REFERENCE SYSTEM FOR TELEPHONE TRANSMISSION 541 



duced sounds, and also the amount of noise accompanying these 

 reproduced sounds. 



For convenience in specifying these requirements, it has been found 

 desirable to impose another requirement, namely, that the system be 

 capable of giving a performance which is as free as possible from dis- 

 tortion and noise. It should be noted that two kinds of distortion 

 must be taken into account: that due to unequal efficiency for sounds 

 of different frequencies and that due to non-linearity causing unequal 

 efficiency for sounds of different magnitudes. This requirement is 

 also of advantage in insuring that the reference system and its parts 

 will have less distortion than any circuit or instrument with which it 

 may be compared. This will permit the simulation of the distortion 

 of such instruments or circuits by the insertion of distortion in the 

 reference system. 



For convenience in use, it is highly desirable that the performance 

 of the reference system and its parts be constant for a reasonable 

 time under normal operating conditions. 



General Features 



The master reference system ^ employs a transmitter and receiver 

 which are capable of a high degree of freedom from distortion. The 

 transmitter is of the condenser type and the receiver is of the moving 

 coil type. Both these instruments are materially lower in efficiency 

 than commercial types of apparatus, but this condition is compensated 

 for by the use of multi-stage vacuum tube amplifiers. These instru- 

 ments, together with their associated amplifiers, constitute reference 

 standards for converters between acoustic and electrical energy. The 

 third necessary element of a telephone transmission system, namely 

 the line, is provided by a network of resistance elements. Such a 

 line can be made to provide uniform attenuation over a wide frequency 

 range, and can be made to control the magnitude of this attenuation 

 over a large range. This line is taken as giving a reference performance 

 for lines. 



The specification of the performance of such a system is based on 

 the principle of the thermophone, which is a converter of electrical 

 energy into acoustic waves by means of the heat generated by the 

 passage of an electrical current through a resistance. From a knowl- 

 edge of the form and physical constants of this resistance element, of 

 the medium in which it is used, and of the electrical input to the 

 element, the acoustic pressure generated in a chamber of known size 



■* A discussion of a preliminary model of this system was given in "A Telephone 

 Transmission Reference System," by L. J. Sivian, Electrical Communication, October, 

 1924. 



