Phase equalisation via the Heq filter cascade in ASN Filter Desgner

An all-pass equalisation filter cascade (Heq) is available for equalising the phase response of the H1 filter cascade and H2 filter respectively.

The cascade is comprised of up to ten second order (biquad) IIR all-pass filters, as defined by:

\(\displaystyle
H(z)=\frac{r^2-2rcos (\frac{2\pi fc}{fs}) z^{-1}+z^{-2}}{1-2rcos(\frac{2\pi fc}{fs}) z^{-1}+r^2 z^{-2}}
\)

Analysing H(z), notice how the numerator and denominator coefficients are arranged as a mirror image pair of one another. This mirror image property has a highly desirable characteristic, namely the modification of phase without affecting the magnitude spectrum, hence its usefulness for phase equalisation.
As with the H1 filter design paradigm, the positions of the conjugate poles may be placed and moved interactively with the mouse. The tool will automatically compute the associated zeros and update the frequency response.