--- /dev/null
+## Copyright (C) 2002 André Carezia <acarezia@uol.com.br>
+##
+## 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/>.
+
+## Usage: chebwin (L, at)
+##
+## Returns the filter coefficients of the L-point Dolph-Chebyshev window
+## with at dB of attenuation in the stop-band of the corresponding
+## Fourier transform.
+##
+## For the definition of the Chebyshev window, see
+##
+## * Peter Lynch, "The Dolph-Chebyshev Window: A Simple Optimal Filter",
+## Monthly Weather Review, Vol. 125, pp. 655-660, April 1997.
+## (http://www.maths.tcd.ie/~plynch/Publications/Dolph.pdf)
+##
+## * C. Dolph, "A current distribution for broadside arrays which
+## optimizes the relationship between beam width and side-lobe level",
+## Proc. IEEE, 34, pp. 335-348.
+##
+## The window is described in frequency domain by the expression:
+##
+## Cheb(L-1, beta * cos(pi * k/L))
+## W(k) = -------------------------------
+## Cheb(L-1, beta)
+##
+## with
+##
+## beta = cosh(1/(L-1) * acosh(10^(at/20))
+##
+## and Cheb(m,x) denoting the m-th order Chebyshev polynomial calculated
+## at the point x.
+##
+## Note that the denominator in W(k) above is not computed, and after
+## the inverse Fourier transform the window is scaled by making its
+## maximum value unitary.
+##
+## See also: kaiser
+
+function w = chebwin (L, at)
+
+ if (nargin != 2)
+ print_usage;
+ elseif !(isscalar (L) && (L == round(L)) && (L > 0))
+ error ("chebwin: L has to be a positive integer");
+ elseif !(isscalar (at) && (at == real (at)))
+ error ("chebwin: at has to be a real scalar");
+ endif
+
+ if (L == 1)
+ w = 1;
+ else
+ # beta calculation
+ gamma = 10^(-at/20);
+ beta = cosh(1/(L-1) * acosh(1/gamma));
+ # freq. scale
+ k = (0:L-1);
+ x = beta*cos(pi*k/L);
+ # Chebyshev window (freq. domain)
+ p = cheb(L-1, x);
+ # inverse Fourier transform
+ if (rem(L,2))
+ w = real(fft(p));
+ M = (L+1)/2;
+ w = w(1:M)/w(1);
+ w = [w(M:-1:2) w]';
+ else
+ # half-sample delay (even order)
+ p = p.*exp(j*pi/L * (0:L-1));
+ w = real(fft(p));
+ M = L/2+1;
+ w = w/w(2);
+ w = [w(M:-1:2) w(2:M)]';
+ endif
+ endif
+
+ w = w ./ max (w (:));
+endfunction