Transmission Features of the New Telephone Sets * 



By A. H. INGLIS 



The new telephone instruments now being introduced by the 

 Bell System result in an outstanding improvement in transmission 

 performance in service. The evidence for this, as obtained by 

 comprehensive laboratory and field tests, is presented here together 

 with a discussion of the factors responsible for this superior per- 

 formance and of the consideration involved in its appraisal. 



NEW telephone instruments are being applied in the plant of the 

 Bell System to the deskstand, wallset and handset, and result in 

 markedly improved transmission performance. The new instruments 

 are associated with the anti-sidetone feature which is also applied to 

 the older sets already in plant. The selection of these particular 

 designs from the wide choice made possible by new design technique, 

 materials and manufacturing methods, has been based on develop- 

 ments in the methods for quantitatively rating the relative merits of 

 different designs. In general there has been consistent effort over a 

 period of years to base these ratings primarily on performance in 

 service rather than on laboratory tests. 



The factors influencing service performance are so many, and so 

 complicated in their relationship, and are in so many cases difficult or 

 even impossible for the designer to evaluate or control, that their net 

 effect on performance cannot be predicted with certainty by laboratory 

 methods. Of necessity such methods involve a limited selection of 

 primary test conditions, and an even more limited selection from the 

 possible combination of these conditions. This Is particularly true 

 in the rating of the transmission performance of a telephone set. 

 Laboratory tests are essential in the study and analysis of design 

 problems, and are invaluable similarly in interpolating, supplementing, 

 and explaining service performance results. In determining the reac- 

 tion on the user of the transmission features of possible designs, 

 however, the field performance test has been found of first Importance 

 in deciding what particular characteristics to include in the new tele- 

 phone instruments and circuits. 



* Presented at A. I. E. E. Summer Convention, Washington, D. C, June 21, 1938 



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