374 



BELL SYSTEM TECHNICAL JOURNAL 



system which, when spoken into, would give the listener in all respects 

 essentially the identical sensation he receives in one ear when facing 

 the speaker directly, with an air path one meter long between the 

 speaker's mouth and the listener's ear, in surroundings without re- 

 verberation or noise. Such a reference transmission system has tenta- 

 tively been called an "orthotelephonic" system. 



15 



-15 



I 



2: -20 



^ 10 



-5 



-20 



-25 



0.5 1.0 1.5 2.0 2.5 3.0 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 7 — Overall orthotelephonic frequency response of 

 Typical Telephone Connections. 



3.5 



The point of interest here is that when measured by any suitable 

 objective method, the frequency characteristic of this "orthotele- 

 phonic" telephone reference system is not "fiat" by a considerable 

 amount in the ordinarily accepted usage of the term. This departure 

 from "flatness" is caused by such factors as the frequency directive 



