182 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



of magnitude as the telephone current. This permitted a much greater 

 reduction of the telegraph-flutter impairments than that which could have 

 been obtained at a reasonable cost by using larger coils, A disadvantage of 

 this system, however, was that it halved the number of superposed telegraph 

 circuits per toll cable quad. 



The other new telegraph system was a voice-frequency carrier system^^ 

 which became commercially available during 1923 soon after the direct 

 current metallic system just mentioned. This provided a total of 10 inde- 

 pendent telegraph channels over the side circuits of special groups of H174- 

 63 4-wire circuits used exclusively for telegraph service, and consequently 

 there could be no telegraph-flutter reactions on telephone transmission. On 

 the other hand, the need for controlling intermodulation effects among the 

 associated carrier-telegraph channels imposed limits upon the allowable non- 

 linear distortion in the loading coils. The loading coils then standard (hav- 

 ing compressed, unannealed, iron-powder cores) satisfactorily met the re- 

 quirements, and with a greater margin than did the repeaters which were 

 standard at that time. Interference considerations, however, prevented the 

 general use of the metallic polar telegraph systems on loaded cable pairs 

 used for carrier telegraphy. 



The voice-frequency carrier-telegraph system soon became the most 

 widely used telegraph system in the long distance toll cables. It did not 

 require special facilities, but made effective use of the whole frequency- 

 band provided for voice telephony. The initial number of channels was 

 expanded to 12 on HI 74-63 facilities, and subsequently to a total of 24 

 channels by using the wider-band H44-25 facilities, previously described. 

 The present-day system is limited to 18 channels, however, in order to 

 permit the ready interconnection of loaded cable and broad-band telephone 

 facilities in tandem. A detailed account of these and other telegraph improve- 

 ments is given in a 1940 B.S.T.J. paper^i. 



The strong-current, composite-grounded d-c telegraph system is very 

 seldom used on modern toll cables. There is, however, a considerable use of 

 the strong-current grounded d-c telegraph system on a simplex-phantom 

 basis. Under this service condition, the telegraph current does not magnetize 

 the loading coil cores and consequently there is no telegraph-flutter inter- 

 ference with telephone transmission. The metallic polar d-c telegraph system 

 is usually limited to a few voice repeater-sections, because of the modern 

 severe limits on telegraph-flutter impairments upon telephone transmission, 

 and partly for economic reasons. 



The improvements in telegraphy over loaded toll cables also included 

 arrangements which were developed in 1922 for the purpose of reducing 

 interference between d.c. telegraph circuits when superposed by the com- 

 positing method on wires of the same loaded cable quad.^ This interference, 



