120 BELL SYSTEM TECHNICAL JOURNAL 



by their normal users in carrying on regular conversations. This re- 

 quires a suitable quantitative method of rating conversations and 

 observations on a sufficient number of conversations over each circuit 

 condition to be investigated to constitute a reliable sample. This does 

 not mean necessarily that all the practicable circuit conditions have to 

 be observed in this manner but rather that sufficient data be so ob- 

 tained for the establishment of correlations with performance measure- 

 ments which are susceptible to laboratory determination. The funda- 

 mental point is that service performance ratings need to be based on 

 service results in order to take proper account of all the factors in- 

 volved. 



Transmission Performance of Circuits 



The distinction has been made between two kinds of transmission 

 performance of a telephone circuit, namely, that indicated by relations 

 between output and input sounds and that indicated by the results ob- 

 tained by the users of the telephone in carrying on their conversations 

 under service conditions. Performance indications of the first kind 

 will be referred to as "transmission characteristics" of the circuit. The 

 second kind of performance may be termed "transmission service per- 

 formance." The distinction between these two kinds of performance 

 is an important one and should be kept clearly in mind. 



The output sounds dealt with in transmission characteristics are not 

 only the reproduced sounds which correspond to the input sounds but 

 also the accompanying extraneous sounds which are delivered by the 

 circuit. Also, the output sounds to be investigated cover not only 

 those delivered by the receiver at the far end of the circuit but also 

 those reproduced by the receiver in the station set containing the 

 transmitter energized by the input sounds. The sounds from the near 

 receiver include both those transmitted through the sidetone path of 

 the set and those returned to the sending end by reflection at impedance 

 irregularities in the circuit. Due to the time required for propagation 

 over the circuit these latter sounds may be delayed with respect to the 

 sidetone and hence appear as echoes. Likewise, echo sounds may be 

 delivered at the far receiver. 



Transmission characteristics do not in themselves show the service 

 performance as realized by the users of the telephone but are essentially 

 indications of the functioning of the circuit in reproducing sounds. 

 They provide, therefore, a means for investigating and specifying the 

 performance of a telephone circuit without involving many, and in some 

 kinds of transmission characteristics any, of the actions of the talker 

 and the listener in conversation. With the establishment of proper 



