Spectral Distortion Using Second-Order Allpass Filters

This work presents a technique for detuning or applying phase distortion to specific spectral components of an arbitrary signal using a cascade of parametric second-order allpass filters. The parametric second-order allpass provides control over the position and slope of the transition region of the phase response, and this control can be used to tune a phase distortion effect to a specific frequency range. We begin by presenting the phase response of a cascade of first-order filters, which we relate to that of the parametric second-order allpass. Time-varying parameters and the time-varying phase response are derived for the second-order case, and we provide examples demonstrating the frequency-selective phase distortion effect in the context of processing of instrumental sounds.