Simplified, Physically-Informed Models of Distortion and Overdrive Guitar Effects Pedals
Proc. of the 10th Int. Conference on Digital Audio Effects (DAFx-07), Bordeaux, France, September 10-15, 2007 SIMPLIFIED, PHYSICALLY-INFORMED MODELS OF DISTORTION AND OVERDRIVE GUITAR EFFECTS PEDALS David T. Yeh, Jonathan S. Abel and Julius O. Smith Center for Computer Research in Music and Acoustics (CCRMA) Stanford University, Stanford, CA [dtyeh|abel|jos]@ccrma.stanford.edu ABSTRACT retained, however, because intermodulation due to mixing of sub- sonic components with audio frequency components is noticeable This paper explores a computationally efficient, physically in- in the audio band. formed approach to design algorithms for emulating guitar distor- tion circuits. Two iconic effects pedals are studied: the “Distor- Stages are partitioned at points in the circuit where an active tion” pedal and the “Tube Screamer” or “Overdrive” pedal. The element with low source impedance drives a high impedance load. primary distortion mechanism in both pedals is a diode clipper This approximation is also made with less accuracy where passive with an embedded low-pass filter, and is shown to follow a non- components feed into loads with higher impedance. Neglecting linear ordinary differential equation whose solution is computa- the interaction between the stages introduces magnitude error by a tionally expensive for real-time use. In the proposed method, a scalar factor and neglects higher order terms in the transfer func- simplified model, comprising the cascade of a conditioning filter, tion that are usually small in the audio band. memoryless nonlinearity and equalization filter, is chosen for its The nonlinearity may be evaluated as a nonlinear ordinary dif- computationally efficient, numerically robust properties.
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