MEASUREMENT OF TELEPHONE NOISE 



155 



connection with this revision, the name has been changed to "Tele- 

 phone Influence Factor." 



The Telephone Influence Factor (T.I.F.) of a voltage or current 

 wave is the ratio of the square root of the sum of the squares of the 

 weighted effective values of all the sine wave components (including, 

 in alternating current waves, both fundamental and harmonics) to 

 the effective value of the wave. The weightings decided upon to be 

 applied to the individual components are as shown in Fig. 2. 



i 4000 



400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400 4800 5200 

 FREQUENCY IN CYCLES PER SECOND 



Fig. 2 — Frequency weighting characteristic for TIF measurements. 



In deriving the revised frequency weighting characteristic, the 

 following factors representing distortion occurring in the various 

 media intervening between the power circuit current or voltage and 

 the telephone subscriber's ear were considered. 



1. Relative interfering effects of single-frequency components in the 



receiver of a subscriber's telephone set. 



2. Distortion occurring between the terminals of the circuit in which 



the noise is induced and the subscriber's receiver. 



3. Variation in coupling between power and telephone circuits with 



frequency. 



4. Variation of effects of telephone circuit unbalances with frequency. 



Data on Items 1 and 2 above were combined to derive the line 

 weighting characteristic of the telephone circuit noise meters indicated 

 by the "line currents" curve of Fig. 1. It was, therefore, possible to 

 use this curve directly to represent the combination of these two 



