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## Copyright (C) 1996 John W. Eaton
##
## This file is part of Octave.
##
## Octave 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 2, or (at your option)
## any later version.
##
## Octave 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 Octave; see the file COPYING. If not, write to the Free
## Software Foundation, 59 Temple Place - Suite 330, Boston, MA
## 02111-1307, USA.
## Compute the frequency response of a filter.
##
## [h,w] = resp(b)
## returns the complex frequency response h of the FIR filter with
## coefficients b. The response is evaluated at 512 angular frequencies
## between 0 and pi. w is a vector containing the 512 frequencies.
##
## [h,w] = resp(b,a)
## returns the complex frequency response of the rational IIR filter
## whose numerator has coefficients b and denominator coefficients a.
##
## [h,w] = resp(b,a,n)
## returns the response evaluated at n angular frequencies. For fastest
## computation n should factor into a small number of small primes.
##
## [h,w] = freqz(b,a,n,"whole")
## evaluates the response at n frequencies between 0 and 2*pi.
## Author: jwe
function [h, w] = freqz(b,...)
if (nargin == 1)
## Response of an FIR filter.
a = 1;
n = 512;
region = "half";
elseif (nargin == 2)
## Response of an IIR filter
a = va_arg();
n = 512;
region = "half";
elseif (nargin == 3)
a = va_arg();
n = va_arg();
region = "half";
elseif (nargin == 4)
a = va_arg();
n = va_arg();
region = va_arg();
endif
la = length(a);
a = reshape(a,1,la);
lb = length(b);
b = reshape(b,1,lb);
k = max([la, lb]);
if( n >= k)
if (strcmp(region,"whole"))
h = fft(postpad(b,n)) ./ fft(postpad(a,n));
w = 2*pi*[0:(n-1)]/n;
else
h = fft(postpad(b,2*n)) ./ fft(postpad(a,2*n));
h = h(1:n);
w = pi*[0:(n-1)]/n;
endif
else
if (strcmp(region,"whole"))
w = 2*pi*[0:(n-1)]/n;
else
w = pi*[0:(n-1)]/n;
endif
h = polyval(postpad(b,k),exp(j*w)) ./ polyval(postpad(a,k),exp(j*w));
endif
endfunction
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