Abstract: Apparatus for extracting the fundamental frequency from a complex audio wave form and for producing a pulse representation of said fundamental frequency includes a low pass filter having a frequency response characteristic which tends to exclude harmonics together with a control circuit including amplifier means, peak detector means and voltage to current converter means for supplying a control signal to the low pass filter thereby to vary the cutoff frequency of the filter and to maintain substantially constant amplitude of the fundmental wave form together with voltage comparator means for comparing the peaks of an alternating current quantity dependent upon the output of said filter with a direct current quantity proportional to the output of the filter, said comparator means thereby producing a pulse per cycle of said fundamental frequency.