REFERENCE SYSTEM FOR TELEPHONE TRANSMISSION 543 



which can be varied over a range of volume and distortion. It well 

 meets the requirements and represents a material advance over the 

 standard cable reference system which it now replaces. In order to 

 tie together ratings established in terms of the old system with those 

 of the new system, comparisons of the two have been made to determine 

 the adjustments of the new system which make its loudness perform- 

 ance correspond with that of the old. 



Fig. 2 — Master reference system for telephone transmission with associated 

 caHbration apparatus. 



Description of Master Reference System 



The master reference system with associated calibration apparatus 

 is shown in Fig. 2. This equipment, mounted on steel panels and 

 racks, and arranged as shown, is installed in a room, shielded from 

 acoustical and electrical disturbances, at the Bell Telephone Labora- 



