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A Digital Model for the Prologue Voltage Control Filter

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The Prologue polyphonic synthesizer features a voltage control filter (VCF) employing an original topology. This paper examines its circuit and extracts a flowchart, which yields a set of analog filter equations. From these, the authors were able to develop a transfer function to describe its ideal linear time-invariant form. Then a linear digital version of the filter to validate the analysis was put forward. Noting that the filter operation within the synthesizer is fairly nonlinear, a number of modifications yielding a nonlinear model of the VCF is proposed. These were compared with the outputs of the original analog filter, and it was found that they approximate them consistently.

JAES Volume 72 Issue 1/2 pp. 79-88; January 2024
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AES - Audio Engineering Society