MAM) WlDl'll AM) I KAXSMISSIOX I'l.KIOKM A.XCE 



495 



however, there need be no separation of channels in eitlier time or freciuency, 

 and a homodyne detection process is required at the receiver for channel 

 selection. The necessarj- precision of instrumentation seems in general 

 more diHicult to achieve than with either frequency or time division, and 

 only minor i)rospects appear for exchange of bandwidth for transmission 

 advantage. 



Of the systems tabulated, the frequency division method (FDM) witli 

 single-sideband suppresscd-carrier transmission is the only method in which 

 bandwidth cannot be traded for some transmission advantage." This 

 system will be used as a standard of comparison. The PAM method, with 

 transmission by AM pulses, can trade upon bandwidth only as a means for 

 reducing interchannel crosstalk. In the other pulse systems, as well as all 

 systems using FM, bandwidth may be expended to gain advantage over 

 noise, intersystem interference, and, generally speaking, intrasystem distor- 

 tion and noise. 



N0N-SIMULT.A.NEOUS Lo.\D Advant.vge in FDM 



The non-simultaneous load advantage pertaining to frequency division 

 multiplex refers to the fact that the channel sidebands rarely add to an 

 instantaneous value even approaching the value N times the peak value for 

 one channel. This means that the required peak capacity of a relay system 

 transmitting the N channels increases slowly with N. Current toll trans- 

 mission practice provides for relative power capacity roughly as follows.^ 



Table I 



NON-SlMUrTANEOUS MULTIPLEX LOAD ADVANTAGE 



To emphasize the strikingly large non-simultaneous load advantage statisti- 

 cally obtainable with conversational speech we may examine Table I and 

 note, for instance, that the capacity of a lOOO-channcl system is completely 



' We have in mind here a system such as Type K or L in which a mininuun separation 

 of adjacent channels in frequency is used. It is true that by spreadins; the channels far 

 apart in frequency, a reduction in cross-modulation falUng in individual channels could be 

 obtained, hut the resulting amount of im[)rovemenl is minor comi)ared with that otTered 

 1)\' a corresj)on(ling band increase in tlie other systems. 



** B. I). Ilolbrook and J. T. Dixon, 'M.oad Rating 'rheor\- for Multichannel .\mpliliers," 

 Bell Sys. Tech. JL, Vol. 18, pp. 624-644, October, 1939. ' The values in the table come 

 from curve C, Fig. 7, taking the single cliannel sine wave power capacity as +9.5 dbm. 



