An Explanation of the Common Battery Anti-sidetone 

 Subscriber Set 



By C. O. GIBBON 



THE telephone transmitter serves to convert sound waves into 

 their electrical facsimile; but in performing this primary function 

 the transmitter also acts as an amplifier. Under some conditions the 

 electrical power output of a transmitter may be more than a thousand 

 times as great as the acoustic power activating it. Part of this greatly 

 augmented power is dissipated in the circuit of the telephone set; part 

 is impressed upon the telephone line, whence it is propagated on to 

 the distant listener; and part finds its way into the receiver of the 

 same set, where it is reconverted into sound waves. Speech or noise, 

 picked up by the transmitter and reproduced by the receiver of the 

 same set, is called sidetone. 



Noise picked up and amplified by the transmitter and heard as 

 sidetone tends to obscure incoming speech, thereby impairing re- 

 ception. Similarly, the sound of his own voice, heard more loudly 

 than normal as sidetone because of transmitter amplification, impels 

 the talker involuntarily to lower his voice; thus impairing the reception 

 of his speech at the far end of the connection. The consequent 

 desirability of reducing sidetone has long been recognized, and operator 

 and subscriber sets which accomplish this have been developed. 



Circuit schematics of the common battery sidetone and anti-sidetone 

 subscriber sets at present standard in the Bell System are shown in 

 Fig. 1. The anti-sidetone set has become increasingly common during 

 the past few years, and because of the improvements in effective 

 transmission which it affords, bids fair ultimately to be well nigh uni- 

 versally employed. It is, therefore, not surprising that numerous 

 requests have arisen for an explanation of this anti-sidetone circuit which 

 may be more easily followed than one based on the methods usually 

 employed in network analysis. The present paper provides an ex- 

 planation by means of diagrams with a minimum of mathematical 

 treatment which it is believed those to whom the mathematical approach 

 does not appeal will find helpful in picturing the behavior of this circuit. 



The explanation given in this paper is, however, confined to idealized 

 conditions. No concern is given to whether the conditions necessary 

 to exact attainment of the balances described are actually feasible; 



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