Allpassphase May 2026

The classic "phaser" guitar pedal is built from a series of allpass filters in parallel with the dry signal. When the phase-shifted signal is mixed back with the original, comb filtering occurs—creating the sweeping, notched "whoosh" sound. The number of allpass stages (4, 6, 12) determines the number of notches. Even the legendary "phase 90" pedal is, fundamentally, an analog allpassphase device.

When measuring a room’s impulse response, engineers use a sinusoidal sweep (e.g., a logarithmic chirp). The recorded response is convolved with the inverse allpass filter of the original sweep. The resulting deconvolution relies entirely on the known allpassphase of the sweep signal to extract the true room response from background noise.

Group delay ( \tau_g(\omega) = -\fracd\phid\omega ). allpassphase

For first-order analog all-pass: [ \tau_g(\omega) = \frac2/\omega_01 + (\omega/\omega_0)^2 ] Peak at ( \omega = \omega_0 ): ( \tau_g = 2/\omega_0 ).

Mathematically, a first-order all-pass filter is defined by the transfer function: The classic "phaser" guitar pedal is built from

[ H(z) = \fraca + z^-11 + a z^-1 ]

Where a is the coefficient (typically between -1 and 1). Notice the symmetry: The numerator and denominator are mirrored. This mirroring is what preserves the magnitude response (gain = 1) while altering the phase. To repair excessive phase smear, use a phase

If your audio system suffers from unexplained "phase problems," consider these diagnostics:

To repair excessive phase smear, use a phase linearization filter (an inverse allpass) or simply minimize the number of cascaded allpass stages.