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BELL SYSTEM TECHNICAL JOURNAL 



is a more difficult and expensive condition to obtain than in direct 

 conversation a few feet from a listener. Something less than perfect 

 reproduction must suffice, for the present anyway, if costs are not to 

 be prohibitive. 



90 

 80 

 70 

 60 

 50 

 40 

 30 

 20 



10 20 30 4Q 50 60 



sound level in decibels 

 (above io"'6 watts per cm2) 



35 



70 80 5 10 15 20 25 30 



difference in decibels (between 

 room noise at ends of telephone 

 connection) 



Fig. 3 — Noise conditions at telephone stations. 



All of these differences involve the acquiring by the user of a set 

 of telephone habits which differ from' those he has acquired in direct 

 conversation. The problem of the transmission design of a practical 

 telephone system requires, then, for a satisfactory solution, not only 

 a determination of the proper speech levels to be delivered, and of the 

 sidetone characteristics which will, under the conditions of a telephone 

 conversation, give optimum results with the noise encountered, but 

 also a decision as to what particular frequency range and characteristic 

 to choose. Properly designed, a telephone transmission system should 

 minimize, to the degree consistent with costs, its inherent differences 

 from direct conversation, and make it easy for the ordinary user to 

 get, without undue effort, results which are satisfactory to him in 

 comparison with direct conversation. 



In the earlier days of telephony, the problem presented appeared 

 much simpler. It was, in effect, uni-dimensional, calling primarily for 

 more efficient instruments and circuits ; more and more power delivered 

 to the listener's ear. While methods for the control of sidetone were 



