IIR Low Pass Filter to Multiband Filter Transformation

 [Num,Den,AllpassNum,AllpassDen] = iirlp2mb(B,A,Wo,Wt)
 [Num,Den,AllpassNum,AllpassDen] = iirlp2mb(B,A,Wo,Wt,Pass)

 Num,Den:               numerator,denominator of the transformed filter
 AllpassNum,AllpassDen: numerator,denominator of allpass transform,
 B,A:                   numerator,denominator of prototype low pass filter
 Wo:                    normalized_angular_frequency/pi to be transformed
 Wt:                    [phi=normalized_angular_frequencies]/pi target vector
 Pass:                  This parameter may have values 'pass' or 'stop'.  If
                        not given, it defaults to the value of 'pass'.

 With normalized ang. freq. targets 0 < phi(1) <  ... < phi(n) < pi radians

 for Pass == 'pass', the target multiband magnitude will be:
       --------       ----------        -----------...
      /        \     /          \      /            .
 0   phi(1) phi(2)  phi(3)   phi(4)   phi(5)   (phi(6))    pi

 for Pass == 'stop', the target multiband magnitude will be:
 -------      ---------        ----------...
        \    /         \      /           .
 0   phi(1) phi(2)  phi(3)   phi(4)  (phi(5))              pi

 Example of use:
 [B, A] = butter(6, 0.5);
 [Num, Den] = iirlp2mb(B, A, 0.5, [.2 .4 .6 .8]);

Package: signal