372 BELL SYSTEM TECHNICAL JOURNAL 



mission system, as indicated both by laboratory and by field test. 

 As stated before, the problem may be more or less arbitrarily separated 

 into three correlated problems — volume, quality and noise. 



As in the case of sidetone, these problems appear, perhaps, more 

 nearly in a proper perspective if considered in comparison with the 

 corresponding factors in a direct conversation. It must be remem- 

 bered that telephone service does not consist in the provision of a 

 mechanism, per se, but in the provision of facilities for conversation, 

 to which the mechanism should be incidental, however important. 

 Since the inherent conditions of such a conversation are quite different 

 in many respects from those of a direct conversation with which, 

 consciously or unconsciously, it will be compared in its overall results, 

 the parallelism in detail should not be too exact. Departures from 

 the conditions of direct conversation in certain respects which are 

 relatively unavoidable, may be best compensated for by deliberate 

 departure in certain other respects. For example, the physical absence 

 of one party to the telephone conversation, and the monaural nature 

 of such a conversation, may be partially compensated for by delivering 

 to the ear of the listener a somewhat higher speech level than he is 

 accustomed to in direct conversation. The limitation of frequency 

 band width imposed on the telephone medium, largely for economic 

 reasons, may be minimized in its effects if the transmission character- 

 istics in the available band are other than a facsimile of the corre- 

 sponding band in direct conversation. All such measures must be 

 employed with knowledge of their effect on the ultimate objective, 

 that the telephone conversation may be easy and natural. 



General Requirements ^ 



It is easily seen that for any particular overall frequency char- 

 acteristic of a telephone transmission system, there are practically an 

 infinite number of ways in which it can be split up between trans- 

 mitting and receiving characteristics. From this standpoint alone, 

 then, there is no particular "best" transmitter or receiver frequency 

 response. From other standpoints, however, certain general types of 

 individual characteristics, both in frequency and efficiency, are to be 

 preferred to others, particularly when considered in their practical 

 application to an already existing telephone system. It has been 

 pointed out ^ that in general, development has been toward a telephone 

 system where both transmitter and receiver are relatively uniform in 

 their frequency characteristics. Induced noise appears to be so evenly 

 distributed with frequency that such response would not appear to 

 magnify the interference problem. 



