H-infinity sub-optimal causal IIR inverse filter

This computes the ptimal inverse filter minimizing the H-infinity norm of the error system.

[psi gopt] = dfdesign_w(phi, w, d);

DFDESIGN_W computes the H-infinity optimal inverse filter of a given filter phi(z).
The resulting filter psi(z) minimizes the H-infinity norm of the error system
E_w(z) = [z^(-d) - psi(z)phi(z)]w(z).

If an error occurs, use "dfdesign_w_lmi.m".

[INPUT]
phi: target system (discrete-time), SS or TF object
w: weighting function, SS or TF object
d: reconstruction delay, non negative number

[OUTPUT]
psi: resulting inverse filter
gopt: optimal value

This is based on the following paper:
M. Nagahara and Y. Yamamoto,
H-infinity optimal approxmation for causal spline interpolation,
Signal Processing, Vol. 91, No. 2, pp. 176-184, 2011.