644 BELL SYSTEM TECHNICAL JOURNAL 



for a small number of channels, the range of overloading voltage is not 

 so restricted and overloading effects may become undesirably severe 

 during the 1 per cent of time when the overload voltage is exceeded. 

 If voltage limiting is not provided in some form, it may be important 

 to reduce the percentage of time during which overloading may occur 

 for small numbers of channels. This is a matter to be determined 

 by experience and, if necessary, would require modification of curve B 

 of Fig. 7 in the direction of requiring more load capacity for a small 

 number of channels, thus increasing the spread between curves B and C. 



Operating Margins, etc. 



The curves which have been given for output capacity versus 

 number of channels apply to a single amplifier, or to a system in which 

 all amplifiers are identical and work at the same output level without 

 appreciable impairment of overall performance. In practice, the 

 number of amplifiers in tandem in a long system may be very large and 

 problems of equalization and regulation may make it difficult to 

 maintain exactly the same level conditions at all amplifiers. In addi- 

 tion, aging of tubes, and other effects will introduce some impairment. 

 It is important, therefore, to allow a margin for these effects in the 

 design of an amplifier for a multi-channel system. The proper margin 

 is essentially a matter of system design and it is often economical to 

 build a liberal margin into the amplifiers in order to allow greater 

 latitude and economy in the design of equalizing and regulating 

 arrangements. 



In addition to the speech loads, there are also impressed on the 

 amplifiers various signaling and pilot frequencies, carrier leaks, etc. 

 It is not always possible in practice to make these negligibly small and 

 the load capacity requirements must be corrected to allow for their 

 presence. Multi-channel telephone systems are also required to 

 transmit other types of communication circuits, such as program 

 channels and voice-frequency telegraph systems, superposed on one 

 or more telephone channels. Modifications of the methods applied 

 to speech loads may readily be made to determine the effect of these 

 on the amplifier load capacity. 



Acknowledgment 



Many members of the Bell Telephone Laboratories, in addition to 

 those mentioned in the text, have contributed to various phases of this 

 work. The authors take this opportunity to acknowledge their 

 indebtedness to these colleagues, and in particular to Dr. G. R. Stibitz, 

 who first developed the theoretical approach here used. 



