Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

SUBCARR1ER IN COMMUNICATIONS 413did not exceed 0.47 for the highest important frequency fm from thepicture material, then the total bandwidth needed is equal to 2 fm.Thisrange (fc — fm ) to (fc + fm ) , where fc is the carrier frequency, is thesame as that for amplitude modulation and reproduces good copy. Thehighest important frequency fm is determined by the speed of scanningand by the "definition" of the graphic material to be reproducedaccording to the I.R.E. standard method. With this bandwidth of 2/monly the first-order sidebands of fm are transmitted, the higher-ordersidebands being sufficiently unimportant to be neglected.Under this condition of bandwidth and M = 0.47 for the top importantfrequency, itcan be shown that for any other lower signal frequencyno sideband components having amplitudes greater than 10 percent of that of the unmodulated carrier are lost although the frequencyswing or deviation is held constant. For example, with /„ = 1800 cyclesper second, fm — 600 cycles per second, and m = 0.47, the first-ordersidebands have amplitudes of 15 per cent of that of the unmodulatedcarrier and the second-order sidebands have amplitudes of 2 per cent.The former will lie just within the 2fm band width while the latter willbe lost. For a lower signal frequency of 300 cycles per second, mwould be 0.94 for the same deviation and the first-order sidebands havingamplitudes of 42 per cent lie well within the transmission band.The second-order sidebands in this case have amplitudes of 10 per centbut they are also inside the 2fm band. Hence no components havingamplitudes greater than 10 per cent are lost. Picture quality is justas good as that obtained with subcarrier amplitude modulation employingthe same bandwidth.Although a subcarrier frequency range 1600 to 2000 cycles has beenused for most of the transoceanic picture traffic,be equally satisfactory.other ranges wouldIt is good practice to have the mid- or carrierfrequencyseveral times the highest signal frequency but good resultscan be obtained with a wide variety of values of frequency swing. Onecommunications system uses the frequency deviation range 2600 to3400 cycles. In a higher-speed, experimental, wire-line test the range7.5 to 10.5 kilocycles gave satisfactory results. From the standpoint ofsuccessful radio transmission the phenomena of selective fading wouldindicate that a comparatively low value of the subcarrier would bedesirable since this would keep the radio-frequency bandwidth relativelynarrow. However, there was no noticeable difference betweenresults with fc— 1800 cycles per second and fc= 3000 cycles per second.Another advantage of subcarrier frequency modulation over the oldconstant-frequency, variable-dot-length method is a more simplified,less critical operating technique. When a picture is to be sent, a band