Allpassphase — Quick

If you have ever wondered why a kick drum loses its punch after equalization, why a stereo image feels "smeared," or how reverb units create dense, natural decay without changing the tonal balance, you have encountered the effects of allpassphase. This article dissects the mathematics, the acoustic perception, and the practical applications of this critical signal processing concept. At its simplest, allpassphase refers to the phase response of an allpass filter . An allpass filter is a unique signal processing block defined by one remarkable property: its magnitude response is flat (0 dB) across all frequencies . It does not boost or cut any frequency. It does not change the equalization of a signal.

Mathematically, the transfer function of a first-order allpass filter is: allpassphase

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

The coefficient a is related to cutoff frequency fc and sample rate fs by: If you have ever wondered why a kick