Allpassphase

In mastering and restoration, engineers sometimes use allpass filters to convert a minimum-phase system (where all energy arrives as early as possible) into a nonlinear-phase system. This can reduce pre-ringing artifacts caused by linear-phase EQs, trading time-domain smear for a more "natural" transient response.

An all-pass filter is a signal processing block that passes all frequencies with unity magnitude gain (0 dB). Its only effect is to change the phase of the input signal as a function of frequency.

Transfer function (analog, 1st order): [ H(s) = \fracs - \omega_0s + \omega_0 ] allpassphase

Digital (1st order): [ H(z) = \fraca + z^-11 + a z^-1, \quad |a| < 1 ]

Magnitude response: [ |H(j\omega)| = 1 \quad \textfor all \omega ] Its only effect is to change the phase

At first glance, a tool that "does nothing" seems useless. But in practice, the Allpass filter is a surgical tool for solving complex audio problems.

You don't need a dedicated plugin to experiment. Many modern EQs, like FabFilter Pro-Q 3 or the stock Logic Pro Channel EQ, have an "Allpass" band option. You don't need a dedicated plugin to experiment

Humans are remarkably sensitive to phase at low frequencies. Here is what allpassphase does to perception:

Far from being a laboratory curiosity, allpassphase is deployed in countless audio systems. Here are the four most common applications.