Multiple Frequency Shift Teletype Systems

Radio teletype transmission is of such a nature that once the signal goes far enough into noise, reliable reception rapidly deteriorates. This paper calls attention to a system for radio teletype communication that has advantages that have been demonstrated both theoretically and experimentally. A new system for teletype communication is described that involves the use of several frequencies instead of the two, called "mark and space," used in convention frequency shift keying (FSK) operation. By so doing, longer pulses are allowed for equivalent transmission rates, and as a result smaller detection bandwidths are possible. The main advantage is a greater correlation between received signal plus noise and the transmitted signal. Probability of error from information theory is used to find the theoretical gain of two multiple frequency shift (MFS) systems over the conventional FSK system. Coding, filters, and special circuitry are described. An experimental comparison of one MFS system and conventional FSK operation is presented on the basis of error counts using both white and impulse noise superimposed on the signal. Theoretical gains are checked by the laboratory experimental work.