One of the most frustrating things that can occur during a phone conversation or conference call is an annoying echo that becomes a distraction to the ongoing discussion. Even with today’s advanced telecommunications systems, it may get to the point where the only solution is to hang up, dial again and hope for a better connection.
There are many reasons why echoes occur during phone conversations. Electrical echoes can result from a hybrid connection (2 to 4 wire conversion) in the telephony device or in the network. Line and network echoes often have different characteristics, but can occur for the same reason. Line echo cancellers work in the same device as the hybrid that causes the echo. Network echo cancellers are often implemented in VoIP gateways and conference servers where the source of the echo occurs somewhere else in the network which creates additional challenges.
A network echo canceller is used in VoIP networks where delays can be introduced by audio encoding and decoding, packet transmission, and the network itself. In networks, the round-trip between the near-end speaker and far-end speaker can easily be on the order of 100ms. When the round-trip delay is no more than 25ms the listener is unable to distinguish the returning echo from the original speech.
In VoIP networks, there could be multiple hybrids, other echo cancellers and even other voice codecs. As such, a network echo canceller must not only be able to handle multiple reflections introduced by the hybrids but also handle no perceived reflections as a result of the other echo cancellers. Voice codecs, in the echo path between the echo canceller and the echo source, transform the echoes, often introducing non-linearities and adding further delays. An echo canceller must be able to handle all of these situations effectively.
The echo canceller is a technology designed to make all of our telecommunications easier. An echo canceller must be sufficiently robust to handle many different situations while providing rapid convergence yet preventing divergence due to doubletalk (both speakers talk simultaneously). In addition, the echo canceller must be able to correctly deal with fax and low speed data transmissions over the same media.
