A Simple Non-linear Transfer Function for a Wiener-Hammerstein Model to Simulate Guitar Distortion and Overdrive Effects

Abstract

Valve amplifiers have been modulated recently by digital signal processing techniques, using the Wiener-Hammerstein cell. The key of this approach is to identify the non-linear static transfer function. In the present contribution we model audio distortion pedal effects and propose a transfer function model derived from a modification of the Shockley equation. Six limiter circuits with different types of diodes (silicon, germanium and LED) were evaluated using a voltage sine wave of 10 Hz and amplitude such as to provide a 10 mA input current. Ten seconds of input and output signals were sampled (100 kSamples/s) and the model was fitted to the data using the Levenberg-Marquardt non-linear least square method. The model worked well, providing a root mean square standard error between the data and best fit less than 10-4, except for the LED limiter circuit. The present approach resulted in an analytical representation of the non-linear transfer function, which can generate directly from a discretised input signal the corresponding output signal according to a desired response of a chosen limiter model.