Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

I398 FREQUENCY MODULATION, Volume Isecondary synchronizing signal to control motor speed between phasepulses is desirable. A simple method of accomplishing this is to transmita control tone (60 cycles in this case) as an amplitude modulationsuper-imposed on the already frequency-modulated sub-carrier. Anamplitude-modulation of 30 per cent was used in this case, and theGO-cycle wave ischannel and attendant filteringthus transmitted without requiring a third multiplexThis complex wave of 18- toproblems.24-kilocycle frequency variation and30 per cent amplitude modulation then carries the picture, phasing,and synchronizing signals. When used in a simplex service and withthe proper limiters and filters, these signals have been found to bepractically immune to any normal signal fluctuations, and to reject all_ .aural _CHANNELIGUARD JBANDFACSIMILE^CHANNELI30/v 18000 20000J240OOPHASEYtMTEBLACKUPPER SIPEBAND LIMIT—OF FAX. KEYIHS 2520O~Fig. 1—Audio spectrum for multiplexed aural and facsimile.noise 6 decibels or more below the signal itself.Occasional interferencesequal to or even greater than the signal usually cause littlemarring of the copy.This composite signal was then used to deviate the r-f carrier 15kilocycles, the aural channel being added at 60 cycles deviation. Theoverall audio spectrum transmitted is then represented in Figure 1.3. Multiplex TheoryIn tone multiplexing on an amplitude modulated system, the additionof the second tone produces an additional pair of side bands displacedfrom the central carrier by the tone frequency itself. All sidebands are proportional to the carrier amplitude and depth of modulation.In multitone frequency modulation the relationship of sidebands is not as simple as this, for each tone in itself produces aninfinite series of side frequencies. The case for two tones involvestwo such series of side frequencies, and their cross beat products.derivation of the side bands produced in such a two-tone system isgiven by Crosby 2 and can be readily applied to this type of multiplexing.2Murray G. Crosby, "Carrier and Side Frequency Relations with MultitoneFrequency or Phase Modulation," RCA Review, July 1938.A