--- /dev/null
+## Copyright (C) 2000 Paul Kienzle <pkienzle@users.sf.net>
+## Copyright (C) 2000 Laurent S. Mazet
+##
+## 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/>.
+
+## Compute chebyshev type I filter order and cutoff for the desired response
+## characteristics. Rp is the allowable decibels of ripple in the pass
+## band. Rs is the minimum attenuation in the stop band.
+##
+## [n, Wc] = cheb1ord(Wp, Ws, Rp, Rs)
+## Low pass (Wp<Ws) or high pass (Wp>Ws) filter design. Wp is the
+## pass band edge and Ws is the stop band edge. Frequencies are
+## normalized to [0,1], corresponding to the range [0,Fs/2].
+##
+## [n, Wc] = cheb1ord([Wp1, Wp2], [Ws1, Ws2], Rp, Rs)
+## Band pass (Ws1<Wp1<Wp2<Ws2) or band reject (Wp1<Ws1<Ws2<Wp2)
+## filter design. Wp gives the edges of the pass band, and Ws gives
+## the edges of the stop band.
+##
+## See also: cheby1
+
+function [n, Wc] = cheb1ord(Wp, Ws, Rp, Rs)
+
+ if nargin != 4
+ print_usage;
+ elseif length(Wp) != length(Ws)
+ error("cheb1ord: Wp and Ws must have the same length");
+ elseif length(Wp) != 1 && length(Wp) != 2
+ error("cheb1ord: Wp,Ws must have length 1 or 2");
+ elseif length(Wp) == 2 && ...
+ (all(Wp>Ws) || all(Ws>Wp) || diff(Wp)<=0 || diff(Ws)<=0)
+ error("cheb1ord: Wp(1)<Ws(1)<Ws(2)<Wp(2) or Ws(1)<Wp(1)<Wp(2)<Ws(2)");
+ end
+
+ T = 2;
+
+ ## returned frequency is the same as the input frequency
+ Wc = Wp;
+
+ ## warp the target frequencies according to the bilinear transform
+ Ws = (2/T)*tan(pi*Ws./T);
+ Wp = (2/T)*tan(pi*Wp./T);
+
+ if (Wp(1) < Ws(1))
+ ## low pass
+ if (length(Wp) == 1)
+ Wa = Ws/Wp;
+ else
+ ## band reject
+ error ("band reject is not implement yet.");
+ endif;
+ else
+ ## if high pass, reverse the sense of the test
+ if (length(Wp) == 1)
+ Wa = Wp/Ws;
+ else
+ ## band pass
+ Wa=(Ws.^2 - Wp(1)*Wp(2))./(Ws*(Wp(1)-Wp(2)));
+ endif;
+ endif;
+ Wa = min(abs(Wa));
+
+ ## compute minimum n which satisfies all band edge conditions
+ stop_atten = 10^(abs(Rs)/10);
+ pass_atten = 10^(abs(Rp)/10);
+ n = ceil(acosh(sqrt((stop_atten-1)/(pass_atten-1)))/acosh(Wa));
+
+endfunction
+
+%!demo
+%! Fs = 10000;
+%! [n, Wc] = cheb1ord (1000/(Fs/2), 1200/(Fs/2), 0.5, 29);
+%!
+%! subplot (221);
+%! axis ([ 0, 1500, -1, 0]);
+%! title("Pass band Wp=1000 Rp=0.5");
+%! xlabel("Frequency (Hz)");
+%! ylabel("Attenuation (dB)");
+%! grid;
+%! plot ([0, 1000, 1000, 0, 0], [0, 0, -0.5, -0.5, 0], ";;");
+%! hold on;
+%! [b, a] = cheby1 (n, 0.5, Wc);
+%! [h, w] = freqz (b, a, [], Fs);
+%! plot (w, 20*log10(abs(h)), ";;");
+%! hold off;
+%!
+%! subplot (222);
+%! axis ([ 0, Fs/2, -250, 0]);
+%! title("Stop band Ws=1200 Rs=29");
+%! xlabel("Frequency (Hz)");
+%! ylabel("Attenuation (dB)");
+%! grid;
+%! plot ([1200, Fs/2, Fs/2, 1200, 1200], [-29, -29, -500, -500, -29], ";;");
+%! hold on;
+%! [b, a] = cheby1 (n, 0.5, Wc);
+%! [h, w] = freqz (b, a, [], Fs);
+%! plot (w, 20*log10(abs(h)), ";;");
+%! hold off;
+%!
+%! subplot (223);
+%! axis ([ 990, 1010, -0.6, -0.4]);
+%! title("Pass band detail Wp=1000 Rp=0.5");
+%! xlabel("Frequency (Hz)");
+%! ylabel("Attenuation (dB)");
+%! grid;
+%! plot ([0, 1000, 1000, 0, 0], [0, 0, -0.5, -0.5, 0], ";;");
+%! hold on;
+%! [b, a] = cheby1 (n, 0.5, Wc);
+%! [h, w] = freqz (b, a, [990:1010], Fs);
+%! plot (w, 20*log10(abs(h)), ";filter n;");
+%! [b, a] = cheby1 (n-1, 0.5, Wc);
+%! [h, w] = freqz (b, a, [990:1010], Fs);
+%! plot (w, 20*log10(abs(h)), ";filter n-1;");
+%! [b, a] = cheby1 (n+1, 0.5, Wc);
+%! [h, w] = freqz (b, a, [990:1010], Fs);
+%! plot (w, 20*log10(abs(h)), ";filter n+1;");
+%! hold off;
+%!
+%! subplot (224);
+%! axis ([ 1190, 1210, -40, -20]);
+%! title("Stop band detail Wp=1200 Rp=29");
+%! xlabel("Frequency (Hz)");
+%! ylabel("Attenuation (dB)");
+%! grid;
+%! plot ([1200, Fs/2, Fs/2, 1200, 1200], [-29, -29, -500, -500, -29], ";;");
+%! hold on;
+%! [b, a] = cheby1 (n, 0.5, Wc);
+%! [h, w] = freqz (b, a, [1190:1210], Fs);
+%! plot (w, 20*log10(abs(h)), ";filter n;");
+%! [b, a] = cheby1 (n-1, 0.5, Wc);
+%! [h, w] = freqz (b, a, [1190:1210], Fs);
+%! plot (w, 20*log10(abs(h)), ";filter n-1;");
+%! [b, a] = cheby1 (n+1, 0.5, Wc);
+%! [h, w] = freqz (b, a, [1190:1210], Fs);
+%! plot (w, 20*log10(abs(h)), ";filter n+1;");
+%! hold off;
git://git.creatis.insa-lyon.fr / CreaPhase.git / blobdiff