370 BELL SYSTEM TECHNICAL JOURNAL 



and receiver. Even if this point is not reached, but is approached 

 within 6 db or so, impairment of quahty results from incipient oscilla- 

 tion. The greater sidetone circuit loss of the anti-sidetone circuit 

 provides an additional margin of safety against any such condition. 



The granular carbon of the transmitter, and the design of the 

 transmitter itself must be carefully controlled, or serious noise — 

 transmitter "burning" — will cause noise in the receiver of the set. 

 The mechanical and electrical wear and tear of service tend to make 

 this transmitter noise worse. In the new transmitters this "burning" 

 has been kept at a low inherent value throughout life. The anti- 

 sidetone circuit, however, provides a margin of safety against the 

 small amount remaining, so that with this set there is less likelihood 

 of transmitter noise causing impairment of reception. 



Reduced Susceptiveness to Interference 



It will be noted from the schematic circuit drawing Fig. 2 that two 

 condensers are used in the new sets, one in the anti-sidetone trans- 

 mission circuit, and a separate one with the ringer. In some types 

 of party line practice the ringer of the set is connected for some 

 parties from one side of the line, and for the others from the other 

 side to ground. 



Figure 6 shows schematically two such ringing arrangements during 

 the conditions of conversation, 6a as used in the new sets, and 66 

 with one condenser common to transmission and ringing circuits. 

 It will be noted that in the standard circuit adopted (Fig. 6a), if any 

 longitudinal noise voltages exist between the central office and station 

 grounds, there is an equal voltage drop from each side of the line to 

 ground through the ringing paths (assuming the two ringer condenser 

 paths to be identical). The voltage drop across the terminals of the 

 talking set is therefore zero and no noise results. 



If the arrangement of Fig. dh, corresponding closely to previous 

 designs, were used, however, this condition would not obtain. The 

 condenser of the station in use being common to the transmission 

 circuit as well as the ringing circuit, the noise voltage drop across this 

 condenser is introduced in the transmission circuit. In addition there 

 are other paths to ground from each side of the line through the 

 transmission circuit which are not of equal impedance. The net result 

 is a residual noise current through the receiver of the talking circuit. 



In the actual case, the impedance of all ringers and condensers is 

 not identical and there are often more parties connected to one side 

 of the line than the other. Even under these relatively unfavorable 

 conditions, however, the two-condenser arrangement adopted reduces 



