Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

414 FREQUENCY MODULATION, Volume Ion the transmitting drum containing alternate black and white sectionsis first scanned. The scanner and modulator are then adjusted to produce2000 cycles and 1600 cycles for black and white, respectively. Atthe radio transmitter the percentage amplitude modulation is set at thedesired value and this remains constant since there is no amplitudevariation in the subcarrier frequency-modulated picture wave. At thereceiving terminal the recording galvanometer light valve or a similardevice isadjusted to the proper settings for the incoming black-andwhitecontrolled signals. Several pictures may then be transmittedwithout readjustment at either terminal.The value and success of the present system of transmitting picturesover long distances by radio is indicated by the large number ofsuch pictures which appear in our newspapers. The quality of thetransmitted photos isfrequently so good that they are indistinguishablefrom the regular news pictures.A means for partially overcoming some of the effectsof selectivefading has been outlined. 1 This consisted in using the second harmonicof the subcarrier frequency which was developed in the receivers whenthe radio-frequency carrier completely dropped out. The harmonicwave which also contains the picture modulation was switched into usewhenever the level of the normal frequencies fell below the usable point.Telegraph Channeling with SubcarrierFrequency ModulationFor several years there has been in operation between New Yorkand Philadelphia a two-way, multiple-channel, telegraph radio-relaycircuit consisting of three links. 2 There are eight narrow-band, voicefrequency,teletype channels and also some wide-band, higher-frequencychannels for facsimile. Each teletype channel is 100 cycles wide andthe channels are spaced 170 cycles apart beginning with 425 cycles forthe lowest frequency. The signal of each channel consists of its frequencykeyed on and off according to the message material and theoutputs of all eight channels are combined directly together throughfilters. The composite signal is then used to amplitude modulate theradio-frequency carrier, but in order to prevent overmodulation frompeaks due to random combination the signal levels in the individualchannels must be kept comparatively low.Although this system has been successful in operation, difficulty isexperienced from time to time because in this system of operation theteletype printers in narrow-band channels are sensitive to variations8 J. Ernest Smith, Fred H. Kroger, and R. W. George, "PracticalApplication of an Ultra-High-Frequency Radio-relay Circuit", Proc. I.R.E.,Vol. 26, pp. 1311-1327, November, 1938.