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:

\(\displaystyleH(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.

## Menu options

Right clicking on the frequency response chart or on an existing all-pass design marker displays an options menu, as shown on the left.

You may add up to 10 biquads (professional version only) and as with the H1 design markers, restrict the movement of the design markers.

The Heq filter is *always implemented* using a Direct Form II Transposed (IIR) in either double or single precision arithmetic. Use the Re-optimise P-Z options ** **to convert an equalised filter cascade into an H1 filter for deployment.