INDUCTIVE LOADING FOR TELEPHONE FACILITIES 463 



and plant expansion in new routes, (2) to facilitate working to new trans- 

 mission limits that occasionally become desirable in consequence of the 

 introduction of improved subscriber sets, or for other reasons, and (3) to fit 

 in with the installation of new central offices and faciUtate the occasional 

 abandonment of old offices during changes from manual to dial operation, 

 or for other reasons. In the plant rearrangements, old loaded circuits using 

 former standard coil spacings or inductances can sometimes be reused to 

 advantage, when engineered with suitable transmission distortion-penalties, 

 as subsequently discussed. 



20.3 Effective Transmission; Distortion Penalties 



The engineering transmission-cost studies that resulted in the standardi- 

 zation of the improved loading systems described in Table XII were made 

 during a period in which a new philosophy ^^^ of the design of complete 

 telephone systems evolved. The basic feature of this philosophy was the 

 acceptance of the rate of occurrence of repetitions requested by the users 

 of a particular circuit in carrying on a regular telephone conversation as a 

 measure of the grade of transmission-service performance of that circuit. 

 This involved the preparation of an adequate new system^^ of transmission 

 engineering data for use in the design of telephone systems, including the 

 effects of all factors that influence the service performance. 



The new technique is of special interest in the present loading review 

 because it made possible for the first time an accurate quantitative appraisal 

 of the effect of the different widths of frequency band transmitted by differ- 

 ent loading systems. This is done in terms of the effective transmission loss 

 relative to that of the trunk in a convenient, working reference system. The 

 speech distortion that results from a reduction of the effective transmission 

 band width in a loaded trunk may be expressed as a loss which is equivalent 

 in transmission-service performance to a definite increase in the distortion- 

 less transmission loss. 



In comparing different types of loading, the differences in distortion 

 penalties must be taken into account along with the attenuation differences. 

 Also, when proving in the use of loading, the distortion penalty of the non- 

 loaded trunk due to the unequal attenuation of frequencies in the speech 

 band must be considered together with the 1000-cycle attenuation loss. 



For present purposes, in appraising the new loading systems under dis- 

 cussion, it is sufficient to say that the distortion penalty ratings of exchange 

 area trunks which use them are zero, or very close to zero, in the longest 

 trunks likely to be required in working to the present or probable future 



^■^ For comprehensive information on these matters reference should be made to an 

 article (34) by W. H. Martin published in 1931, and an article (35) by Messrs. F. W. 

 McKown and J. W. Emling published in 1933. 



