Patent US 4,156,797

A filter for reducing the effective keying passband of a VOX (voice operated relay) responsive to output signals comprising pulses which follow the peak frequency pattern of the audio input pulses includes a pair of retriggerable monostable multivibrators, a "D" type flip-flop circuit, and a NOR gate which produces positive output pulses used to trigger ON the voice operated transmitter only when the input signals occur within the specified passband.