Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

SERVICE RANGE OF FREQUENCY MODULATION 217Effect of Pre-Emphasizing the Higher Modulation FrequenciesAs has been pointed out for the case of amplitude modulation 5 ,use of pre-emphasis circuit at the transmitter and a de-emphasis circuitat the receiver produces an overall gain in signal-noise ratio.This gain depends upon the fact that the higher modulation frequenciesof voice and program material are of such a small amplitude thattheir accentuation does not increase the peak voltage of the appliedmodulating wave as much as the restoring circuit at the receiverreduces the noise. Thus, when the pre-emphasis circuit is inserted atthe transmitter; the peak voltage of the modulating wave may increasesomewhat so that the modulation level must be lowered, but this lossat the transmitter is overshadowed by the gain at the receiver. Consequentlythere are two quantities which must be evaluated—the lossat the transmitter and the gain at the receiver.In order to determine the losstheintroduced by the pre-emphasis circuitat the transmitter, the following experimental observations weremade: The equivalent of a two-string oscillograph was arranged byusing two electronically switched amplifiers to feed the vertical platesof an oscilloscope. The outputs of the amplifiers were common, withone of the separate inputs being fedby program material through apre-emphasis circuit. The gain of the two amplifiers was equalizedat a low modulation frequency where the pre-emphasis circuit was noteffective. The two amplifiers were alternately switched on by meansof a 60-cycle square-wave form and the oscilloscope sweep circuitsynchronized with the 60 cycles. The resulting pattern on the oscilloscopescreen consisted of two segments of sweep one of which wasactuated directly from the program material and the other throughthe pre-emphasis circuit.Hence, an accurate method of comparing thepeak voltages of the two waves could be provided by inserting anattenuator in the pre-emphasized circuit and setting for equal peakvoltages indicated by the two traces on the oscilloscope.The observations made with the electronic-switch oscillograph coveredall types of program material and were made for two audio-bandwidths of 5 and 12 kilocycles. The results of the observations indicatedthat in general the insertion of the pre-emphasis circuit increases thepeak voltage of the level about 2.5 decibels for both the 5 and 12-kilocycleband widths. On certain material such as guitar, harmonica, andpiano solos, the level is raised about 4.5 decibels total, but the occasionof such rises is rather infrequent and their duration very short.Hence,a permanent attenuation of 2.5 decibels might be inserted and thevolume-limiting equipment relied upon to take care of the occasionalhigher peaks.