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cceps.m

## Copyright (C) 1994 Dept of Probability Theory and Statistics TU Wien
##
## 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 2 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:  cceps (x [, correct])
##
## Returns the complex cepstrum of the vector x.
## If the optional argument correct has the value 1, a correction
## method is applied.  The default is not to do this.

## Author: Andreas Weingessel <Andreas.Weingessel@ci.tuwien.ac.at>
## Apr 1, 1994
## Last modifified by AW on Nov 8, 1994
  
function cep = cceps (x, c)

  if (nargin == 1)
    c = 0;
  elseif (nargin != 2)
    error ("usage: cceps (x [, correct])");
  endif

  [nr, nc] = size (x);
  if (nc != 1)
    if (nr == 1)
      x = x';
      nr = nc;
    else
      error ("cceps: x must be a vector");
    endif
  endif

  bad_signal_message = ["cceps:  bad signal x, ", ...
      "some Fourier coefficients are zero."];
  
  F = fft (x);
  if (min (abs (F)) == 0)
    error (bad_signal_message);
  endif

  # determine if correction necessary
  half = fix (nr / 2);
  cor = 0;
  if (2 * half == nr)
    cor = (c && (real (F (half + 1)) < 0));
    if (cor)
      F = fft (x(1:nr-1))
      if (min (abs (F)) == 0)
      error (bad_signal_message);
      endif
    endif
  endif

  cep = fftshift (ifft (log (F)));

  # make result real
  if (c)
    cep = real (cep);
    if (cor)      
      # make cepstrum of same length as input vector
      cep (nr) = 0;
    endif
  endif

endfunction

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