How to use xcorr function in matlab as autocorr

Hello all,

I am trying to use the xcorr() function as an autocorrelation function in Matlab. But I am getting the wrong output. Can any one correct me if I am doing any mistake in the below procedure?

I am doing auto-correlation with the shift of 24(i.e. every samples is correlated with the 25th samples) using the function given below,

out= xcorr(x, 24);  //assume x is a complex variables.

Answer 1

[Greg Dionne]

out = xcorr(x,x(25:end))

Answer 2

[Image Analyst]

The two slashes need to be replaced by a percent symbol, which is a comment indicator in MATLAB.

To autocorrelate x, you need to pass in x twice:

out= xcorr(x, x)  % Assume x is a complex variables.

I don't know what you mean by a shift of 24. As you know, autocorrelation shifts one array by all possible shifts that result in an overlap of the signals, so basically the result is an array that is as long as the sum of the two lengths of the component vectors. If you want a shift of 24, simply pick out that element.

[Tamil selvan]

Sorry for confusing.

a = repmat((1:9).*(1+1i), 1, 10);
/************** AutoCorrelation Function ***********
for m=1:10;
    out1(m) = a(m) .* conj(a(m+24));
end
******************************************************

I tried to implement the above function using xcorr function. That's why I mentioned 24 shift the previous comment.

[Image Analyst]

Why is that "wrong"? It does what you told it to do. It's not an autocorrelation but it's doing what you tell it to.