Recent work has shown that if the coefficients of a first-order allpass filter are modulated over time, it behaves as a dynamic Phase Distortion (PD) device. A key advantage of this arrangement is that the filter input and the modulation signal can, within stability constraints, be arbitrary. Furthermore, no heterodyning or interpolation, as used for the techniques of Adaptive PD and Adaptive FM, respectively, is required. This now broadens the palette of sound effects that allpass filters are capable of generating and opens up a new perspective through which they can be configured for sonic modification purposes. However, a rigorous analysis of this dynamic device is not available and the purpose of this paper is to draw on the theory of time-varying filters to thoroughly understand its properties in the time and frequency domains. The significant influence of filter topology on the transient behavior of the output is also investigated. Finally, a procedure for mapping from a desired PD function to a modulation signal is also illustrated. This can be applied to derive modulation functions that impart particular timbral properties to the input, and it is used to create examples demonstrating the capabilities of this new technique.
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