autocorrelation

[Francesco Potorti`]

What is the reason why the autocorrelation (autocor.m) is computed with
a for loop rather than using fft?

This is a function that does it for a single column vector.  For
matrices, one could iterate over the vectors.  For long vectors (my ones
are 200 000 samples long each) this is hundreds time faster.

===File ~/math/octavelib/signal/autocorr.m==================
function A = autocorr(x)

# Given a vector X, computes a vector of the same shape, whose length
# is one less than the argument, representing the autocorrelation of X.
# The autocorrelation is simply the normalised autocovariance.

  A = x - mean(x);              # detrend
  A = fftconv(A, rot90(A,2));   # fft
  A = A(length(x) : 2*length(x)-1); # one-side fft
  A /= length(x);               # this is the autocovariance
  A /= A(1);                    # this is the autocorrelation
endfunction
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