Abstract: A digital underwater communication system includes a transmitter and a receiver. An oscillator bank in the transmitter generates a plurality of signals, each having a unique frequency. A timed switching matrix selects predetermined combinations of signals from this plurality and a gate passes only those signals within a selected combination that corresponds to the binary value of a bit to be transmitted. A timer commands the switching matrix to select different combinations at each bit time. The receiver contains a filter bank feeding a plurality of channels, each responsive to a different one of a plurality of signals generated in the oscillator bank. A receiver timer, synchronized with the transmitter timer, enables combinations of receiving channels corresponding to the combinations of signals selected by the transmitter switching matrix during that bit time.