Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

112 FREQUENCY MODULATION, Volume Iover 100 u-h-f transmitting stations and it seemed that a trend wasdeveloping toward u-h-f broadcasting, but this trend was not sustained.Interest has been revived in recent months through wide-spread discussionof the advantages of frequency modulation on the ultra highfrequencies.Amplitude-modulated u-h-f stations can provide greater coveragethan standard broadcasting shared-channel stations limited by nighttimeinterference from distant stations operating on the same channel.This interference usually causes such a station's useful service areato shrink to a small fraction of its daytime area and many of thesestations are, in addition, required to reduce power at night to minimizesimilar interference to other stations. Few such stations are free fromnight-time interference within their 2000 microvolt contour and manyare limited to 8000 microvolts. The ultra high frequencies offer anescape from such limitations by virtue of practical freedom from staticand shared-channel interference. Frequency modulation can providea much greater degree of improvement than can amplitude modulationon these ultra high frequencies.Purpose of F-M Field TestThe National Broadcasting Company has been one of the groupswhich viewed realistically the possibilities of the ultra high frequenciesand has pioneered in conducting experimental operations inthat fieldfor many years. It has been NBC's belief that at some time in thefuture thesa ultra high frequencies would come into much widor use.It is the policy of NBC to investigate every technical developmentaffecting its field and apply it where possible to provide better publicservice.Toward that end 18 months ago NBC formulated plans for anexacting field test of frequency modulation similar to the field testof television which preceded the dedication of that service on April 30,1939. The purposes of the field tests were to quantitatively evaluatethe advantages of frequency modulation over amplitude modulation onthe ultra high frequencies, not by confined laboratory measurementswhich had already been made by others, nor merely by operating afrequency-modulated station, but by painstakingly measuring and comparingthe hvo systems under allkinds of actual service conditions inthe field and determining how much of the theoretical advantage ofFM could be obtained in practice.The impression has been gained by some, that only by tne use ofFM may the public now enjoy high fidelity sound reproduction in thehome. With ultra high frequencies, the same fidelity can be providedwith either amplitude or frequency modulation. Improved fidelity is