Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

BAND WIDTH AND READABILITY 63The curves of Figure 5 were taken in the same manner as those ofFigure 4, but the two systems compared were the narrower frequencymodulationsystem of Figures 1 and 2 and its equivalent amplitudemodulationsystem. The maximum frequency deviation of the frequency-modulationsystem was 6 kilocycles and the audio band was 5kilocycles. It can be seen that the narrower frequency-modulationsystem gives a readability which ison the amplitude-modulation system.always greater than that obtainedIt is apparent from these tests that the optimum frequency deviationfor a frequency-modulation system designed to obtain maximumdistance for full readability is that which corresponds to a deviationratio of one.Fig.5—Readability comparison of a frequency-modulation system and itsequivalent amplitude-modulation system. Maximum frequency deviation = 6kilocycles. Audio band = 5 kilocycles.modulation system. To use a deviation ratio less than one would ofcourse impair the efficiency without a compensating benefit inSuch a system produces very nearly the same intermediate-frequencyand audio channel widths as the corresponding amplitude-reductionof band width since it would be equivalent to an amplitudemodulationsystem which employed a modulation percentage less than100.The results of other listening tests 2conducted by the writer haveindicated that the use of pre-emphasis and de-emphasis, as isused inthe present broadcast frequency-modulation systems, is of doubtfulvalue when the object is the mere transmission of intelligibility insteadof the enjoyment of a high-fidelity program. The peculiar triangularnature of the noise spectrum in the output of a frequency-modulationreceiver is apparently more tolerable than the same noise spectrumafter it has been made practically flat by the de-emphasis circuit. Theresults of the averaged observations of three observers showed thatabout 8 decibels more noise could be tolerated with the triangularfrequency-modulation noise than with the rectangular amplitude-modu-