DAFX: Digital Audio Effects, Second Edition

12.3 Circuit-based Valve Emulation

Digital emulation of valve- or vacuum-tube amplifiers is currently a vibrant area of research with many commercial applications. As explained in a recent review article [PY09], most existing digital valve-emulation methods may roughly be divided into static waveshapers, custom nonlinear filters, and circuit-simulation-based techniques. The first type of these methods, static waveshapers (e.g., [Sul90, Kra91, AS96, Shi96, DMRS98, FC01, MGZ02, Jac03, Ame03, SST07, SM07]), use memoryless nonlinear functions for creating signal distortion and linear filtering before and after the nonlinearity for tuning the magnitude response. Oversampling is usually used to avoid signal aliasing.

12.3.1 Dynamic Nonlinearities and Impedance Coupling

Although the valve component itself is mainly a memoryless device that can in principle be approximated with static nonlinear functions, reactive components (typically capacitors) in the circuit make the nonlinearity act as dynamic. This means that in reality, the shape of the nonlinearity changes according to the input signal and the internal state of the circuit. Custom nonlinear valve emulation filters [Pri91, KI98, GCO+04, KMKH06] simulate this dynamic nonlinearity, for example by creating a feedback loop around the nonlinearity.

Another important phenomenon in real valve circuits is the two-directional impedance coupling between components and different parts of the circuit. This causes, for example, an additional ...