TRANSMISSION DATA FOR RATING TELEPHONE CIRCUITS 341 



ence condition. Somewhat different losses would be obtained with 

 other loops and trunks, but the values obtained under the reference 

 conditions give sufficiently good approximations for practical purposes. 



The intermediate junction losses are corrections to be added to the 

 other elementary losses when the trunk is made up of more than one 

 type of facility. They include the volume reflection loss at the junc- 

 tion of the two facilities and a distortion correction which together 

 with the other distortion losses of the elements will give a total equal 

 to the distortion rating of the complete circuit relative to the reference. 



The data covering the six types of losses discussed above are set 

 up on the basis of the same electrical line noise and the same room 

 noise as those specified for the working reference circuit. 



Effective losses due to circuit noise may be presented in the form 

 of curves showing the loss due to any amount of circuit noise. These 

 losses are added to the other effective losses when the noise at the 

 receiving loop terminal differs from the noise on the reference circuit. 

 The amount of circuit noise on a particular circuit cannot usually be 

 predicted accurately from the design constants of the circuit, since it 

 depends largely on the characteristics of the disturbing circuits, the 

 coupling between disturbing and disturbed circuits, and characteristics 

 of the disturbed circuit which include random unbalances. Effective 

 losses due to circuit noise, therefore, must, in general, be based on 

 noise measurements rather than on the design constants of the tele- 

 phone circuits. 



Effective losses due to room noise may be added to the other effective 

 losses when the room noise at the receiving end differs from the room 

 noise associated with the reference system. Since the magnitude of 

 the room noise at a particular station is in no way a function of the 

 design of the telephone circuit this type of loss is in a somewhat 

 different class from the others. More than one curve is required for 

 presenting room noise loss since the loss depends to some extent on 

 the sidetone of the telephone set. 



The determination of a circuit rating from effective loss data is 

 simpler than may appear from the description of the data. Exchange 

 area circuits involve at most eight types of losses and most of these 

 circuits involve a smaller number. Three of these losses, the trans- 

 mitting and receiving loop losses and the terminal junction losses, are 

 determined from curves similar to Figs. 3 and 5. The remaining 

 losses, that is, the trunk, office, intermediate junction and noise losses, 

 are obtained from simple tables or curves. 



The definitions of losses have been set up so that the rating of an 

 element is obtained when the element is substituted for the corre- 



