--- /dev/null
+## Copyright (C) 1999 Paul Kienzle <pkienzle@users.sf.net>
+## Copyright (C) 2003 Doug Stewart <dastew@sympatico.ca>
+##
+## This program 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.
+##
+## This program 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
+## this program; if not, see <http://www.gnu.org/licenses/>.
+
+## Generate an Chebyshev type I filter with Rp dB of pass band ripple.
+##
+## [b, a] = cheby1(n, Rp, Wc)
+## low pass filter with cutoff pi*Wc radians
+##
+## [b, a] = cheby1(n, Rp, Wc, 'high')
+## high pass filter with cutoff pi*Wc radians
+##
+## [b, a] = cheby1(n, Rp, [Wl, Wh])
+## band pass filter with edges pi*Wl and pi*Wh radians
+##
+## [b, a] = cheby1(n, Rp, [Wl, Wh], 'stop')
+## band reject filter with edges pi*Wl and pi*Wh radians
+##
+## [z, p, g] = cheby1(...)
+## return filter as zero-pole-gain rather than coefficients of the
+## numerator and denominator polynomials.
+##
+## [...] = cheby1(...,'s')
+## return a Laplace space filter, W can be larger than 1.
+##
+## [a,b,c,d] = cheby1(...)
+## return state-space matrices
+##
+## References:
+##
+## Parks & Burrus (1987). Digital Filter Design. New York:
+## John Wiley & Sons, Inc.
+
+function [a,b,c,d] = cheby1(n, Rp, W, varargin)
+
+ if (nargin>5 || nargin<3) || (nargout>4 || nargout<2)
+ print_usage;
+ endif
+
+ ## interpret the input parameters
+ if (!(length(n)==1 && n == round(n) && n > 0))
+ error ("cheby1: filter order n must be a positive integer");
+ endif
+
+ stop = 0;
+ digital = 1;
+ for i=1:length(varargin)
+ switch varargin{i}
+ case 's', digital = 0;
+ case 'z', digital = 1;
+ case { 'high', 'stop' }, stop = 1;
+ case { 'low', 'pass' }, stop = 0;
+ otherwise, error ("cheby1: expected [high|stop] or [s|z]");
+ endswitch
+ endfor
+
+ [r, c]=size(W);
+ if (!(length(W)<=2 && (r==1 || c==1)))
+ error ("cheby1: frequency must be given as w0 or [w0, w1]");
+ elseif (!(length(W)==1 || length(W) == 2))
+ error ("cheby1: only one filter band allowed");
+ elseif (length(W)==2 && !(W(1) < W(2)))
+ error ("cheby1: first band edge must be smaller than second");
+ endif
+
+ if ( digital && !all(W >= 0 & W <= 1))
+ error ("cheby1: critical frequencies must be in (0 1)");
+ elseif ( !digital && !all(W >= 0 ))
+ error ("cheby1: critical frequencies must be in (0 inf)");
+ endif
+
+ if (Rp < 0)
+ error("cheby1: passband ripple must be positive decibels");
+ end
+
+ ## Prewarp to the band edges to s plane
+ if digital
+ T = 2; # sampling frequency of 2 Hz
+ W = 2/T*tan(pi*W/T);
+ endif
+
+ ## Generate splane poles and zeros for the chebyshev type 1 filter
+ C = 1; # default cutoff frequency
+ epsilon = sqrt(10^(Rp/10) - 1);
+ v0 = asinh(1/epsilon)/n;
+ pole = exp(1i*pi*[-(n-1):2:(n-1)]/(2*n));
+ pole = -sinh(v0)*real(pole) + 1i*cosh(v0)*imag(pole);
+ zero = [];
+
+ ## compensate for amplitude at s=0
+ gain = prod(-pole);
+ ## if n is even, the ripple starts low, but if n is odd the ripple
+ ## starts high. We must adjust the s=0 amplitude to compensate.
+ if (rem(n,2)==0)
+ gain = gain/10^(Rp/20);
+ endif
+
+ ## splane frequency transform
+ [zero, pole, gain] = sftrans(zero, pole, gain, W, stop);
+
+ ## Use bilinear transform to convert poles to the z plane
+ if digital
+ [zero, pole, gain] = bilinear(zero, pole, gain, T);
+ endif
+
+ ## convert to the correct output form
+ if nargout==2,
+ a = real(gain*poly(zero));
+ b = real(poly(pole));
+ elseif nargout==3,
+ a = zero;
+ b = pole;
+ c = gain;
+ else
+ ## output ss results
+ [a, b, c, d] = zp2ss (zero, pole, gain);
+ endif
+
+endfunction