how to calculate time lag in two signal using cross correlation

hi all,i m facing some problem with finding the time lag between two signals. i imported the data and used the cross correlation function.it gives 74 but according to my calculations it should be 32. i don't understand where i m wrong. please chk it out

t=end100(:,1);
>> ux228=start228(:,2);
>> ux100=end100(:,2);
>> Y=xcorr(ux228,ux100);
>> plot(Y);
>> find(Y==max(Y))

ans =

      74

>> N=length(t);
>> R=Y(1:N);
>> Rx=fliplr(R);
>> Plot (Rx);

Answer 1

[Wayne King]

You have to keep in mind:

1.) xcorr is returning negative lags as well as positive, but for real-valued inputs the cross-correlation sequence is even

2.) you can return the lags argument

Create an example with a lag of 10 between the two vectors.

 x = randn(100,1);
 y = [zeros(10,1); x];
 % xcorr returns 2*length of the longest vector-1 lags by default
 % therefore lag 110 is the zero-th lag
 [xc,lags] = xcorr(y,x);
 xc = xc(110:end);
 lags = lags(110:end);
 [val,index] = max(abs(xc));
 lags(index)

The above gives a lag of 10, which is the correct lag.

Also see the examples and how tos for estimating delay on this page:

< < https://www.mathworks.com/help/signal/convolution-and-correlation.html>