When the coefficients of a first-order allpass filter are time modulated by a periodic function, the filter behaves as a dynamic phase distortion device that produces timbral modification of its input. This filter belongs to the class of Periodic Linear Time-Varying (PLTV) systems, and thus it has different properties to the original LTI allpass filter. Such properties include a magnitude response that is not necessarily flat over all time, i.e. not allpass in the usual sense. Many interesting features of these time-varying filters can be discovered by analyzing them in the time and frequency domains. It is important to know too how the time-varying transfer function can be decomposed into recursive and nonrecursive elements, which leads to a criterion for system stability. Additionally, filter topology impacts the output wave shape; only certain configurations produce a low crest factor. These filters offer novel sonic elements for sound designers and the makers of audio effects processors.
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