function [mag, phase] = subspz(Editor, w, mag, phase, PZold, PZnew)
%SUBSPZ  Updates mag/phase by swapping pole/zero groups.
%
%   [MAG,PHASE] = SUBSPZ(EDITOR,W,MAG,PHASE,PZold,PZnew)
%   returns the updated MAG,PHASE data when swapping the 
%   pole/zero group PZOLD for PZNEW.

%   Author(s): P. Gahinet, Bora Eryilmaz
%   Copyright 1986-2002 The MathWorks, Inc. 
%   $Revision: 1.8 $  $Date: 2002/04/10 05:05:14 $

% RE: MAG, PHASE is associated to a (normalized) ZPK model and the 
%     update is therefore independent of the format.
Ts = Editor.LoopData.Ts;

% Construct corrective factor
zcorr = [PZold.Pole ; PZnew.Zero];
pcorr = [PZold.Zero ; PZnew.Pole];

% S or Z vectors
s = 1i*w(:).';
if Ts,
  s = exp(Ts * s);
end
ls = length(s);

% Compute correction. Corrective term is prod(s-zj)/prod(s-pj)
sz = s(ones(1, length(zcorr)),:) - zcorr(:, ones(1, ls));
sp = s(ones(1, length(pcorr)),:) - pcorr(:, ones(1, ls));
a = prod(sz, 1);
b = prod(sp, 1);
Correction = ones(size(a));
nzb = find(b);
Correction(:, nzb) = a(:, nzb) ./ b(:, nzb);

% Update mag and phase
mag = mag .* abs(Correction(:));
phase = phase + (180 / pi) * unwrap(angle(Correction(:)));