268 BELL SYSTEM TECHNICAL JOURNAL 



users' own performances — the subject material of conversation, how 

 they talk into the transmitters, and how they hold the receivers — 

 and upon the room noise conditions. In other words, there are a 

 number of factors random in nature which, while beyond the control 

 of those who design and engineer the telephone plant, must be taken 

 account of in rating the service performance. 



A large amount of thought and effort has been given to the problem 

 of how best to determine transmission service performance. Very 

 briefly stated, we have been led to the following steps: In order to 

 take suitably weighted account of all the factors involved, service 

 performance ratings should be based on service results, that is, trans- 

 mission service performance should be measured by the success which 

 users of the telephone circuit have in carrying on conversations over 

 the circuit. With the various factors in mind, we have fixed upon 

 what we have termed "effective transmission" ratings for transmission 

 plant design. These ratings are based on a determination of the 

 repetition rate in normal telephone conversations. 



As the effect of a change in a circuit depends upon its initial char- 

 acteristics, it is necessary in order to be able to compare numerical 

 results to have a basic circuit for reference. By suitable choice of 

 basic circuit, it is possible to express the effects of changes in any one 

 transmission characteristic in terms of the attenuation of the trunk. 

 For example, the effect of changes in sidetone level in the subscriber's 

 set can be expressed as so many decibels change in trunk attenuation. 

 Mr. W. H. Martin's paper, "Rating the Transmission Performance 

 of Telephone Circuits," in the Bell System Technical Journal, January, 

 1931, discusses the method and general principles. It should be noted 

 that the application of the method requires careful consideration of 

 many factors and the accumulation and analysis of a very substantial 

 amount of data. Based on these data, we have arrived at the fol- 

 lowing relationship: 



Relative effective loss in db = 50 logio (r) 



where r is the ratio of the repetition rates for the two conditions 

 compared. 



Association of Transmitter and Receiver 



In order to furnish a convenient two-way talking circuit over a 

 single pair of wires, the transmitter and the receiver at each end of 

 the circuit must be continuously associated in the circuit. This has 

 been accomplished by various circuit arrangements since the early 

 days of the telephone, and as every user of the telephone knows, leads 



