DAFX: Digital Audio Effects, Second Edition

4.1 Introduction

Audio effect algorithms for dynamics processing, valve simulation, overdrive and distortion for guitar and recording applications, psychoacoustic enhancers and exciters fall into the category of nonlinear processing. They create intentional or unintentional harmonic or inharmonic frequency components which are not present in the input signal. Harmonic distortion is caused by nonlinearities within the effect device. For most of these signal processing devices, a lot of listening and recording experience is necessary to obtain sound results which are preferred by most listeners. The effect parameters have to be adjusted carefully by ear while monitoring the output signal by a level meter. The application of these signal processing devices is an art of its own and of course one of the main tools for recording engineers and musicians.

The terms nonlinear processing or nonlinear processors are used for all signal processing algorithms or signal processing devices in the analog or digital domains which do not satisfy the condition of linearity. A system with input x(n) and output y(n) is called linear if the property

4.1

is fullfilled. In all other cases it is called nonlinear. A difference can be made between static or memoryless nonlinearities, that can be described by a static input-to-output mapping, and dynamic nonlinear systems.

If a sinusoid of known amplitude ...