Sequential tone acoustic command link

DETAILED DESCRIPTION The receiver portion includes a transducer means which is responsive to the acoustically transmitted multi-tone code signal for providing a corresponding electrical multi-tone code signal serving as an input signal.
A phase lock loop circuit is provided and includes a voltage controlled oscillator (VCO) and a phase comparator which receives not only the input signal but the output signal of the VCO.
The phase comparator provides at least two output signals one being a control signal for governing the frequency of the VCO and another, a phase output signal indicative of the degree of lock between the input signal and the VCO output signal with the degree of lock being indicative of the signal-to-noise (S/N) ratio of the input signal.
A gating circuit, when enabled, is operable to pass the VCO output signal and a lock detector circuit will enable the gating circuit when the S/N ratio is above a predetermined level.
A multi-tone code decoder is responsive to the passed VCO output signal for providing control signals in accordance with the transmitted code signal to thereby govern certain operations of the remote underwater system.