Ever since Voice over Internet Protocol (VoIP) was introduced, all calls going to the Public Switched Telephone Network (PSTN) have been plagued with line or electrical echo. I often get asked where these electrical echoes come from and why it is inevitable when using VoIP. We’ve given many seminars on the subject, especially in the Asterisk space where people are suddenly confronted with this problem when building equipment. I’ve put together a basic introduction explaining why echo cancellation is needed when using VoIP over PSTN.

Electrical or line echo is inherent to the PSTN, it has always been present and cannot be avoided because of the nature of the lines connected to homes and offices. These local or subscriber loop circuits use two wires to carry the voice signals while the voice channels use four wires for bi-directional communications beyond the first switch that is the local exchange. The conversion between the four wire and two wire electrical circuits is done by transformer called a telephone hybrid whose goal is to separate the signals’ directions and adapt the impedance of both circuits. Like all analog circuits, the hybrids can’t perfectly match the impedances and that causes part of the signal …