i.o.inixc hOR ri'LiU'jfoxii circuits 



271 



of xariations in the frcciuency distortion cuul Nolunie efficiency of a 

 telephone circuil on the capability of the circnit to transmit and re- 

 produce inteiligil)le speech. 



In the cost studies, allowance was made for the reduced costs of the 

 new types of cable facilities, and the use of less expensive types of 

 coils proportioned to be in approximate cost-equilibrium with these 

 facilities. On the basis of these new cost relations, it was found that 

 an increase in the cut-off frequency of exchange area loading could be 

 justified. Further studies showed that if the cut-off frequency should 

 be raised materially above 3,000 cycles the increased costs would be 

 large in proportion to the resultant impro\'ement in transmission. 

 On this basis, it was decided to adopt 2,900 cycles as the cut-off 

 frequency standard for the new loading, when used on higher capaci- 

 tance cables. This corresponds to a cut-off frequency of approxi- 

 m.ately 3,200 cycles on the older types of low capacitance cables. 



New Standard Loading Systems. Having decided upon a new 

 cut-off frequency standard, the next step was to choose coil spacings 

 and inductance \alues. Obviously, in order to make full use of 

 available loading manholes and vaults, it is desirable to adhere to the 

 established spacing standards. Also, it is desirable to make as much 

 use as practicable of the old standard loading coil inductance values, 

 in order to minimize the expense of rearranging the existing loadmg 

 to conform to the new standards. Furthermore, there are important 

 advantages in having a graded series of standards. This avoids 

 economic waste otherwise involved in the use of expensive loading on 

 trunks where a less expensive loading is good enough. 



The foregoing general considerations resulted in the standardization 

 of certain new loading systems, the principal transmission features of 

 which are summarized in Table XV. 



TABLE XV 



New Loading Standards for Excliange Area Trunks 



Loading 

 Designation 



M-88 

 H-135 

 H-175 

 D-175 



Coil 



.Spacing 



Feet 



9000 

 6000 

 6000 

 4500 



Coil 

 Code 



Nos. 



602 

 603 

 57-t 

 574 



Approx. Cut-off Frequency 

 Cycles 



High 



Capacitance 



Cable 



2900 

 2800 



2900 



Low 



Capacitance 



Cable 



3200 

 3200 

 2800 

 3200 



Note. High capacitance cable has approximately 0.083 ixf per mile. Low capacit- 

 ance cable has approximately 0.066 nf per mile. 



