B.WP W I Dill AM) /'A'.IA.S 1//.V.S70.V I'l'.KlOKM AWE 



V)') 



the instaiilauooiis mmpamior advantage is substaiitiall}- zero because, at 

 full system load, compaucling aclually increases the total noise. In time 

 division the noise is increased at full load by comi>an(ling but, as discussed 

 earlier, this is permissible because full load occurs only with loud talkers 

 who mask the noise. In a frequency divided group transmitted by pulse 

 methods nearly- full load may be produced when a number of loud talkers 

 are momentarily active; the weak talkers then enjoy no improvement due 

 to companding but may, on the contrary, suffer some degradation. 



NON-SIMUI.T.VXEOUS LOAD AdVANTACK IN PULSK TRANSMISSION 



Transmission of a freciuency-divided group by i)ulse methods does, how- 

 ever, permit the realization of a portion of the non-simultaneous load ad- 



" 1 2 4 6 8 10 20 40 60 80 100 200 400 600 1000 



NUMBER OF CHANNELS IN FREQUENCY-DIVISION MULTIPLEX 



Fig. 3 — Quantizing noise in each channel when PCM (128 equal steps) is applied to 

 an FDM group. 



vantage in lieu of the instantaneous compandor advantage realizable in time 

 division. A pulse system, designed to carry N channels in time-division 

 with a certain full load signal-to-noise ratio (without companding) may also 

 be used to carry N channels in frequency division. These iY channels in 

 frequency division may be treated as a single channel .Y times wider than 

 the time-divided channels and sampled X times faster. The ratio of the 

 full-load signal in this wide band to the noise in the wide band turns out 

 to be the same as the corresponding ratio in each of the narrow, time-divided 

 channels. This fact makes the transmission of large groups of frequency- 

 divided channels advantageous compared to small groups. For instance, in 

 a 100-channel frequency-divided group, a single channel would have avail- 

 able a total load capacity which is 9 db less than that for the multiplex 

 group. This comes from Table I, which shows that 100 channels require 



