--- /dev/null
+## Copyright (C) 2010, 2011 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 -*-
+## @deftypefn {Function File} {@var{sys} =} frd (@var{sys})
+## @deftypefnx {Function File} {@var{sys} =} frd (@var{sys}, @var{w})
+## @deftypefnx {Function File} {@var{sys} =} frd (@var{H}, @var{w}, @dots{})
+## @deftypefnx {Function File} {@var{sys} =} frd (@var{H}, @var{w}, @var{tsam}, @dots{})
+## Create or convert to frequency response data.
+##
+## @strong{Inputs}
+## @table @var
+## @item sys
+## LTI model to be converted to frequency response data.
+## If second argument @var{w} is omitted, the interesting
+## frequency range is calculated by the zeros and poles of @var{sys}.
+## @item H
+## Frequency response array (p-by-m-by-lw). H(i,j,k) contains the
+## response from input j to output i at frequency k. In the SISO case,
+## a vector (lw-by-1) or (1-by-lw) is accepted as well.
+## @item w
+## Frequency vector (lw-by-1) in radian per second [rad/s].
+## Frequencies must be in ascending order.
+## @item tsam
+## Sampling time in seconds. If @var{tsam} is not specified,
+## a continuous-time model is assumed.
+## @item @dots{}
+## Optional pairs of properties and values.
+## Type @command{set (frd)} for more information.
+## @end table
+##
+## @strong{Outputs}
+## @table @var
+## @item sys
+## Frequency response data object.
+## @end table
+##
+## @seealso{dss, ss, tf}
+## @end deftypefn
+
+## Author: Lukas Reichlin <lukas.reichlin@gmail.com>
+## Created: February 2010
+## Version: 0.1
+
+function sys = frd (H = [], w = [], varargin)
+
+ ## NOTE: * There's no such thing as a static gain
+ ## because FRD objects are measurements,
+ ## not models.
+ ## * If something like sys1 = frd (5) existed,
+ ## it would cause troubles in cases like
+ ## sys2 = ss (...), sys = sys1 * sys2
+ ## because sys2 needs to be converted to FRD,
+ ## but sys1 contains no valid frequencies.
+ ## * However, things like frd (ss (5)) should
+ ## be possible.
+
+ ## model precedence: frd > ss > zpk > tf > double
+ superiorto ("ss", "zpk", "tf", "double");
+
+ argc = 0; # initialize argument count
+
+ switch (nargin)
+ case 0 # empty object sys = frd ()
+ tsam = -2; # undefined sampling time
+
+ case 1
+ if (isa (H, "frd")) # already in frd form sys = frd (frdsys)
+ sys = H;
+ return; # nothing more to do here
+ elseif (isa (H, "lti")) # another lti object sys = frd (sys)
+ [sys, alti] = __sys2frd__ (H);
+ sys.lti = alti; # preserve lti properties
+ return; # nothing more to do here
+ else # sys = frd (H) *must* fail
+ print_usage ();
+ endif
+
+ case 2
+ if (isa (H, "lti")) # another lti object sys = frd (sys, w)
+ [sys, alti] = __sys2frd__ (H, w);
+ sys.lti = alti; # preserve lti properties
+ return; # nothing more to do here
+ else # sys = frd (H, w)
+ tsam = 0; # continuous-time
+ endif
+
+ otherwise # default case
+ argc = numel (varargin); # number of additional arguments after H and w
+ if (issample (varargin{1}, -10)) # sys = frd (H, w, tsam, "prop1", val1, ...)
+ tsam = varargin{1}; # sampling time, could be 0 as well
+ argc--; # tsam is not a property-value pair
+ if (argc > 0) # if there are any properties and values ...
+ varargin = varargin(2:end); # remove tsam from property-value list
+ endif
+ else # sys = frd (H, w, "prop1", val1, ...)
+ tsam = 0; # continuous-time
+ endif
+ endswitch
+
+ [H, w] = __adjust_frd_data__ (H, w);
+ [p, m] = __frd_dim__ (H, w); # determine number of outputs and inputs
+
+ frdata = struct ("H", H, "w", w); # struct for frd-specific data
+ ltisys = lti (p, m, tsam); # parent class for general lti data
+
+ sys = class (frdata, "frd", ltisys); # create frd object
+
+ if (argc > 0) # if there are any properties and values, ...
+ sys = set (sys, varargin{:}); # use the general set function
+ endif
+
+endfunction