a method of telegraphy using signals in the subaudio-frequency range (0-100 hertz [Hz]) carried by telephone lines. It permits different telephone messages, which occupy a frequency band from 150 to 3,500 Hz, and telegraph signals, which occupy a frequency band from 0 to 87 Hz, to be transmitted simultaneously on one communication line without mutual interference.
A composite system was developed in the USSR in the mid-1920’s by P. A. Azbukin. In this system, telephone and telegraph circuits were separated at the terminals of the communication line by high-pass and low-pass electric filters. The system was used from 1926 to 1928 during the organization of telephone and telegraph communications linking Moscow with Leningrad, Kharkov, and other cities. In view of the complexity and bulk of the electric filters, composite telegraphy has not become widespread.