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Wikipedia gives the cross-correlation as

$ \begin{align*} (f \star g)[n] = \sum^{\infty}_{m = -\infty} f^{*}[m] g[n+m] \end{align*} $

MATLAB's documentation gives xcorr(x, y) as

$ \begin{align*} R_{xy}[n] = E(x[n+m]y^{*}[m]) \end{align*} $ where $E$ is the evaluation function and I have switched $m$ and $n$ so it's consistent with Wikipedia's notation.

They're obviously opposite with respect to one another. If Wikipedia's definition is correct, why did MATLAB implement their function to be the opposite of the mathematical definition?

Am I missing something here? Because this difference just caused me a lot of pain while working on a project.

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    Yes, which tripped me up for a while. Just wondering if this is just an annoying "feature", or am I missing out on some critical reason for MATLAB's method of implementation.2012-04-29

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It's just a matter of notation - similar to the definition of the Fourier Series (does one include $2\pi$ ? ). All properties of the cross correlation stay the same.