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BELL SYSTEM TECHNICAL JOURNAL 



determination is made of the equivalence of loudness or volume of 

 these two circuits by talking alternately over them and adjusting the 

 reference circuit until the sounds coming out of the two receivers are 

 judged to be equal. For the conditions where volume is the important 

 controllable characteristic of telephone circuits, these loudness com- 

 parisons constitute a practicable and effective means of indicating the 

 performance of these circuits. 



The reference circuit adopted about twenty-five years ago for these 

 loudness comparisons consisted of transmitters, receivers, station sets, 

 cord circuits and a line, of types which were then used commercially. 

 In this reference circuit the line was an adjustable artificial line 

 simulating a 19 AWG. cable circuit having a capacity per loop 

 mile of 0.054 mf. The amount of cable in this line to give a loudness 



ST D. COMMON BATT 

 STATION SET 



25-A 

 REPEAT COIL 



25-A 

 REPEAT COIL 



ST D. COMMON BATT 

 STATION SET 



A A 

 L-AA/\rt^MA/W~-VWV-M/W^r-M/Wi^^^^^~>A^AAr^ — I 

 "■229 TRANS. ST'D- ADJUSTABLE LENGTH OF 



*20 IND. COIL ARTIFICIAL CABLE 



*122 REC ST'D. 88 OHMS 0-054 MF PER MILE 



2MF COND. A = 22"*' 



Fig. 1 — Standard cable reference system. 



balance was taken as the rating of the circuit under comparison. 

 This reference system, shown schematically in Fig. 1, has been called 

 the Standard Cable Reference System. 



In addition to this rating of the performance of a telephone circuit, 

 this standard cable reference system has had other applications. 

 Certain settings of this reference system were selected as specifying 

 the standards of transmission which were to be provided in the design 

 and operation of commercial circuits for the several kinds of service, 

 such as local and toll. The effect of introducing or changing any 

 part in a commercial circuit was rated in terms of the amount of cable 

 by which it was necessary to change the line of the reference system 

 to produce the same effect on the loudness of the reproduced speech 

 sounds. Likewise, the transmitters and receivers of this reference 

 system were used as reference instruments for the comparison and 

 rating of other transmitters and receivers. 



