I migrated from a FHT frequency meter for 150-4000Hz that has an error of less than 10 percent in the idea that I can measure it more precisely. The author said he can get even decimal but not specified as just for two hundred or three hundred Hz. Looks like I'm out of math. The formula " f square / fs" " is very weird for me and I still want to know where it comes from unless I ask too much. Do I understand that autocorrelation is convenient to implement in general only for low frequencies ? Anyway thanks for the information. For now I understand that I am in a direction that surpasses me and I return to FHT that even if I do not understand how it works, at least it gives very good results.