xcorr returning a triangular shaped plot
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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
3 个评论
ChristianW
2013-3-4
Can you normalise a and b with T=(a-mean(a))/std(a) and show the plot? Alternatively give us some data.
Marek
2013-3-4
TJ
2013-5-24
Could you elaborate on what fixed your problem? How did you normalize your data and what did that do?
回答（1 个）
Image Analyst
2013-5-24
0 个投票
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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