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Paraunitary FiltersC.4

Another way to express the allpass condition $ \left\vert H(\ejo )\right\vert=1$ is to write

$\displaystyle \overline{H(\ejo )} H(\ejo ) = 1, \quad\forall\omega.

This form generalizes by analytic continuation (see §D.2) to $ {\tilde H}(z)H(z)$ over the entire the $ z$ plane, where $ {\tilde H}(z)$ denotes the paraconjugate of $ H(z)$ :

Definition: The paraconjugate of a transfer function may be defined as the analytic continuation of the complex conjugate from the unit circle to the whole $ z$ plane:

$\displaystyle {\tilde H}(z) \isdef \overline{H}(z^{-1})

where $ \overline{H}(z)$ denotes complex conjugation of the coefficients only of $ H(z)$ and not the powers of $ z$ . For example, if $ H(z)=1+jz^{-1}$ , then $ \overline{H}(z) = 1-jz^{-1}$ . We can write, for example,

$\displaystyle \overline{H}(z) \isdef \overline{H\left(\overline{z}\right)}

in which the conjugation of $ z$ serves to cancel the outer conjugation.


We refrain from conjugating $ z$ in the definition of the paraconjugate because $ \overline{z}$ is not analytic in the complex-variables sense. Instead, we invert $ z$ , which is analytic, and which reduces to complex conjugation on the unit circle.

The paraconjugate may be used to characterize allpass filters as follows:

Theorem: A causal, stable, filter $ H(z)$ is allpass if and only if

$\displaystyle {\tilde H}(z) H(z) = 1

Note that this is equivalent to the previous result on the unit circle since

$\displaystyle {\tilde H}(\ejo ) H(\ejo ) \isdef \overline{H}(1/\ejo )H(\ejo ) = \overline{H(\ejo )}H(\ejo )

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``Introduction to Digital Filters with Audio Applications'', by Julius O. Smith III, (September 2007 Edition)
Copyright © 2024-05-20 by Julius O. Smith III
Center for Computer Research in Music and Acoustics (CCRMA),   Stanford University