Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

USE OF SUBCARRIER FREQUENCY MODULATIONIN COMMUNICATION SYSTEMS*!ByWarren H. Bliss!RCA Communications, Inc., Terminal Facilities Research Group,New York, N. Y.Summary— When subcarrier frequency modulation having a frequencyrange of 1600 to 2000 cycles was used for transoceanic facsimile transmission,pictures were obtained with finer detail and better half-tone qualitythan those transmitted by previous systems; the speed of transmission couldalso be increased. An extension of the system to a two-way multiplechannelradio-relay circuit providing teletype service between New Yorkand Philadelphia gave improved stability of operation when variations insignal strength occurred.GeneralA S THE art of frequency modulation has developed, ithas natu-/=\ rally produced many by-products and has found applicationsA- -^- and uses not at first apparent to radio engineers. One of theseis in the field of point-to-point communication where an applicationknown as subcarrier frequency modulation has been investigated andfound to have important advantages. When a subcarrier wave is frequency-modulatedby signal material and is then used to modulate aprimary carrier, the system is referred to as subcarrier frequencymodulation.Two applications of subcarrier frequency modulation which arehighly successful or show definite promise are (a) the transmission offacsimile material over long radio circuits and (b) the transmissionof telegraph characters (printer-type signals in particular) over shortwaveradio-relay circuits. The former has been used by RCA Communications,Inc., for over two years with results far surpassing thoseof any other method. 1Experimental tests recently conducted with thelatter prove there is a definite advantage in stabilizing teletype signallevels when the different tones from multiple-channel operation arecombined together tofrequency modulate a common subcarrier wave.In both of these cases the major advantage is due to the great reduction* Decimal Classification : R581 X R630.f Reprinted from Proc. I.R.E., August, 1943.| Now with the Research Department, RCA Laboratories Division,Princeton, N. J.1R. E. Mathes and J. N. Whitaker, "Radio Facsimile by SubcarrierFrequency Modulation", RGA REVIEW, Vol. 4, pp. 131-154; October, 1939.410