Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

DUPLEX SOUND AND FACSIMILE 409synchronized by a simultaneous amplitude modulation of the picturefrequency modulation. This resultant signal could also be used tofrequency-modulate a transmitter at the same time as a sound programto give duplex transmission. The required spectrum space forduplex operation is greatest for very low modulating frequencies, andfor such frequencies, is likely to exceed the channel width.The test indicated several practical difficulties which may be expectedin duplex sound and facsimile operation.1. Necessity for adequate filtering to separate signals beforepassing through any audio amplifier.This could be done on receiversdesigned for duplex operation, but would be necessary on all f-mreceivers whether designed for facsimile reproduction or not, if crossmodulation of the sound by the facsimile is to be avoided.Thus, receiversdesigned for sound reproduction only would require design provisionsnot now needed.2. Necessity for continuous uninterrupted reception of an undesiredsound program to obtain a desired facsimile program and viceversa.3. Necessity for tuning accuracy beyond the capabilities of theaverage user for acceptable freedom from cross-talk.4. Necessity for unusually high degree of oscillator frequencystability to maintain this tuning accuracy.5. Necessity for unusually high degree of linearity in any circuitcarrying both sound and facsimile signals.These considerations result in the opinion that duplex operation off-m sound and f-m facsimile is technically possible, but has certaindifficulties both technical and commercial which would appear to makeitundesirable.The two services have too many conflicting requirements in receiverdesignand sale,in program design and in user's desire to change one,but not the other, to be tied together thus intimately. It would appearmore desirable to operate with independent modulation of sound andfacsimile on a time division basis,the facsimile being transmitted ata period when the station is not in use for sound transmission.The authors are indebted to W. A. R. Brown and T. J. Buzalski ofthe National Broadcasting Company for furnishing and operating thetransmitter facilities for the field tests, and to W. L. Carlson, V. D.Landon, and H. Kihn of the RCA Manufacturing Company for a partof the laboratory tests described. Credit is also due J. A. Rankin ofthe RCA License Laboratories for able assistance in the field andlaboratory tests, and to M. G. Crosby of R.C.A. Communications forhis helpful discussions of the theoretical aspects of the problem.