An Explanation of the Common Battery Anti-sidetone Subscriber Set

HE 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 reception. 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 universally employed.