Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

DUPLEX TRANSMISSION OF FREQUENCY-MODULATED SOUND AND FACSIMILE*!ByMaurice ArtztRCA Manufacturing Company, Inc., Camden, N. J.and Dudley E. FosterRCA License Laboratory, New YorkSummary— Laboratory and field tests of multiplexing aural and facsimileprograms on a frequency-modulated wave have been made. Theresults obtained show that while technically possible, difficulties are encounteredthat will probably make such operation undesirable. Enough ofthe theory of multitone modulation is discussed to show that circuitlinearity requirements are severe, and that the side bands can easily exceedthe 200-kilocycle channel width if the aural deviation is not reduced.Quantitative data are given on the types and percentages of cross-talkobtained."S 3.228 Facsimile Broadcasting and Multiplex Transmis-The Commission may grant authority to a high-frequency broad-Tsion.1. Introductiony }f ^HE F.C.C. rule defining requirements of high-frequency, auraland facsimile broadcasting stations is as follows:cast station for the multiplex transmission of facsimile and auralbroadcast programs provided the facsimile transmission isincidentalto the aural broadcast and does not reduce the quality of or the frequencyswing required for the transmission of the aural program.frequency swing for the modulation of theTheaural program should beNomaintained at 75 kilocycles and the facsimile signal added thereto.transmission outside the authorized band of 200 kilocycles shall resultfrom such multiplex operation nor shall interference be caused to otherstations operating on adjacent channels."Little practical experience has yet been obtained with multiplexoperation of the kind called for in this regulation. For this reason ithas seemed worthwhile to report the resultsof some laboratory andfield tests carried out by KCA in Camden and New York. These testswere made with the object of determining first, if any limitations andpracticaldifficulties were likely to be encountered, and second, to whatextent the introduction of multiplex facsimile signals might effectdesign of receivers for aural service only.It is generally conceded that in such a multiplex system the signalto-noiselevelthein the aural program should be favored as much as possible.In order to maintain and protect adequately the high signalto-noiseratio of the aural service, the facsimile signal must use a much* Decimal Classification : R581X R630f Reprinted from RCA REVIEW, July, 1941.396