REFERENCE SYSTEM FOR TELEPHONE TRANSMISSION 539 



This cable reference system has played a very important and 

 necessary part in the development of telephone transmission, in that 

 it has provided a ready means of rating the performance of the various 

 parts of the system and of any changes, and made it possible to design 

 commercial circuits to provide a predetermined grade of service. 

 The performance of this system was specified by stating the kinds of 

 apparatus and circuits used. The performance of the elements of the 

 electrical portion of the system could be checked by voltage, current 

 and impedance measurements, but for the transmitters and receivers 

 reliance for constancy of performance was placed primarily upon the 

 careful maintenance and frequent cross comparisons of a group of 

 transmitters and receivers which were specially constructed to reduce 

 some of the sources of variation in the regular product instruments. 

 In this way, reasonable assurance of the performance of the reference 

 system was secured. This system has been widely used both in this 

 country and in other parts of the world, and the performances of the 

 various systems have been kept in accord by frequent circulation of 

 calibrated transmitters and receivers. 



As the telephone art has developed, modifications have been found 

 to be desirable in this reference system to make it more suitable for 

 its purpose. Telephone instruments and circuits have been designed 

 and used which have less distortion than existed in the corresponding 

 parts of the cable reference system. For this reason it is desirable to 

 have as a new reference system one with which the transmission over 

 the most perfect telephone circuit or over some less perfect one may 

 be simulated at will. The change of the unit of transmission from 

 the mile of standard cable to the decibel ^ has brought about the need 

 for a change in the line of the reference system. 



In selecting a new reference system, it is obviously desirable to 

 eliminate as far as possible the factors which are not subject to exact 

 measurement, or which may possibly vary with time. For this 

 reason, the elements of the new system have been chosen so that their 

 performance may be definitely measurable at any time, and may 

 remain as far as possible invariable. This applies also to those 

 elements which are provided for insertion in the system when it is 

 wished to produce some distortion which will make more easily possible 

 a loudness balance between the reference system and the circuit under 

 investigation. 



A reference system such as that described here, in which the essential 

 elements are so constructed as to reproduce speech with a high degree 



^ "Decibel" (db) is the name for the Transmission Unit which has superseded the 

 "mile of standard cable." W. H. Martin, Bell System Technical Journal, January, 

 1929, and A. I. E. E. Journal, March, 1929. 



