Disclosed is a system for receiving and detecting propagated mechanical-wave analog signals in which the analog signals are converted to a digital signal wherein pulse spacing is proportional to the corresponding instantaneous frequency of the detected mechanical wave signals. A digital processor compares the pulse-spacing of successive pulses of the incoming digital signal with selected pulse-spacing limits corresponding to the frequency limits of the selected bandwidth, and an output signal is generated indicating the receipt of analog signals within the selected frequency band whenever all of the successive pulses during a sampling interval lie within the selected pulse-spacing limits. Generation of an output signal is inhibited when any pair of successive pulses has a pulse spacing outside of the pulsespacing limits during the sampling interval.