MATLAB interpretation of Xcorr2

Question

I have two vectors of matching lengths. They are readings from two different sensors (one is from a smartphone and the other is from a wiimote) of the same hand movement. I am trying to find the time offset between them to synchronise the readings for further processing. The readings I get are of the format (Time(ms) Value) for accelerations in the X,Y and Z direction.

For the synchronization, I plotted the cross-correlation function xcorr2() between the two sets. I am getting the same graph (a weird triangle peak and a straight line at the bottom) for Accelerations along the x, y and z directions (which I guess is good) but I don't know how to interpret it. What do the axes in the graph represent?

Can anyone explain to me what xcorr2() means in a qualitative sense. From the correlation function, how do I determine the offset (i.e. how many seconds is sensor1 behind sensor2)?

Answer 1

I concur with the comment made above by Predictor. To align the time series against each-other, I would pick xcorr() without the 2. Consider correlating only the acceleration magnitudes. For example:

a_mag_wii = sqrt(a_x_wii.^2 + a_y_wii.^2 + a_z_wii.^2);
a_mag_phone = sqrt(a_x_phone.^2 + a_y_phone.^2 + a_z_phone.^2);
res = xcorr(a_mag_wiimote, a_mag_smartphone);