Modeling of the EMS VCS3 Voltage-Controlled Filter as a Nonlinear Filter Network

A previously developed nonlinear differential equation system, representative of the diode-based voltage-controlled filter on board of the EMS VCS3 analog synthesizer, can be modeled in terms of a filter network. The properties of this network can be converted block-by-block into a discrete-time version of the same model, in which the relationships existing between the system variables take the form of delay-free loops. In the presence of such loops, the discrete-time filter network is solved by iterating over a fixed-point scheme. Simulations running at 176.4 kHz show reasonable fidelity of the model in the regimes of interest, as well as shorter computation time compared to previous implementations of the same system. While reducing aliasing, such a sampling rate optimizes also the computation time as it requires fewer fixed-point iterations. The new model represents an improved alternative to previously developed voltage-controlled filter designs in the field of virtual analog.