126 BELL SYSTEM TECHNICAL JOURNAL 



ness, may lead the users In the course of their conversations to make 

 favorable or adverse comments regarding the circuit performance. 

 These comments can be noted by the repetition observers and used, 

 together with any notations on effort and annoyance, to supplement the 

 repetition rating in arriving at a better picture of the service. 



Effective Transmission Ratings for Plant Design 



To provide for the transmission design of the telephone plant along 

 the lines of the previous discussion, ratings, termed "effective trans- 

 mission" ratings, are being determined which are based on the repeti- 

 tion rate in normal conversations. Circuits of different transmission 

 characteristics are considered to have the same effective transmission if 

 their repetition rates are equal when they are used for the same kind of 

 service. Furthermore, two changes in the transmission characteristics 

 of a circuit are taken as equivalent on the same basis. The effects of 

 such changes, however, are a function of the initial transmission char- 

 acteristics and it is therefore desirable to take as a basis for rating such 

 changes, a circuit which has characteristics typical in the various 

 respects of the ranges encountered in practice. 



As a standard reference circuit for determining an expressing effective 

 transmission ratings, it is proposed to use a modification of the Master 

 Reference System, inserting in this certain amounts of distortion, 

 sidetone and noise to give it transmission characteristics comparable to 

 those of present commercial circuits. Pending the development of this 

 standard reference circuit, however, use will be made of a circuit con- 

 sisting of station sets and instruments of kinds in general use, loops of 

 typical length and construction connected by typical cord circuits to a 

 trunk of specified transmission characteristics. For this latter it is 

 convenient to assume a trunk having a cutoff typical of the loading 

 systems in use and having a frequency characteristic which is flat below 

 the cutoff point. It is also convenient to assume that the attenuation 

 of this trunk can be varied uniformly for all frequencies below the cutoff 

 point. This circuit may also be assumed to deliver at the two ends a 

 typical amount of line noise and to have typical room noise at the 

 terminals. Such a circuit then specifies a complete combination of 

 transmission characteristics which are typical of the telephone plant in 

 commercial use and may be considered as a working reference circuit. 

 The transmission service performance of such a circuit in commercial 

 use can be changed by varying the attenuation of the trunk and this 

 attenuation, expressed in decibels with respect to some reference value, 

 can thus be taken as constituting a scale for expressing different grades 

 of service performance. 



