Freeverb Allpass Approximation
In Eq.(3.2) we defined the allpass notation
by
![$\displaystyle \hbox{AP}_{N}^{\,g} \isdef \frac{-g + z^{-N}}{1 - g z^{-N}}
$](http://www.dsprelated.com/josimages_new/pasp/img738.png)
![$\displaystyle \hbox{AP}_{N}^{\,g} \approx \frac{-1 + (1+g)z^{-N}}{1 - g z^{-N}}.
$](http://www.dsprelated.com/josimages_new/pasp/img739.png)
![$ \hbox{FBCF}_{N}^{\,g}$](http://www.dsprelated.com/josimages_new/pasp/img740.png)
![$ \hbox{FFCF}_{N}^{\,-1,1+g}$](http://www.dsprelated.com/josimages_new/pasp/img741.png)
![\begin{eqnarray*}
\hbox{FBCF}_{N}^{\,g} &\isdef & \frac{1}{1 - g\,z^{-N}}\\ [5pt]
\hbox{FFCF}_{N}^{\,-1,1+g} &\isdef & -1 + (1+g)z^{-N}.
\end{eqnarray*}](http://www.dsprelated.com/josimages_new/pasp/img742.png)
A true allpass is obtained only for
(reciprocal of the ``golden ratio''). The default value used in
Freeverb (see revmodel.cpp) is
. A detailed discussion
of feedforward and feedback comb filters appears
in §2.6, and corresponding Schroeder allpass filters are
described in §2.8.
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Conclusions
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Lowpass-Feedback Comb Filter