TRANSMISSION FEATURES OF NEW TELEPHONE SETS 365 



in addition to three transformer windings, a balancing network. The 

 circuit, made up of the four elements: transmitter, receiver, line, and 

 network, coupled by the transformer, functions in such a manner that 

 the transmitter and receiver are in conjugate relationship, i.e., voltages 

 produced by the transmitter are balanced out and do not affect the 

 receiver. Theoretically, such a circuit, with pure resistance elements, 

 can be perfectly balanced at all frequencies with complete elimination 

 of sidetone, and at the same time be as efficient as can any transformer 

 coupling in an invariable telephone set,^ for the transfer of power from 

 the transmitter to the line, and from the line to the receiver. 



This type of circuit is not new in principle, and many varieties are 

 known and have been described.® Many of these arrangements, for 

 one reason or another, are not suitable for application. Some, for 

 example, call for impedances of transmitters or receivers differing 

 widely from those available. Certain others are not economical for 

 common battery service, where the transmitter must receive its battery 

 supply from the line. Still others require relatively complicated and 

 expensive cording and switchhook arrangements. The circuit which 

 has been chosen for general common battery subscriber station applica- 

 tion, and shown schematically in Fig. 2, is not only as simple and as 

 easily adapted to Bell System conditions as any, but permits a coil 

 design which is economical to manufacture as well as efficient in 

 performance. Other types of anti-sidetone circuit have been adopted 

 for local battery station service and for operators' telephone sets. 



The theory of operation of this anti-sidetone circuit has already 

 been discussed elsewhere.'^ It is intended here to show the general 

 purposes of the application, some of the considerations involved in the 

 design, and the kind of results accomplished. 



While in theory complete elimination of sidetone is possible, as well 

 as ideal efficiency of transformation, in practice neither objective 

 can be entirely realized. The unavoidably wide variations in line 

 impedance looking from the set, ranging from high positive to high 

 negative phase angle, and from a few hundred to more than a thousand 

 ohms in magnitude, together with other practical departures from 

 ideal conditions, necessitate a choice between a high degree of side- 

 tone balance and the standardization of a minimum number of coil 

 designs. The variations in loop length and resistance, by their effect 

 on transmitter battery supply, and consequently on transmitter re- 

 sistance, furthermore cause variations in the absolute transmitting 

 and sidetone efficiency of the terminal set, which must be taken into 

 account in the station circuit design. 



The actual design chosen is so arranged as to favor sidetone balance 



