Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

DUPLEX SOUND AND FACSIMILE 403ity of listeners. In the case of modulation by two different types ofprogram, however, non-linearity anywhere in the system results incombination tones or cross modulation. The cross modulation of oneprogram by the other has the characteristics of interference which isreadily noticed by the listener. An interfering program 30 db belowthe desired program is easily apparent, whereas distortion of a singleprogram 30 db below (approximately 3 per cent of) the fundamental,is seldom noticed.Consequently, duplex frequency-modulation transmission of soundand facsimile requires a somewhat greater bandwidth than a sound.zo60-KC DEV.2500 rvJLl50 8060-KC. DEY. SSOO^y PLUS15-KC. DEV. 20,00O~20 50 60 70KILOCYCLESFig. 6—Sideband amplitude.80I I Iprogram alone of comparable deviation, particularly for low modulatingfrequencies, and imposes much more stringent requirements on systemlinearity.4. Duplex EeceiverThe receiver for duplex transmission of sound and facsimile, inaddition to the requirement of exceedingly linear circuits, must beprovided with filters for separating the two programs following thedetector.A block diagram of a receiver constructed for tests of duplex transmissionis shown in Figure 7. Two selective circuits were used aheadof the converter and an i.f. of 8.25 Mc was employed to minimize likelihoodof spurious responses. The i-f passband was made somewhatwider than normal to guard against non-linearity from that source.