cross-correlation of two signal always shows unexpected 0 lag

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.

load b.mat
load c.mat
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);

Answer 1

[Honglei Chen]

You may want to try

[r,lag] = xcorr(b-mean(b),c-mean(c),'coeff');

HTH

[YANAN ZHU]

Thank you so much! It works! Can I know what really happened under the hood after subtracting the mean?

[Honglei Chen]

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