anti-sidetone circuit

An anti-sidetone circuit is an electronic component within a telephone instrument designed to reduce "sidetone." Sidetone is the distracting sound of a speaker's own voice that is picked up by the transmitter and fed back into the receiver.12

This circuit uses transformers, resistors, and capacitors to achieve its function. It splits the transmitter's signal into two paths with opposite polarities. When these signals meet at a transformer, they largely cancel each other out before reaching the receiver. This prevents the speaker from hearing their own voice too loudly, while still allowing the incoming speech from the other end of the line to be heard clearly. The anti-sidetone circuit also helps match the electrical impedance between the telephone instrument and the telephone line for better current flow.2