/usr/share/octave/packages/signal-1.3.0/buttord.m is in octave-signal 1.3.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 | ## Copyright (C) 1999 Paul Kienzle <pkienzle@users.sf.net>
##
## 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/>.
## -*- texinfo -*-
## @deftypefn {Function File} {[@var{n}, @var{Wc}] =} buttord (@var{Wp}, @var{Ws}, @var{Rp}, @var{Rs})
## @deftypefnx {Function File} {[@var{n}, @var{Wc}] =} buttord ([@var{Wp1}, @var{Wp2}], [@var{Ws1}, @var{Ws2}], @var{Rp}, @var{Rs})
## Compute butterworth 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] = buttord(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] = buttord([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.
##
## Theory: |H(W)|^2 = 1/[1+(W/Wc)^(2N)] = 10^(-R/10)
## With some algebra, you can solve simultaneously for Wc and N given
## Ws,Rs and Wp,Rp. For high pass filters, subtracting the band edges
## from Fs/2, performing the test, and swapping the resulting Wc back
## works beautifully. For bandpass and bandstop filters this process
## significantly overdesigns. Artificially dividing N by 2 in this case
## helps a lot, but it still overdesigns.
##
## @seealso{butter}
## @end deftypefn
function [n, Wc] = buttord(Wp, Ws, Rp, Rs)
if nargin != 4
print_usage;
elseif length(Wp) != length(Ws)
error("buttord: Wp and Ws must have the same length");
elseif length(Wp) != 1 && length(Wp) != 2
error("buttord: 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("buttord: Wp(1)<Ws(1)<Ws(2)<Wp(2) or Ws(1)<Wp(1)<Wp(2)<Ws(2)");
endif
if length(Wp) == 2
warning("buttord: seems to overdesign bandpass and bandreject filters");
endif
T = 2;
## if high pass, reverse the sense of the test
stop = find(Wp > Ws);
Wp(stop) = 1-Wp(stop); # stop will be at most length 1, so no need to
Ws(stop) = 1-Ws(stop); # subtract from ones(1,length(stop))
## warp the target frequencies according to the bilinear transform
Ws = (2/T)*tan(pi*Ws./T);
Wp = (2/T)*tan(pi*Wp./T);
## compute minimum n which satisfies all band edge conditions
## the factor 1/length(Wp) is an artificial correction for the
## band pass/stop case, which otherwise significantly overdesigns.
qs = log(10^(Rs/10) - 1);
qp = log(10^(Rp/10) - 1);
n = ceil(max(0.5*(qs - qp)./log(Ws./Wp))/length(Wp));
## compute -3dB cutoff given Wp, Rp and n
Wc = exp(log(Wp) - qp/2/n);
## unwarp the returned frequency
Wc = atan(T/2*Wc)*T/pi;
## if high pass, reverse the sense of the test
Wc(stop) = 1-Wc(stop);
endfunction
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