Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

412 FREQUENCY MODULATION, Volume Ivariations in amplitude can be completely removed by means of a limitingdevice. Since changes in amplitude as great as 70 decibels can beeliminated, signal fading can have very little influence on the reproducedcopy. Only rarely does the signal drop below the usable level.Although frequency modulation is capable of giving a definite gainin signal-to-noise ratio, this advantage increases with increasing deviationratio. In the present picture-transmission system this advantageis not fully used because a comparatively low deviation is utilized;actually, the bandwidth is no greater than that for amplitude modulation.Some time ago a series of tests was made in one of the research* •T-Fig. 1—One of the first test radiophotos sent from Moscow toNew York, via RCA Communications, Inc.laboratories of RCA Communications, Inc., to determine the bandwidthrequirement for picture transmission by subcarrier frequencymodulation.Various depths of modulation and bandwidths were triedand oscillograms and transmitted copy were compared and elosely examined.The comparative importance of various sideband components wasalso investigated.The results proved that if the index of modulation m, which is theratio of frequency deviation of the carrier to the modulating frequency,