Patent US 3,947,633

In a data transmission system which uses frequency-differential phase modulation including a composite signal comprising a plurality of carriers apparatus is described for receiving and demodulating the aforementioned signal. In the receiver the signal, so modulated, is coupled to a plurality of correlators which produce d.c. voltages having values indicative of the phase states of the carriers. Binary words, similarly indicative of the phase states, are derived from the d.c. voltages, and these are stored in consecutive form in binary stores. The differences between the binary words yield the transmitted data. The outputs of the aforementioned correlators are connected to inputs of a time multiplexer which acts to connect outputs of two correlators to two multiplexer outputs. The outputs of the time multiplexer are connected to a phase modulator which produces a phase modulated signal responsive to the time multiplexer output signals. The zero crossovers of the phase modulated signal are indicative of the phase states of the carriers. A phase-sensitive analog-digital converter derives from the aforementioned zero crossovers binary words which indicate the phase states of the carriers.