This section provided only a ``surface scratch'' into the large topic of digital filter design based on an arbitrary frequency response. The main goal here was to provide a high-level orientation and to underscore the high value of such an approach for encapsulating linear, time-invariant subsystems in a computationally efficient yet accurate form. Applied examples will appear in later chapters. We close this section with some pointers for further reading in the area of digital filter design.
Some good books on digital filter design in general include [346,365,291]. Also take a look at the various references in the help/type info for Matlab/Octave functions pertaining to filter design. Methods for FIR filter design (used in conjunction with FFT convolution) are discussed in Book IV , and the equation-error method for IIR filter design was introduced in Book II . See [283,284] for related techniques applied to guitar modeling. See  for examples of using matlab functions invfreqz and invfreqs to fit filters to measured frequency-response data (specifically the wah pedal design example). Other filter-design tools can be found in the same website area.
The overview of methods in §8.6.2 above is elaborated in , including further method details, application to violin modeling, and literature pointers regarding the methods addressed. Some of this material was included in [452, Appendix I].
In Octave or Matlab, say lookfor filter to obtain a list of filter-related functions. Matlab has a dedicated filter-design toolbox (say doc filterdesign in Matlab). In many matlab functions (both Octave and Matlab), there are literature citations in the source code. For example, type invfreqz in Octave provides a URL to a Web page (from ) describing the FFT method for equation-error filter design.