# cross-correlation of two signal always shows unexpected 0 lag

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YANAN ZHU on 20 Nov 2018
Commented: Honglei Chen on 20 Nov 2018
Hi,everyone, I got a very strange issue.
b is a signal of 251 samples. c is got by shifting b by 5 sample. (b and c are attached). But when I do xcorr on b and c, the max correlation always occurs at 0 lag. which I do not understand. Can someone help?
Below is my code. Thank you.
figure;
plot(b);hold on;
plot(c);
[r,lag] = xcorr(b,c,'coeff');
[m, i_max]=max(r);
shift=lag(i_max);
% plot(lag,r);
display(shift);

Honglei Chen on 20 Nov 2018
You may want to try
[r,lag] = xcorr(b-mean(b),c-mean(c),'coeff');
HTH
##### 2 CommentsShowHide 1 older comment
Honglei Chen on 20 Nov 2018
Since your goal is to look for similarity between two signals, I think in theory the cross correlation really requires both signal to be zero mean. Otherwise, the DC component will affect your result. For example, let's say two signals are uncorrelated and have all positive values. In theory, the correlation coefficient should be 0. However, without removing the means, the result will always be positive.
HTH

R2018a

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