This article presents a nonlinear discrete-time model of the EMS VCS3 voltage-controlled filters. The development of the model is based on the study of the filter circuitry and its behavior in the time domain. From this circuitry a system of nonlinear differential equations has been derived describing the dynamics in regime of large signals. The digital implementation of the filter is based on a numerical approximation of those equations. The resulting Matlab model has been compared with a structurally identical simulation running under PSpice. Finally, a real-time realization of the VCF has been implemented under the Pure Data processing environment.