128 BELL SYSTEM TECHNICAL JOURNAL 



larly, loop loss curves can be drawn up for the combination of instru- 

 ments, set, loop and cord circuit such as has been used in the past on a 

 volume basis. On the new basis, these curves will include not only the 

 ratings of volume losses but also the ratings for the distortions in the 

 loop and instruments and the effect of the sidetone on transmitting and 

 receiving. In this manner, the transmission design of the plant can be 

 carried out in about the same manner as it has been on the volume rat- 

 ing basis but the effects of distortion, noise and sidetone can all be 

 included in these effective transmission ratings which are based directly 

 on service performance. 



This in outline is the method of determining effective transmission 

 ratings which is now being worked to, its method of formulation and its 

 application. The complete discussion and description of these matters 

 involves innumerable details which, as already stated, it is not the pur- 

 pose to set forth here. From this outline it is seen that this method 

 provides the following outstanding things: 



1. A scale for indicating different grades of effective transmission, 



which scale is expressed in decibels and is directly correlated 

 with service performance by means of a typical circuit selected 

 as a reference. This permits the specification of grades of 

 service. 



2. The use of this same scale as a means of assigning to each element 



of practical telephone circuits an index, expressed in decibels, 

 which measures its contribution to the effective transmission of 

 the circuit, these indices being of such a nature that those cor- 

 responding to the elements in a circuit can be combined in a 

 simple way to give an overall performance index for that circuit. 

 Such a system of indices is necessary for plant design. 



3. A means of correlating effective transmission service and circuit 



transmission characteristics. This correlation is advantageous 

 in setting up the indices of (2) and in development and design 

 work in determining the desirability of possible changes in the 

 performance of the various elements. 



The selection for the present of the typical practical circuit described 

 above, as a working reference circuit, has two important advantages, 

 which will be restated. First, by using a reference circuit having 

 typical transmission characteristics, the indices established for changes 

 in the various characteristics within the range of practical interest, are 

 directly applicable to the present plant and can be combined in a 

 simple manner to provide an overall circuit index. Second, and by no 

 means of minor importance in the earlier stages of the application of the 



