BAND WIDTH AND TRANSMISSION PERFORMANCE 553 



fare alike in respect to frequency requirements imposed by antenna direc- 

 tivity, the bandwidth figures do not adequately reflect the merits of the 

 systems. This is because the band-spacing factors are different and, in 

 addition, only the regenerative systems can be expected to achieve the 

 halving of occupancy accruing from dual polarization and from one-fre- 

 quency routes. The crosshatched bars of Table VHI include the effect of 

 multiplying by the estimated band-spacing factors shown beneath the bars. 

 These band spacing factors are in some cases smaller than those previously 

 tabulated for the less rugged systems of Tables IV and V. Only the PCM 

 methods are shown for the case of very high-grade channels, since the non- 

 PCM methods are so strikingly less effective here. 



These conclusions depend for validity on the assumptions made and par- 

 ticularly on those concerning antermas, route disposition and fading, and 

 apply when the converging systems are of the same kind. In a real situation, 

 departures from the assumed conditions could markedly affect the conclu- 

 sions. For instance, the meritorious showing of PCM in respect to efl&cient 

 utilization of frequency space in the face of route congestion depends heavily 

 on the assumption that all routes in the occupied space employ PCM. 

 Any routes employing a modulation method that is highly vulnerable to 

 interference like some of the narrower bandwidth methods would have to 

 employ higher power to operate in the face of interference from the PCM 

 routes. This higher power, concentrated in a narrower band, could destroy 

 the PCM routes. In some cases it would obviously be impossible to assign 

 values of power which permit the two kinds of routes to share the same 

 frequency band. 



Our calculations should be taken to illustrate the factors involved and 

 the philosophy by which such problems may be approached rather than 

 to find an unequivocally best system. 



V. More About The Non-Simultaneous Load Advantage 



The transmission advantage enjoyed by multiplexing many single side- 

 band telephone channels in frequency division, discussed in the introduction, 

 stems from several factors: 



1. During the busiest period, only a small percentage (of the order of 

 12 to 15%) of the channels are actually transmitting speech (''talk spurts") 

 at one time, on the average. 



2. There are only a few loud talkers; the remaining ones range downward 

 to a volume 35 to 40 db lower. 



3. In the addition of the sideband voltages representing the talkers actu- 

 ally producing talk spurts, only a fraction of the grand maximum occurs 

 often enough to be significant. 



With frequency division all of these factors jointly contribute in a natural 



