LOAD RATING THEORY 625 



mental diflferences between single-channel and multi-channel systems. 

 In the first place, the modulation produced in a single-channel amplifier 

 depends only upon the input to that channel and occurs only when the 

 channel is energized. In addition, the most important frequencies 

 resulting from modulation fall directly back upon frequencies already 

 impressed and the net effect appears as a distortion of the original 

 input, rather than as noise. The situation is entirely different in a 

 multi-channel system. In this case, the modulation products falling 

 into one particular channel are in the main unrelated either to the 

 impressed frequencies or to the volume of impressed speech in that 

 channel. Thus it is no longer possible to think of the interference as 

 distortion ; the effect must rather be considered as that of a particular 

 kind of noise whose level depends upon the load on the other channels 

 of the system. For a given grade of service, the ratio of signal to 

 noise must be much larger than the ratio of signal to modulation prod- 

 ucts resulting in distortion; thus it is to be expected that the non- 

 linearity requirements will be more stringent for multi-channel opera- 

 tion than for single-channel operation. 



The second fundamental difference between single-channel and 

 multi-channel systems arises from the character of the load which each 

 system must be designed to handle. A single-channel amplifier must 

 be capable of handling one channel at the maximum volume normally 

 expected. Inasmuch as the amplifier will be loaded only about one- 

 fourth of the time, even in the busiest hour, and as the average im- 

 pressed volume will be some 15 db below the maximum that must be 

 provided for, the ratio of maximum to average load of such an amplifier 

 is inherently very high. In a multi-channel system, however, the 

 several channels will very rarely be heavily loaded simultaneously. 

 There is thus a favorable diversity factor, increasing with the number 

 of channels, and multi-channel amplifiers may accordingly be worked 

 successfully at lower ratios of maximum to average load. 



Occasionally, of course, there will be short periods of excessive 

 loading during which the interchannel interference in multi-channel 

 systems will rise above the value normally permitted. This sort of 

 thing often occurs when it is desired to make economical use of facil- 

 ities of any kind in common. In machine switching systems, for ex- 

 ample, it is common practice to associate a large number of lines with 

 a smaller number of switches and trunks. The number of switches and 

 trunks provided is sufficient to ensure a satisfactory service, with a very 

 small probability of requiring more facilities than are available. The 

 multi-channel amplifier problem presents a situation identical in prin- 

 ciple, though the methods of solution are necessarily very different. 



