--- /dev/null
+## Copyright (C) 2010, 2012 Lukas F. Reichlin
+##
+## This file is part of LTI Syncope.
+##
+## LTI Syncope is free software: you can redistribute it and/or modify
+## it under the terms of the GNU General Public License as published by
+## the Free Software Foundation, either version 3 of the License, or
+## (at your option) any later version.
+##
+## LTI Syncope is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+## GNU General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with LTI Syncope. If not, see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## Frequency response of FRD models :-)
+
+## Author: Lukas Reichlin <lukas.reichlin@gmail.com>
+## Created: October 2010
+## Version: 0.2
+
+function H = __freqresp__ (sys, w, resptype = 0, cellflag = false)
+
+ [H, w_sys, tsam] = frdata (sys, "array");
+
+ if (! isempty (w)) # freqresp (frdsys, w), sigma (frdsys, w), ...
+ tol = sqrt (eps);
+ w_idx = arrayfun (@(x) find (abs (w_sys - x) < tol), w, "uniformoutput", false);
+ w_idx = vertcat (w_idx{:});
+
+ ## NOTE: There are problems when cellfun uses "uniformoutput", true
+ ## and find returns an empty matrix,
+
+ if (length (w_idx) != numel (w))
+ error ("frd: freqresp: some frequencies are not within tolerance %g", tol);
+ endif
+
+ H = H(:, :, w_idx);
+ endif
+
+ [p, m, l] = size (H);
+
+ if (resptype == 0)
+
+ if (cellflag)
+ H = mat2cell (H, p, m, ones (1, l))(:);
+ endif
+
+ else
+
+ if (m != p)
+ error ("tf: freqresp: system must be square for response type %d", resptype);
+ endif
+
+ H = mat2cell (H, p, m, ones (1, l))(:);
+ j = eye (p);
+
+ switch (resptype)
+ case 1 # inversed system
+ H = cellfun (@inv, H, "uniformoutput", false);
+
+ case 2 # inversed sensitivity
+ H = cellfun (@(x) j + x, H, "uniformoutput", false);
+
+ case 3 # inversed complementary sensitivity
+ H = cellfun (@(x) j + inv (x), H, "uniformoutput", false);
+
+ otherwise
+ error ("frd: freqresp: invalid response type");
+ endswitch
+
+ if (! cellflag)
+ H = cat (3, H{:});
+ endif
+
+ endif
+
+endfunction