626 BELL SYSTEM TECHNICAL JOURNAL 



efificiency, however, are attended by many difficulties and compli- 

 cations. The efficiency at the point of maximum response could, of 

 course, be increased by piling up the several resonances which have been 

 referred to, but this would give serious distortion effects which would 

 more than offset any increase in loudness which was obtained. In 

 general, it has been found that any improvements which might permit 

 higher maximum responses can be utilized to give greater benefit in 

 the reduction of distortion. Moreover, any large increase in the trans- 

 mitter efficiency would require measures such as the reduction of the 

 efficiencies of the receivers in order to avoid increased crosstalk effects 

 between circuits and uncomfortably loud transmission over short 

 connections. 



When the program of the joint development and research work of 

 the N. E. L. A. and Bell System was formulated there was the idea 

 that in view of the resonant characteristic of the telephone receiver, 

 some benefit might be obtained in the performance of the telephone 

 circuits in the presence of line noise by shifting the point of maximum 

 response of this instrument. Some cases had arisen where pronounced 

 harmonics in the power system in the neighborhood of 1,000 cycles 

 caused serious troubles in nearby telephone circuits and it was felt 

 that if this condition were found to be prevalent in power circuits, 

 some relief in the interference situation might be obtained by shifting 

 the point of maximum response away from this region. The investi- 

 gations which have been carried out under Project 4 of the N. E. L. A.- 

 Bell System Joint Development and Research Subcommittee, of the 

 noise on telephone lines in different parts of the country have shown 

 that the average distribution of energy with frequency is approxi- 

 mately uniform over the range from 300 to 2,000 cycles with, however, 

 a pronounced dip in the region around 1,000 cycles. A similar de- 

 crease in the energy of the components around 1,000 cycles is also 

 shown by the results of the investigations made under Project 5 on the 

 wave shapes of electrical power machinery. With this situation, 

 shifting the maximum response of the telephone receiver away from 

 its present location would thus in the average case be placing it in a 

 region in which larger amounts of interfering currents are to be found. 

 Moreover, examination of the data showing the distribution with 

 frequency of noise currents, indicates that on particular circuits this 

 distribution is by no means uniform but in many cases is materially 

 higher in the region below 1,000 cycles and in other cases materially 

 higher in the upper regions. It would not appear, therefore, that a 

 shift in the maximum response of the telephone receiver would on the 

 average give an improvement from the interference standpoint. 



