Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

408 FREQUENCY MODULATION, Volume Iresultant components, especially 1000 cycles, which was the most prominentin listening. Even with the receiver so tuned, the output to theear was readily evident as being different from pure 4000 cycles.The results of these laboratory tests confirm the theoretical considerationsthat two tones applied to a system having any non-linearitywhatsoever will result in combination frequencies exceeding in amplitudethe harmonics which occur with a single modulating tone.Whenthese two tones differ widely in frequency, the low modulating frequencies,while producing greatest side-band extent, produce combinationtones outside the audible range, hence are lessdisturbing to thesound program. The higher-frequency audio tones, combining withthe facsimile frequencies, however, do produce serious interference inthe sound channel.6. Field TestsA series of tests was run on the system of duplex sound and facsimileon W2XWG (W51NY) frequency-modulation transmitter locatedon the Empire State Building in New York City during January1941. The receiver used in the tests was the one described above andwas tried at three different locations in Radio City, about one milefrom the transmitter.The facsimile and sound programs were applied in parallel to thegrid of the reactance tube modulator of the f-m transmitter and wereindependently monitored as to deviation. The response of the facsimilesystem was flat from scanner to transmitter within 0.5 db up to 24 kc.With careful tuning of the receiver and limitation of the deviationto 60 kc for the sound and 15 kc for the facsimile, substantial freedomfrom interference between the two was possible.Even with these conditions, a sound program with prominentsibilants produced a short skip (white space) in the facsimile copyfor each "s" sound, and during pauses in sound programs the facsimilecould be heard in the background.If the receiver was not precisely tuned, little increase in interferencewith facsimile was apparent, but the facsimile backgroundbecame markedly evident in the sound reproduction.During the progress of the test one practical operating factor wasnoticed that illustrated the interdependence of sound and facsimile induplex transmission.There was a tendency for unwarned individualsto tune to a different sound program for check purposes while receivingfacsimile. This caused the facsimile to drop out of synchronism whichinchhad to be regained upon returning to W2XWG, so that about y±of copy was lost for even momentary tuning to a different station.7. ConclusionsThe laboratory and field tests indicated that the frequency-shiftfacsimile method was capable of producing good copy and could be