Use of Subcarrier Frequency Modulation in Communication Systems
W. H. Bliss · Proceedings of the IRE · 1943
When subcarrier frequency modulation having a frequency range of 1600 to 2000 cycles was used for transoceanic facsimile transmission, pictures were obtained with finer detail and better half-tone quality than those transmitted by previous systems; the speed of transmission could also be increased. An extension of the system to a two-way multiple-channel radio-relay circuit providing teletype service between New York and Philadelphia gave improved stability of operation when variations in signal strength occurred.