408 BELL SYSTEM TECHNICAL JOURNAL 



New Telephone Transmission Reference System 



With the standartlization of the distortionless unit of transmission 

 it is desirable also to ado])! for a transmission reference system a 

 telephone circuit which will be distortionless from sound input lo 

 the transmitter to sound output from the receiver. This system will 

 consist of three elements, a transmitter, a line and a receiver. Each 

 will be designed to be practically distortionless and the operation 

 of each will be capable of being defined in definite physical units so 

 that it can be reproduced from these physical \aiues. Thus the 

 transmitter element will be specified in terms of the ratio, over the 

 frequency range, of the electrical power output to the sound power 

 input, this ratio being expressed in transmission units. The receiver 

 element will be specified likewise in terms of the ratio of sound power 

 output to electrical power input. The output impedance of the 

 transmitter and the input impedance of the receiver elements will be 

 UOO ohms resistance. The line will be distortionless with adjust- 

 ments calibrated in transmission units and will li,i\r a iharacter- 

 istic impedance of 600 ohms resistance. 



Such a reference system is now being constructed. The trans- 

 mitter clement consists of a condenser type transmitter and multi- 

 stage vacuum tube amplifier. The receiver element consists of an 

 amplifier and specially damped receiver. Each element is adjusted 

 to give only negligible distortion over the frequency range. 



It is proposed when this system is completed and adjusted that 

 it will be adopted as the Transmission Reference System for telephone 

 transmission work. Other secondary reference systems, employing 

 commercial-type apparatus will be calibrated in terms of the primary 

 system and used for field or laboratory tests when such commercial 

 type systems ari' needed. 



