Master Reference System for Telephone Transmission ^ 



By W. H. MARTIN and C. H. G. GRAY 



The telephone transmission system described here is the Master Reference 

 System of the Bell System for the expression of transmission standards and 

 the ratings of the transmission performance of telephone circuits. The 

 transmitter and receiver elements of this system are reference standards for 

 the ratings of the transmitting and receiving performance of terminal 

 station sets. 



A replica of this reference system, installed in Paris, has been adopted as 

 the Master Reference System of the International Advisory Committee on 

 Long Distance Telephone Communication in Europe. The establishment 

 of these two master systems provides a common reference for the telephone 

 transmission work of the Bell System and the telephone administrations 

 which are members of this International Advisory Committee. 



THE Master Reference System for Telephone Transmission, as its 

 name indicates, is to serve as the fundamental circuit in the 

 ratings of the transmission performance of telephone circuits. In de- 

 scribing this system, therefore, it will be advantageous to outline first 

 the general considerations underlying the methods of determining 

 and specifying these ratings and their applications. 



The conversions and transfers of energy which constitute the 

 process of telephone transmission result in general in a difference 

 between the speech sounds at the sending end of the telephone circuit 

 and the sounds reproduced at the other end in the ear of the listener. 

 These reproduced sounds may dififer from the original in three im- 

 portant respects; their loudness, their distortion or degree to which 

 their wave shape departs from facsimile reproduction, and the amount 

 of extraneous sound or noise which accompanies them. From the 

 standpoint of telephony, the major importance of a difference between 

 the original and reproduced sounds is determined by its effect on 

 "intelligibility," that is, the degree to which the latter sounds can be 

 recognized and understood by the listener when carrying on a telephone 

 conversation. The tolerable departure of the reproduced from the 

 original sounds is limited also by certain effects which are noticeable 

 to the listener before they materially affect intelligibility, such as 

 loss of naturalness. 



Measurements of intelligibility are of utmost importance in rating 

 the performance of telephone circuits, but they are unduly cumbersome 

 for direct use in the detailed development and design of telephone 

 circuits and their many parts, particularly where small effects are 

 concerned. It has been desirable, therefore, to handle telephone 



^ Presented before A. I. E. E. Summer Convention, June 24-28, 1929. 



536 



