124 BELL SYSTEM TECHNICAL JOURNAL 



This testing method thus offers a means of indicating the distortion 

 effects of circuits in terms of the recognizabiHty of the reproduced 

 sounds of speech. Probably one of its first applications^ was in 

 determining the cutoff frequency to be used in the design of coil 

 loaded circuits. 



The articulation testing method provides, of course, quantitative 

 measures in terms of the recognizabiHty of the reproduced sounds of 

 speech not only of distortion effects, but also of the effects of the loud- 

 ness of these sounds and of the noise which may accompany them. 

 This method has provided a very powerful tool for investigating the 

 effects of changes in the reproduction characteristics of telephone 

 circuits on the recognition of the reproduced sounds and has been par- 

 ticularly useful in indicating the lines to be followed in reducing causes 

 of distortion in circuits and apparatus and in evaluating the impair- 

 ment caused by noise on telephone circuits. It has been recognized, 

 however, that while such measurements indicate the capabilities of the 

 circuits in reproducing recognizable speech sounds, they do not in them- 

 selves give direct measures of the degree of success which the users of 

 the telephone obtain in conversations where their actions are free from 

 the control which is necessary in articulation testing and where the 

 contextural relation of the words plays such a large part in their recog- 

 nition. To make the results of this type of testing approach more 

 nearly the conversational results, words and sentences have been used 

 in place of the meaningless syllables but it is evident that even with 

 sentences, the control on the actions of the testers and on the ideas to be 

 communicated presents a condition which is quite different from those 

 of regular conversations. 



All these ways of investigating and measuring the performance of 

 telephone circuits in reproducing sounds have useful applications in 

 present day transmission work. Frequently it is convenient to use 

 different methods for the various parts of a circuit in specifying the 

 complete functioning of the circuit in reproducing sounds. 



Transmission Service Performance 



From the standpoint of the users of the telephone circuit, the trans- 

 mission performance is measured by the success which they have in 

 carrying on conversations over the circuit. Different degrees of success 

 in this process may be taken as being indicated by the number of 

 failures to understand the ideas transmitted over the telephone and by 

 the amount of effort required on the part of the users to impart and 

 receive these ideas. Service performance is of course affected also by 



* "Telephonic Intelligibility," Campbell, Phil. Mag., Jan., 1910. 



