches, spread unevenly, give the phaser a softer, rounder, more liquid swirl.

The one-line version: a flanger is a comb of many even notches and sounds metallic, while a phaser is a handful of uneven notches and sounds vocal. If a moving effect sounds sharp and jet-like it is probably a flanger; if it sounds smooth and swirly it is probably a phaser.

The phaser in patchd

In patchd, the Phaser is one node in the Studio voice-fx rack, the same rack that holds reverb and delay. You add it to a channel like any other node and drag it into the order you want. It ships as a classic 4-stage phaser by default, so you start with the familiar two-notch swirl and open it up from there.

Its controls are the ones this page describes: Rate and Depth for the speed and reach of the sweep, Feedback to sharpen and intensify the notches, Stages to trade the gentle two-notch character for more notches and a thicker effect, and Mix to blend the swirled signal against the dry one.

One honest note about the LFO: in patchd it is not a separate module you route around. Every modulation node carries its own LFO inside it, and you meet it through that node's Rate (its speed) and Depth (its amount). There is no global modulation matrix to wire up; the Phaser's sweep belongs to the Phaser. If you want the underlying idea on its own, see what is an LFO, and for the notch-shaping relatives see audio filters and the flanger.

Because the engine runs in real time over the low-overhead Windows audio paths, adding a Phaser node costs only single-digit milliseconds of engine-internal latency at a small ASIO buffer, so you can hear the swirl on yourself while you talk or play. The real-time engine is identical on the Free and Studio tiers.

The takeaway

A phaser passes your sound through a cascade of allpass filters, which shift phase without touching volume, then mixes that copy back with the dry signal so the two cancel at a few points and leave notches behind. A 4-stage phaser makes just two of them, spaced unevenly, and an LFO sweeps them up and down for that liquid swirl. The quick way to tell it from a flanger: few uneven notches and a smooth swirl mean phaser, while many even notches and a metallic whoosh mean flanger.

patchd is in development: a Windows audio mixer with a full effects rack on every channel and a Studio voice-fx rack that includes the Phaser among its modulation nodes. Join the waitlist to get notified when it ships. The Studio tier is $39.99 per year, and the real-time engine is identical on both tiers.