Antisidetone Circuits

Antisidetone Circuit Using a Balancing Network.^6,7 The balancing network of Fig. 8 is used in several antisidetone sets. It is the circuit of Fig. 7 with the addition of the balancing network C-N.

Figure 8. An antisidetone telephone circuit with network C-N for reducing sidetone.

In the circuit of Fig. 7 (a) the transmitter forces speech currents through coil B, and also through coil A and the receiver, this current causing sidetone. Coil C of Fig. 8 is wound on the magnetic core of the induction coil (transformer) in such a direction that the voltage induced in it tends to cause speech currents to flow through the receiver in the direction opposite to the flow of Fig. 7(a). This cancellation reduces sidetone. The resistor N, which may be the resistance of coil C, is effective in producing the correct phase relations in the circuit so that the sidetone cancellation is effective over a wide frequency range.

Antisidetone Circuit Using a Hybrid Coil. The theory of the hybrid coil is considered on page 401. Briefly, the circuit of Fig. 9 is so adjusted that the output of the transmitter divides, and, for the correct values of C_w and R_w (which may be the resistance of coil P), the speech currents flowing through coils P and S are equal. Since these currents will also be opposite in direction, the output of the transmitter produces (theoretically, and at a given frequency) no induced voltage in coil A, and no sidetone.

Figure 9. Simplified connections of an antisidetone circuit using a hybrid coil, or bridge transformer.