Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

Iis»sIj| /9SS]'SUBCARRIER IN COMMUNICATIONS 415in incoming signal level. This requires a very careful adjustment oflevels at both terminals and at the relay points. If for any reason thereis a signal-level variation of a few decibels, then printer failures occur.Because of the great success of subcarrier frequency modulation inpicture transmission, it seemed reasonable to expect an improvement ifit were applied to subcarrier telegraph operation. Tests were accordinglymade and a marked improvement was noted. Figure 2 is a blockdiagram showing how a subcarrier frequency-modulation unit was usedbetween the group of outgoing channels and the radio-frequency transmitterand a combined limiter and frequency discriminator were includedbetween the receiver and^^incoming channeling niters. By directcomparison it was found that where a 4-decibel drop in equivalentS9Sh;PHiLAHlLPHIAT£P.WMALTRANSM/TTER93S//OS1 /S7SSCfMmodulatorlowF/LTfASZOO'-H/GNFILT£ftt2.S KCLOWFtLTCRf*CSJ&/i.£ *X/J.S/CCH/GHFiLT£RFig. 2—Diagram of test circuit showing subcarrier frequency modulationapplied to multichannel teletype operation.signal level caused failure in the amplitude-modulation method, a 22-decibel drop was required to cause failure when subcarrier frequencymodulation was used.This shows an appreciable advantage.In both cases the signal-to-noise ratio was of secondary importance.The advantageous gain in allowable signal-level fluctuation beforeprinter dropouts occur is due primarily to elimination of most of theeffect of this fluctuation before it gets to the actual printer machine.As in the case of pictures, amplitude variations encountered in transmissionare reduced to nil by the limiter device included at the receivingterminal.