Measurement of Telephone Noise and Power 

 Wave Shape * 



By J. M. BARSTOW, P. W. BLYE and H. E. KENT 



IN Studies of the inductive coordination of power and telephone 

 systems from the noise standpoint, a knowledge of the magnitudes 

 of the harmonic currents and voltages on the power circuits and of the 

 harmonic components of the telephone circuit noise is necessary. It 

 is also necessary that there be available a means of rating and summing 

 up these individual components to give an overall indication of their 

 effects on a person using a telephone connected to one of the exposed 

 circuits. This paper discusses methods which have been developed 

 for making such overall measurements. 



The effect on a listener of a given amount of noise on a telephone 

 circuit is a complex one, and it is not practicable in the day-by-day 

 maintenance of telephone circuits to measure separately all the factors 

 involved. Rather, it is necessary to make some overall measurement of 

 the circuit noise which may be related to its effect on telephone trans- 

 mission. It is, of course, desirable that the measuring devices used 

 should measure different circuit noises as equal when they produce 

 equal interfering effects on telephone transmission. 



Two methods of measuring telephone circuit noise are at present 

 in use in the Bell System. One of these methods is subjective, that is, 

 uses the human hearing mechanism as a part of the measuring ap- 

 paratus. This method consists of comparing, in a telephone receiver, 

 the noise to be measured with a noise generated by means of a standard 

 buzzer. The observer adjusts the magnitude of the buzzer noise by 

 means of a calibrated potentiometer until, in his judgment, it is as 

 disturbing as the noise to be measured. 



The objective method of noise measurement which has been made 

 available within the last few years employs an electrical network for 

 weighting the various single frequency components of a noise as closely 

 as practicable in accordance with their interfering effects on telephone 

 transmission, and a calibrated amplifier to raise the energy level of the 

 weighted components sufficiently to operate an electric meter. The 

 chief operating advantages of the objective method are the repro- 



* Digest of a paper published in the December 1935 issue of Electrical Engineering 

 and scheduled for presentation at the A.I.E.E. Winter Convention, New York, N. Y., 

 January 28-31, 1936. 



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