Hybrid phone echo cancellation circuit

Question

I'm using a single board computer to talk on a phone line through two coils, one for the TX part, the other for the RX. The phone line is on another coil and then it's connected to a two-wire circuit that leads to an handset. As you know, the problem with this config is that if you build a full-duplex communication system with VoIP in the middle, you hear the echo from what you just transmitted on the network coming back See this connection scheme

Is there a circuit which is able to reduce or eliminate such echo? Right now I'm using a DSP to implement a simple subtractor (Rx=Rx-Tx), but maybe this common problem has been solved by clever analogue engineers back in the past.

Connection scheme with echo path in red

Answer 1

The analogue circuit is known as a "two-wire hybrid" and does exactly what you are doing with your DSP. Strictly speaking this is just sidetone elimination rather than echo cancellation which Kellenjb's link addresses. Below is a simple example (which is a discrete replacement for an obsolete Mitel part). This arrangement only works if the load impedance is precisely controlled - in this case 754\$\Omega\$.