334 BELL SYSTEM TECHNICAL JOURNAL 



of the relations. These ratings of complete circuits can then be divided 

 up into ratings of individual circuit elements. 



Form of Effective Data 



As stated before, the purpose of the effective data is to give a means 

 of computing a transmission performance rating of a complete tele- 

 phone circuit from the physical makeup of the circuit. These ratings, 

 which are called effective transmission equivalents, are based on the 

 definition that two complete circuits have the same effective trans- 

 mission equivalent when under the same conditions of use they give 

 the same grade of service as indicated by the repetition rate. The 

 term complete circuit as used here includes the transmitter and receiver 

 as well as the other elements of the electrical circuit. Circuit noise is, 

 of course, one of the characteristics of a circuit and room noise may be 

 treated as if it were a circuit characteristic, since it affects the trans- 

 mission results obtained by the users of the circuit. 



Reference System 



In addition to adopting a criterion for the equality of two circuits, 

 it is necessary to adopt a scale for the rating of circuits which differ 

 in performance over a wide range. This has been done by a method 

 analogous to that used for expressing volume equivalents. A refer- 

 ence circuit has been selected, to which a rating has been assigned as 

 discussed below. This reference system may be varied from its normal 

 adjustment by distortionless changes in the loss of the trunk, which 

 forms a part of the system, until the reference system is equal in per- 

 formance to the circuit being rated. Each change of 1 db in this 

 trunk by definition changes the effective equivalent of the reference 

 system by 1 db. Thus, the effective equivalent of circuits may be 

 determined by comparison with the reference system. 



The requirements of a reference system for effective transmission 

 equivalents are: (1) that it be reproducible from simple physical meas- 

 urements, and (2) that its performance can be compared with the per- 

 formance of the circuits to be rated. Theoretically these are the two 

 requirements for the reference system if it is to be used simply for 

 rating complete circuits. As discussed later, since the system is to 

 be used for determining effective losses of circuit elements as well as 

 effective equivalents of complete circuits, there is the additional re- 

 quirement (3) that it have characteristics similar to the commercial 

 circuits to be rated. No system is available at present which fully 

 meets all three requirements or ev^en the first two, since present systems 

 meeting requirement (1) are essentially laboratory devices and cannot 



