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## Re: autocorrelation

**From**: |
Vinayak Dutt |

**Subject**: |
Re: autocorrelation |

**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: address@hidden
* address@hidden
* address@hidden
*
*/
#include "disclaimer.h"