Frequency-modulated carrier telegraph system

Voice-frequency carrier-current telegraph systems used in this country and abroad have until now employed amplitude modulation, analogous to single current signalling in d-c telegraphy. A variety of so-called two-tone telegraph systems has been tried out by various workers, but none of these was adopted on a commercial scale because all required the employment of at least double the frequency spectrum space ordinarily assigned for amplitude modulation. The system described employs true frequency modulation to derive the advantages of polar current signalling, with the same spectrum efficiency as conventional amplitude systems, and secures at the same time freedom from attenuation change in the transmission medium and greater immunity to extraneous disturbing currents.