500 BELL SYSTEM TECHNICAL JOURNAL 



9 db more range than one channel. This makes the signal-to-noise ratio in a 

 channel 9 db lower than if all of the entire load capacity were devoted to that 

 channel. However, in the 100-channel group a single channel receives only 

 1% of the noise power in the entire band, so a 20 db improvement accrues 

 on this score. The net improvement is 20 - 9 = 11 db. Applied to a 128 

 step PCM system in which the full-load signal-to-noise ratio is 45 db'^ (7 

 digits binary PCM), the full-load signal-to-noise ratio in one channel of a 

 100-channel group thus becomes 45 + 11 = 56 db. With smaller groups 

 than 100 channels the signal-to-noise ratio falls to 45 db while for larger it 

 reaches 59 db, as shown in Fig. 3. Better results than these are obtainable 

 with time division and instantaneous companding, as shown in Fig. 2, but 

 these results may have significance in relation to the transmission of tele- 

 vision by a pulse method, such as PCM. If a 128-step system were used, a 

 large frequency-divided group of telephone channels filling the television 

 band could be substituted for television when desired. 



A more powerful application of the non-simultaneous load advantage in 

 time division will be discussed later in Section V. 



Signal Band Width and Frequency Occupancy 



We define signal band width as the width of the signal spectrum (or more 

 realistically as that portion of the signal spectrum which must be preserved 

 in order to make the signal sufficiently undistorted). Frequency occupancy 

 is greater than signal bandwidth in two respects: 



First, the frequency range accepted by the receiving filter at the end of 

 each span must be greater than the signal band for reasons of filter imperfec- 

 tion. In all of the pulse or FM systems it would be advantageous from the 

 circuit point of view to make the receiving filter much wider in order thereby 

 to reduce the phase distortion over a small central frequency range occupied 

 by the signal band. The assigned frequency space must include the entire 

 band accepted by the receiving filter. Our comparisons will assume that the 

 filters make use of an appropriate amount of refinement to conserve fre- 

 quency space. 



Second, frequency occupancy must include the multiplication of assign- 

 ments made necessary to avoid interference between converging or inter- 

 secting radio relay routes, between the two directions of a single route, or 

 between a main route and a spur. 



Our procedure in evaluating these systems will be to plot for each system 

 certain curves relating power, signal bandwidth and channel signal-to-noise 

 ratio or signal-to-interference ratio for various associated transmission 



1^ Appendix I shows that the quantizing noise power at the minimum sampling fre- 

 quency is the same for wide and narrow signal bands. This illustrates the general prin- 

 ciple used here. 



