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### Formulations

Below are various physical-model representations we will consider:

We will briefly introduce these topics below, and the remaining chapters and appendices will develop further details.

ODEs and PDEs are purely mathematical descriptions (being differential equations), but they can be readily ``digitized'' to obtain computational physical models.2.5Difference equations are simply digitized differential equations. That is, digitizing ODEs and PDEs produces DEs. A DE may also be called a finite difference scheme. A discrete-time state-space model is a special formulation of a DE in which a vector of state variables is defined and propagated in a systematic way (as a vector first-order finite-difference scheme). A linear difference equation with constant coefficients--the Linear, Time-Invariant (LTI) case--can be reduced to a collection of transfer functions, one for each pairing of input and output signals (or a single transfer function matrix can relate a vector of input signal z transforms to a vector of output signal z transforms). An LTI state-space model can be diagonalized to produce a so-called modal representation, yielding a computational model consisting of a parallel bank of second-order digital filters. Impedance networks and their associated equivalent circuits are at the foundations of electrical engineering, and analog circuits have been used extensively to model linear systems and provide many useful functions. They are also useful intermediate representations for developing computational physical models in audio. Wave Digital Filters (WDF) were introduced as a means of digitizing analog circuits element by element, while preserving the ``topology'' of the original analog circuit (a very useful property when parameters are time varying as they often are in audio effects). Digital waveguide networks can be viewed as highly efficient computational forms for propagating solutions to PDEs allowing wave propagation. They can also be used to ``compress'' the computation associated with a sum of quasi harmonically tuned second-order resonators.

All of the above techniques are discussed to varying extents in this book. The following sections provide a bit more introduction before plunging into the chapters that follow.

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