This article describes the use of second-order all-pass filters as components in a feedback network, with parameters made time varying to enable effects such as phase distortion in a generative audio system. The term “audio” is used here to distinguish from generative “music” systems, emphasizing the strong coupling between processes governing the production of high-level music and lower-level audio. The classical time-invariant implementation of an all-pass filter is subject to issues of instability that can arise when time-invariant filter parameters are allowed to vary over time. These instabilities are examined, along with the adoption of a power-preserving rotation matrix formulation of the all-pass filter to ensure stability and ultimately an improved synthesis for a generative audio system.

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