Mainfrequency finds the main frequency (and its standard deviation) of a signal y, sampled in a interval x (of time) with Fs as sampling rate. If you don't know the sampling rate, set Fs equal to zero (if so, x must be in seconds). TDx is the flag of x, and it can be "Second" or "Data". Also it returns the plots "amplitude vs. time" & "magnitude vs. frequency". If there are two or more peaks with the same height, it chooses the lower one in frequency. Finally it returns the total power P of the signal.