From help-octave-request at che dot utexas dot edu  Thu Apr 27 15:55:44 1995
Subject: Re: autocorrelation
From: vdp at us0 dot mayo dot EDU (Vinayak Dutt)
To: help-octave at che dot utexas dot edu
Date: Thu, 27 Apr 95 10:54:10 CDT

Ted Harding wrote:

#There's nothing wrong with the answer from fftconv. This computes
#a CONVOLUTION, which is defined as
#
#  Sum[over s] x(s) * x(t-s)
#
#i.e. one of the factors is taken in the reverse order.
#The CORRELATION [at lag t = -(N-1), .. 2, 1, 0, 1, 2, .. , (N-1)]
#which you want is defined by
#
#  Sum[over s] x(s) * x(s-t) = R(t) = R(-t)
#
#You can get it the right way roud by reversing one of the arguments
#to fftconv, if you wish. This gives the result you expected.
#

Yup. This is the correct explaination for the problem. I checked fftconv()
and found that it does padd to  length(Vector1) + length(Vector1) - 1.
So there is no need to pad as I had earlier suggested :-(



--vinayak-
/*
 * vinayak dutt
 * graduate student, ultrasound research
 * mayo graduate school, rochester mn
 *
 * e-mail: vdp at mayo dot edu
 *         dutt dot vinayak at mayo dot edu
 *         vdp at us0 dot mayo dot edu
 *
 */
#include "disclaimer.h"