1. The method of filtering a signal represented by a first series of indicia wherein each indicium of the series represents the amplitude comprising the steps of serially averaging the amplitudes of the indicia of sets of indicia of the first series wherein each set includes a given number n of indicia, n being a positive integer less than the number of indicia in the first series of indicia, to form a second series of indicia and serially amplitude comparing each subsequently occurring indicium of the second series with a previously occurring indicium thereof to form a third series of indicia comprising those indicia which result in comparisons having an amplitude difference greater than a predetermined amount.