xcorr returning a triangular shaped plot

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Marek
Marek 2013-3-4
I'm currently trying to find a the time delay between peaks in CO2 concentration and the peaks in intensity gained from and MRI scan. But so far when I use xcorr and plot the results I just get a triangular shaped peak at 0 lag which I know can't be right.
Has anyone any idea what I am doing wrong?
So far my code it pretty simple:
a=load(CO2 file)
b=load(MRI file)
T=a/norm(a)
Y=b/norm(b)
time=92 % There are 92 data point
figure 1
plot(time, T, time, Y)% Just to see what the normalised data looks like on the same graph
C=xcorr(a, b, 20, 'coeff') % Only interested in 20 lags either way
time2=-20:20
plot(time2, C) % Results in a triangular-shaped plot
http://tinypic.com/r/2v9utdk/6
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Marek
Marek 2013-3-4
Ahh no need, normalising the data in the way you described has solved the problem now and I'm getting more reasonable results. Thanks!!
TJ
TJ 2013-5-24
Could you elaborate on what fixed your problem? How did you normalize your data and what did that do?

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Image Analyst
Image Analyst 2013-5-24
Maybe you should use normalized cross correlation, done by normxcorr2() in the Image Processing Toolbox (IPT). IPT functions also work with 1D signals as well as 2D images and sometimes 3 dimensions or more.
Often non-periodic signals will correlate and give a roughly triangular, or mountain-shaped, result so what you're seeing is not uncommon. In fact the correlation of two uniform signals will give a perfect triangle - just consider how the area of overlap changes as two boxes slide past each other.

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