Abstract: A detecting apparatus for tuning musical instruments receives an input signal from a sound transducer and removes second harmonic content therefrom by means of a filter responsive to a note and octave selection. A phase lock pulse generator receives the filtered output and generates a signal in step with the input sound signal. The generated signal is counted by a note counter during a gating period derived by counting a predetermined number of output cycles of an oscillator started in step with the input signal. Three outputs indicating "on-frequency", "sharp" or "flat" are supplied in the alternative at the end of the gating period in accordance with the count in the note counter at that time.