# What is the significance of first zero upcrossing of a cross correlation?

What is the significance of the first zero up-crossing of a cross-correlation? The signals being correlated are cosines signals with some phase difference.

EDIT 1 : Cross-correlation plot attached. The signals are cosines with phase difference and may be with some low frequency components.

Sampling frequency of the measured signal is taken as 50Hz

Cross correlation obtained using:

Cxx=fftshift(ifft(fft(x,N).*conj(fft(y,N))))/(norm(x) * norm(y));

• I'm not quite sure what you mean by the "first zero upcrossing" of the cross correlation. Can you post a picture? – Jim Clay Jun 12 '13 at 2:20
• Well, the first peak occurs at the lag at which the wave constructively interferes with itself, and the first negative peak occurs at the lag at which it destructively interferes with itself, so the first zero crossing is... the midpoint between those? – endolith Jun 13 '13 at 17:30
• @endolith A minor emendation: zero crossings occurs between positive and negative peaks, but don't need to occur at midpoints between the two. – Dilip Sarwate Jul 4 '13 at 18:24