182 BELL SYSTEM TECHNICAL JOURNAL 



effects of interference on telegraph signals and at the same time cause 

 as little disturbance as possible to associated telegraph or telephone 

 circuits. In the Bell System, the four-wire cable circuits used for 

 telegraph purposes are for the most part devoted to this use exclusively 

 and are organized with terminal repeater gains adapted for this special 

 service. In the case of open-wire carrier-telephone circuits, on the 

 other hand, the overall gain from modulator input to demodulator 

 output is fixed by the telephone requirements, and the telegraph must 

 be adapted thereto. The power per telegraph channel in dbm.* now 

 used on cable and open-wire circuits is shown in Fig. 1 at various points. 



The effect of changing the power on the line is illustrated qualita- 

 tively in Fig. 11, in which the variations of the received current with 

 increasing transmitted current are sketched diagrammatically for 

 various operating circumstances. By nominal power {a), is meant 

 the power which would be received if transmission took place over a 

 linear network having a fixed gain equal to the nominal gain of the 

 circuit. Owing principally to the reduction of repeater gain which 

 takes place with increasing load, the current actually received with all 

 channels marking, is less than this (c). If only one channel is marking, 

 some intermediate values (6) will be obtained of course, while if, as in 

 the case of regular operation, some of the channels are spacing and 

 others marking, still other values {c') will result. This saturation 

 effect is sometimes called "crowding." 



One of the contributions to spacing interference consists of ambient 

 noise due to the combined crosstalk from all the other circuits in the 

 cable and to external induction. This current is represented by curve 

 e, which is shown as independent of the power transmitted ; this corre- 

 sponds to the situation existing where telegraph is a small part of the 

 total traffic in the cable under consideration, for evidently if the power 

 were increased on all the circuits the noise power would increase almost 

 proportionally. 



A more serious source of spacing interference consists of parasitic 

 currents due to third-order modulation products arising directly or 

 indirectly from the interaction of the several channels of the same 

 system when passing through the non-linear elements of the circuit. 

 Second-order modulation products are taken care of quite effectively 

 by the receiving filters, due to the fact that the carriers are odd har- 

 monics of 85 cycles, while these products being even harmonics thereof 

 fall midway between channel frequencies. Since the attenuation of 

 the receiving filters in the frequency range occupied by neighboring 



* The symbol dbm. as used in this paper may be read "db referred to 1 milliwatt." 

 It is intended to denote the ratio expressed in db of the power under consideration to 

 1 milliwatt; e.g., — 6 dbm. = .2512 milliwatt. 



