164 BELL SYSTEM TECHNICAL JOURNAL 



The purpose of this paper is to describe the principal transmission 

 developments which have taken place in the voice-frequency system 

 since the first commercial installation, to present some of its operating 

 characteristics, and to outline advances in maintenance methods which 

 have developed during this period. 



There has been marked and steady improvement in the quality of 

 the service rendered by the voice-frequency telegraph circuits during 

 the last decade within the Bell organization, and while it would be 

 unfair to overlook the part which imaginative management, employee 

 cooperation, and similar factors have played in securing this desirable 

 result, it appears certain that a good deal of it is to be credited to those 

 physical improvements and advances in testing and operating pro- 

 cedures which we are about to recite. 



Extensions in Utilized Frequency Range 



While the greater part of past experience has been had with the 

 application of voice-frequency systems to extra-light-loaded four-wire 

 cable circuits,* a considerable mileage now utilizes other types of 

 telephone facilities, particularly high-frequency carrier open-wire lines ^^ 

 through the less densely populated regions. These latter applications 

 are interesting to the transmission engineer because the association of 

 telephone and telegraph in the same repeaters brings into view new 

 problems which will doubtless grow in importance as the use of broad- 

 band carrier systems becomes more extensive. A typical voice- 

 frequency carrier-telegraph circuit is shown in Fig. 1. It consists of a 

 section of four-wire cable connected in tandem with a three-channel 

 type "C" carrier-telephone circuit,^*- ^^ without mechanical repetition 

 at the junction point. The telegraph power is appropriately modified 

 to suit the requirements of the two media by means of pads and 

 amplifiers P at the point where they join. In addition to this, the 

 remaining telephone circuits operating through the same carrier re- 

 peaters are equipped with volume limiters ^^ to prevent voice-energy 

 peaks, which contribute little if any to telephone quality, from over- 

 loading the amplifiers, thereby causing excessive distortion to the 

 telegraph. 



Shortly after the initial voice-frequency telegraph installation in 

 cables the number of channels was extended from 10 to 12 by the 

 addition of two channels at the upper end of the frequency range, 

 corresponding to carrier frequencies of 2125 and 2295 cycles. 



* These circuits are designated as H44. They consist of 19 AWG conductors, 

 loaded with 44-millihenry coils 6000 ft. (1830 m.) apart, with four-wire repeaters 

 spaced at approximately 50-mile (80.5 km.) intervals.^^ 



