LOAD RATING THEORY 



629 



For calls on typical toll circuits, the best present values for the param- 

 eters are Vo = — 16.0 db and a = 5.8 db. These parameters de- 

 pend, of course, upon the character of the local plant and upon the 

 habits of telephone users, and changes in either will affect their values. 

 Curve A of Fig. 1 shows this distribution of talker volumes at a point 

 of zero transmission level. Curve B of Fig. 1 is the talker volume 

 distribution used for load rating computations when a particular 

 amount of peak amplitude limiting occurs in the terminal equipment. 

 This will be discussed later. Although the mean volume is Vo, the 



1.0 



lij 0.7 

 U 



<£ 0.6 

 < 



CD 0.3 



-25 -20 -15 -10 -5-5 5 



TALKER VOLUME IN DECIBELS (ABOVE REFERENCE VOLUME) 



Fig. 1 — Talker volume distribution. 



volume corresponding to the mean power of the distribution (3) is 

 equal to Vo + .115a•^ as may be seen by converting the volume scale 

 of the distribution to power ratios, averaging, and reconverting the 

 average to volume in db. For the values of the parameters given 

 above, Vo + M5a^ = - 12.1 db. 



Instantaneous Voltage Distribution 

 Finally, the voltage in an active channel fluctuates widely even at 

 constant volume. Not only the differences between successive syl- 

 lables and the differences between vowel sounds and consonants, but 

 also the fane structure of single sounds, are important in this connec- 



