Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

''DUPLEX SOUND AND FACSIMILE 399Applying the Bessel function expression to the facsimile signalonly, with no aural signal, gives the plot in Figure 2 of the relativeamplitudes of carrier and side bands to transmit this signal withdeviations up to 25 kilocycles.It can be seen that for deviations below15 kilocycles the second-harmonic side band (J 2 ) is small and canprobably be neglected without seriously affecting the facsimile signalitself.The radio-frequency spectrum for the facsimile signal when operatingsimplex is shown in Figure 3. Suppose now that an aural toneis introduced and with such a low-frequency period that any smallsection of the wave may be considered as constant infrequency com-+*. -' -_-^- "J&&- —l«BSC5Sg- = ::rJ "~ DEVIATION-—*P*" " —i;r"8 10Fig. 2—Facsimile sideband amplitudesIS ' ' 26 • • - - j5KC(No aural).pared to the rapid deviation caused by the facsimile signal. Figure 4will then represent the wave. This is strictly true only when the auraltone introduced has zero frequency, for frequency modulations cannotbe added directly; but for frequencies in the aural band up to severalhundred cycles there is very little error. If only the first-harmonicside bands of the facsimile signal (±20 kilocycles) are considered,the band width under this condition will be ± 60 ± 20 or ± 80 kilocycles.If the second-harmonic side bands of ± 40 cycles are included,the width is ± 100 kilocycles.With an audio frequency of 3125 cycles having a frequency deviationof 75 kilocycles applied alone to the frequency modulator thespectrum appears as in Figure 5 for frequencies on one side of thecarrier, the side bands for frequencies on the opposite side of thecarrier being similar.