The Echoplex is a tape delay unit featuring fixed playback and erase heads, a movable record head, and a tape loop moving at roughly 8 ips. The relatively slow tape speed allows large frequency shifts, including "sonic booms" and shifting of the tape bias signal into the audio band. Here, the Echoplex tape delay is modeled with read, write and erase pointers moving along a circular buffer. The model separately generates the quasiperiodic capstan and pinch wheel components and drift of the observed fluctuating time delay. This delay drives an interpolated write simulating the record head. To prevent aliasing in the presence of a changing record head position, an anti-aliasing filter with a variable cutoff frequency is described.
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