REFERENCE SYSTEM FOR TELEPHONE TRANSMISSION 537 



transmission work in two steps. One natural division is suggested 

 by the statement that intelHgibiUty is a function of the relation between 

 the output and input speech sounds, and of the psychological reaction 

 of the listener to these output sounds. Because of the complex nature 

 of the speech sound waves, however, it has been found more practicable 

 to treat the transmission performance of telephone circuits in the 

 following two parts: (1) the physical performance of the circuit, and 

 (2) the relation between physical performance and intelligibility. The 

 physical performance of a circuit is taken here to cover the transmission 

 characteristics which can be specified in terms of the performance for 

 single frequencies, a number of frequencies being taken to cover the 

 range which is important for the reproduction of speech sounds. 

 These measurements of physical performance cover such things as the 

 response-frequency characteristic of the circuit over the range of 

 speech frequencies, the distortion due to non-linear elements, phase 

 distortion and the extraneous currents which cause noise. These 

 determinations of physical performance do not include measurements 

 of the speech sounds themselves, nor of the functioning of the talker 

 and listener. This differentiation is advantageous in segregating the 

 studies of speech sounds and of the psychological phases of the work, 

 and permits the design of the operating plant and a large portion of the 

 development work to be carried out on a physical basis. 



The determination of the relation between intelligibility and the 

 physical performance of a telephone circuit is a laborious process, 

 because persons play the parts of generators and meters and a number 

 of people must be used in both parts to take into account the normal 

 ranges of their performance. The goal of this portion of the work has 

 been, therefore, to establish suitable relations which will permit the 

 determination of the intelligibility of a circuit by computations which 

 start with the physical characteristics of the circuit. This work ^ 

 has involved determinations of the capabilities of circuits having 

 various kinds of physical characteristics to reproduce intelligible 

 speech, investigations of the nature of speech sounds and hearing, and 

 of people's customs in using the telephone. 



Prior to the time when suitable means were available for measuring 

 the physical performance of telephone circuits, and when the kinds of 

 circuits in commercial use were quite similar in their distortion charac- 

 teristics, the practice was adopted of rating the performance of a 

 circuit by comparing it on a loudness basis with a reference circuit 

 which was adjustable in attenuation, and whose distortion was closely 

 similar to that of the commercial circuits. In such a comparison, a 



^ "Speech and Hearing," by H. Fletcher, pubHshed by D. Van Nostrand Co. 



