INDUCTIVE LOADING FOR TELEPHONE FACILITIES 179 



repeaters and their associated distortion-corrective networks, and to learn 

 how to use regulating repeaters to control satisfactorily the very large 

 transmission changes that resulted from temperature variations in long cir- 

 cuits. 



During the mid 1920's aerial cable came into use extensively along new 

 cross-country routes where underground cable conduits would have been 

 unduly expensive. Such cables, however, presented added difficulties and 

 expense in transmission regulation since the temperature range variations 

 are about three times as great as in underground cable. During 1930 the use 

 of buried cables started. With respect to transmission regulation and service 

 continuity, they compare well with cables in underground conduit and are 

 less expensive, when the number of cables along the route is small. However, 

 buried toll cable generally tends to be more expensive than aerial cable. At 

 the end of 1949 the aggregate wire mileage in buried toll cables was nearly 

 one-third of that in aerial toll cables. 



A substantial amount of H44-25 loading was also used on 16-gauge 2-wire 

 repeatered facilities, for circuit lengths intermediate between the trans- 

 mission limits of HI 74-106 and HI 74-63 loading and lengths where echo- 

 impairment difficulties made necessary the use of 19-gauge 4-wu-e facilities. 

 In such two-wire circuits, very good line-balance was of course required at 

 the intermediate repeaters. An important economic factor in this "medium" 

 long-haul practice was the 50% lower loading cost per unit of facility length 

 of the two-wire circuits. This intermediate-length usage of 16-gauge 2-wire 

 circuits, however, tapered off in the long-distance plant of the A.T.&T.Co. 

 during the late 1920's, so as to obtain the important plant flexibility and 

 operating advantages that were inherent in the general use of 19-gauge 

 4-wire circuits for medium-haul and long-haul toll cable facilities. 



Notwithstanding the 2 : 1 ratio in loading cut-off frequencies, the width of 

 the frequency-band transmitted over long H44-25 circuits was not much 

 wider than that transmitted over the much shorter HI 74-106 facilities. 

 This was largely due to the filters which were used to suppress the upper 

 half of the H44-25 transmission band, primarily for the purpose of reducing 

 the very serious velocity-distortion transmission impairments that would 

 have otherwise resulted in very long circuits. This suppression of the higher 

 frequencies also eased the transmission equalization and regulation problems. 



The attenuation-frequency distortion characteristics of the old and new 

 toll cable loading are shown in Figs. 6 and 7. 



Figure 6 (p. 175) shows the attenuation in db per mile. Figure 7 (p. 180) 

 gives the bareline attenuation-frequency curves under specified circuit length 

 and repeater conditions hi which the total 1000-cycle attenuation is 10 

 db. The effect on frequency distortion of raismg the loading cut-off fre- 

 quency, and of usmg distortion corrective-networks, is clearly indicated, 

 (d) H 174-63 Loading: Before concluding this summary of the basic develop- 



