... Abel1
Work supported in part by San Jose State University Cooperative Agreement NCC-2-327.
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... plane2
Note that the image of the conformal map corresponds to the domain variable $\zeta $ of the allpass transformation, while the input of the map corresponds to the range variable $z$.
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... desired.3
In general, the unit circle is mapped once to itself by any allpass transformation for which the number of poles $N_p$ minus the number of zeros $N_z$ inside the unit circle is $\vert N_p-N_z\vert=1$. Therefore, higher order allpass transfer functions can be used having $N_p$ poles inside the unit circle, say, and $N_z = N_p\pm 1$ poles outside the unit circle. However, such a transformation cannot be used for audio digital filter design, our principle application, because it results in an unstable final filter $H^*[{\cal A}_{-\rho }(z)]$. It similarly cannot be used in any applications requiring time-domain implementation of the unstable allpass filter in place of a unit delay element.
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... Barks4
The normalized warped-frequency interval $\omega\in[0,\pi]$ was converted to Barks $b$ by the affine transformation $b = (\omega/\pi)*(N_b-1)+0.5$, where $N_b$ is the number of Bark bands in use. For example, $N_b=25$ for a $31$ kHz sampling rate.
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...JOST:5
Matlab functions bark2lin.m and lin2bark.m for transforming between linear and bark-warped frequency representations are available on the internet at http://ccrma.stanford.edu/~jos/bbt/bbt.html.
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