A recent development in communication technique

The paper provides a short introduction to a new system of communication. An attempt is made to show how the system was evolved as a logical development from earlier systems, and to suggest how it may find useful practical application. A short review of the growth of technique and theory of communication demonstrates the continued search for new systems of high efficiency and leads inevitably to the conception that the ultimate theoretical efficiency of utilization of frequency bandwidth has now been established by Shannon. The view is taken, however, that practical systems still need to be developed in order to approach more nearly to ideal performance. A discussion of features of pulse-code modulation (p.c.m.)-the most efficient system in current use-shows how suspected deficiencies have led to suggestions for a new system of communication. Examination of this system, tentatively called the "ambiguous index system," demonstrates that the suspected deficiencies were not, in fact, very real and that the purely theoretical performance of p.c.m. has not been improved upon. However, certain practical advantages are revealed, and it is suggested that one variation of the new system, which is very nearly equivalent to the theoretical 2-digit multiple-level p.c.m. system, could be of great value when insufficient frequency bandwidth is available for binary coding by as many as 6 digits. It is shown that the new system permits a new and satisfactory classification of the system commonly known as "pulsed f.m." which represents a special case of the ambiguous index system, which, in turn, is very closely related to signal coding systems. Hence, the very high theoretical performance of pulsed f.m. is not in any sense anomalous. A modification of the pulsed f.m. system probably represents an almost ideal system of transmission for the case where the bandwidth available for transmission is only about twice that which would be required for single-sideband working.