502 BELL SYSTEM TECHNICAL JOURNAL 



Transmission Characteristics of Line and Repeater. Fig. 7 shows 

 the attenuation frequencN' characteristic of a typical repeater section 

 expressed in TU? The ampHfication frequency characteristic of 

 the telephone repeater shown in Fig. 8 is such as to compensate for 

 the inverse characteristic of the line circuit so that the n\iT-al] trans- 

 mission characteristic of the circuit will be uniform o\cr the im- 

 Ijortant frequencies of the speech range as illustrated by Fig. 9. 



Signaling^. On the shorter of these circuits employing only a few 

 repeaters, signaling current is relayed at each repeater point, new 

 energy being sent into the adjacent section of the line by the oper- 

 ation of relays associated with the repeater. 135-cycle current is 

 used for the signaling current sent over the line, this being the fre- 

 quency commonly used for signaling over composited circuits. On 

 longer circuits employing several repeaters, the time lag of the ring 

 can be decreased by a system employing a combination of amplified 

 and relayed ringing at alternate repeater stations. At points where 

 the ring is amplified, it is necessary to increase the repeater amplifi- 

 cation at 135 cycles in order that sufficient ringing energy may reach 

 the relaying repeater point to operate the ringing relays. This is 

 accomplished by making slight changes in the input circuit of the 

 repeater to increase its efficiency at the lower frequencies as illus- 

 trated by the dotted curve in Fig. 8. Best results arc obtained by 

 relaying at alternate repeater points. 



At each relayed ringing point a certain time inlerxal is rc(|uired 

 f<jr the operation of the relays and for this reason the length of time 

 during which the ringing current is applied to the line may become 

 less and less for each succeeding repeater. If the ring, therefore, 

 is not of sufficient duration, it is likely that sufficient ringing energy 

 to operate the line signal will not be received at the distant terminal. 

 This has introduced some operating difficulties and made it neces- 

 sary to exert great care in the maintenance of the apparatus at the 

 intermediate as well as at the terminal stations invohed and careful 

 oNcrall checking and lining uj) of the circuit as a whole. 



There has been developed a system employing signaling currents 

 of voice frequency which has largely oNcrcome these difficulties. 

 The signaling current is amplified b\' the repeaters with approximately 

 the same efficiency as the voice currents so that relaying is unneces- 

 sary. Particular attention has been given in the design of the system 

 to preventing false operation of the signals from Noicc or extraneous 

 currents. 



'See article in tills Issue "The Transmission I'liit .iml l\'K|ilioiu' rr.iiisiiiissioii 

 Reference System," by W. H. Martin. 



