TRANSMISSION FEATURES OF NEW TELEPHONE SETS 373 



Transmitting efficiency should be as high as required to keep the 

 speech well above induced noise but not so high as to cause excessive 

 crosstalk into other telephone circuits. The maximum desirable re- 

 ceived level is determined for a given telephone system by the limita- 

 tions of the human ear in accepting with comfort speech levels above 

 a certain intensity. Finally, the practical necessity of working as 

 satisfactorily as possible in conjunction with the telephone trans- 

 mitters, receivers, and sets in the existing plant during the period of 

 transition, places a practical limitation on the amount of change that 

 is desirable in relative levels of either transmitting or receiving. 



With regard to frequency range, previous work ^° indicated the 

 desirability of designing circuits to transmit frequencies from 200 or 

 300 cycles up to about 3,000 cycles. Gains in articulation and natural- 

 ness are realized by increases in this band width, but are progressively 

 smaller for successive equal increments in frequency. A 3,000-cycle 

 band properly used gives good transmission both in articulation and 

 naturalness, but frequency limitation is essentially an economic one, 

 subject to change as conditions change. Recent work on the new 

 multiple channel carrier systems has indicated justification in these 

 systems for providing a somewhat wider band, from about 150 to 

 about 3,500 cycles. ^^ 



Overall Frequency Response 



In describing the frequency characteristic of a transmission system 

 it has become customary to refer to it as more or less "flat," where 

 "flat" is assumed to be synonymous with "perfect" as far as the 

 relative transmission of various frequencies is concerned. In meas- 

 urements of the elements of an electrical circuit, from which this 

 terminology came, the word is useful since, when the measurements 

 are properly made, at any rate, the basis of comparison implied by 

 the word "flat" is generally understood. This is also true, although 

 probably to a more limited extent than is generally realized, when the 

 term is applied to electro-acoustic transmission systems, where free 

 progressive, plane air waves of various frequencies are transferred to 

 an electrical system, or vice versa, by means of microphones or loud 

 speakers. 



In the case of a telephone system, however, where a transmitter is 

 placed close to the lips, and a receiver directly to one ear, and where 

 the air waves are not free progressive, or plane, use of the word "flat" 

 implies a basis of comparison which is not self-evident. Much effort 

 has been given recently to establishing an appropriate reference system, 

 sufficiently simple in concept and ease of specification, to be useful in 

 this connection. The result of this work has been a reference telephone 



