486 BELL SYSTEM TECHNICAL JOURNAL 



The selection of the 250-cycle point for the specified frequency range 

 is on the basis of maintaining reasonable naturalness. 



It has been found that with present commercial station sets little is 

 gained either in intelligibility or naturalness by extensions of the trans- 

 mitted frequency range beyond the limits which have been set. This 

 range, moreover, permits effective utilization, particularly from the 

 standpoint of intelligibility, of the capabilities of much better station 

 instruments even if this improved apparatus should approach the ideal 

 in performance. With such terminal apparatus, major extensions 

 beyond the upper frequency limits give improvements from the stand- 

 point of naturalness largely as the result of better reproduction of the 

 fricative consonants and of some of the incidental sounds which accom- 

 pany speech. The^extension necessary to effect a material improve- 

 ment in this respect is a matter of a thousand cycles or more, rather 

 than hundreds of cycles. It has been considered that such an extension 

 for message circuits is not now justified. 



Further in this connection, it must be borne in mind that an exten- 

 sion of the transmission range will in general increase the amount of 

 noise on the circuit and magnify the crosstalk problem. For trans- 

 mission systems such as carrier and radio where the noise may be 

 assumed to be uniformly distributed over the sideband range, the added 

 noise may be particularly important. Also a widening of the range 

 increases the difficulties of securing proper impedance balances and of 

 equalizing amplitude and phase distortion. 



On the basis of these considerations, it has been decided that new 

 designs of telephone message circuits for the Bell System should have 

 an effective transmission band width of at least 2,500 cycles, extending 

 from about 250 to 2,750 cycles. Furthermore, this band width will be 

 made greater in those cases where this can be accomplished without 

 material increase in costs. 



