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Digitizing Bridge Reflectance

Converting continuous-time transfer functions such as $ \hat{\rho}_b(s)$ and $ \hat{\tau}_b(s)$ to the digital domain is analogous to converting an analog electrical filter to a corresponding digital filter--a problem which has been well studied [346]. For this task, the bilinear transform7.3.2) is a good choice. In addition to preserving order and being free of aliasing, the bilinear transform preserves the positive-real property of passive impedancesC.11.2).

Digitizing $ \hat{\rho}_b(s)$ via the bilinear transform (§7.3.2) transform gives

$\displaystyle \hat{\rho}_b^d(z) \isdefs \hat{\rho}_b\left(c\frac{1-z^{-1}}{1+z^{-1}}\right)
$

which is a second-order digital filter having gain less than one at all frequencies--i.e., it is a Schur filter that becomes an allpass as the damping $ \mu $ approaches zero. The choice of bilinear-transform constant $ c=1/\tan(\omega_0T/2)$ maps the peak-frequency $ \omega_0$ without error (see Problem 4).


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``Physical Audio Signal Processing'', by Julius O. Smith III, W3K Publishing, 2010, ISBN 978-0-9745607-2-4.
Copyright © 2014-03-23 by Julius O. Smith III
Center for Computer Research in Music and Acoustics (CCRMA),   Stanford University
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