Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

SUBCARRIER IN COMMUNICATIONS 411in signal-amplitude variation rather than to any increase in signal-tonoiseratio resulting from the frequency-modulation method.Radiophoto PracticeAll facsimile systems use a scanning device at the sending terminalfor converting graphic material intovoltage.a variable amplitude current orIn the subcarrier-frequency-modulation process this variablequantity representing the point-by-point density of the copy is used toswing or shift a subcarrier tone over a definite predetermined range.This constant-amplitude, frequency-modulated tone is then sent out bymeans of a standard amplitude-modulation radiotelephone transmitter.At the distant terminal all amplitude variations resulting from fadingand multipath effects are removed by means of a limiter, and a simplefrequency-discriminator circuit converts the received signals back tothe conventional varying amplitude wave. The copy is then recordedphotographically, or by other means.Since May, 1939, when subcarrier frequency modulation firstwentinto public service between London and New York, additional terminalsof this type have been put into commercial use.Berlin did and BuenosAires does provide this service now and just recently transmittingfacilities have been put into operation in Moscow.Figure 1 shows oneof the first photographs sent by radio from Moscow over a 5000-milecircuit.The two outstanding advantages of subcarrier frequency modulationover dot-pattern and other systems are better quality of receivedcopy and increased speed. Much finer picture detail can be reproducedand contrast in the darker half tones is better. Eight- and ten-pointtype are now transmitted successfully whereas fourteen-point type formerlywas the lower limit in size. Streaks and irregularities due tonoise are not so noticeable in the radiophotos.Because of this betterquality and freedom from a screen, the radio pictures can be handledthe same as conventional photographs by publishers. An eight- by teninchpicture can be transmitter in twenty minutes under radio-circuitconditions which three years ago would require a period of one hour.Improved operating technique and advancements in radio communicationindicate that the speed may be increased still more.The primary reason for this great improvement is due to the factthat subcarrier frequency modulation offers the equivalent of a linearamplitude-variation system which functions independently of signallevelfluctuations caused by fading and multipath phenomena whichalways occur on long-distance radio circuits.Since the picture intelligenceis conveyed by frequency modulation of the subcarrier tone,