Abstract: A non-ringing circuit for detecting the presence of predetermined frequency components in input signals and for delivering an output signal when a predetermined component is dominant, the circuit employing plural tuned filters with different adjacent cross-over frequencies, and using coincidence detectors connected to pairs of these filters in such a way as to detect when there exists a dominant frequency component having a frequency so located with respect to said cross-over frequencies that the relative phases of the signals fed to the coincidence detectors from the filters will produce phase coincidences therein, the circuit further using multiple different paired filters having staggered cross-over frequencies such that the tendencies of these filters to ring as a result of transient excitations will produce diverse and non-coincident ringing wave forms.

Abstract: A detector for detecting the presence of selected frequency components of an input signal and operative to deliver output responsive thereto by providing two separate signal paths in the detector, at least one path including filter circuitry having a distinct cross-over point operative to pass frequencies on one side of the cross-over point with one phase relative to the input and to pass frequencies on the other side of the cross-over point substantially 180.degree. out of phase relative to the input, and the detector further including a coincidence circuit receiving the signals from the two paths and generating one level of output for signals which are substantially in phase and generating another lower level of output for signals substantially out of phase. The coincidence circuit may have provisions for establishing an input threshold level which the signals must overcome.

Abstract: A portable underwater signalling apparatus for use by divers having self-contained underwater breathing apparatus (scuba) to signal to other divers as well as surface and underwater vessels. The apparatus utilizes compressed air from the diver's tanks to drive a hammer against a diaphragm to cause sound waves to be transmitted through the water.