Acoustic Echo Cancellation
Echo Cancellation is the process of removing echo from transmitted audio signals. This process is carried out with the help of echo suppressors and echo cancellers. Sometimes they are used simultaneously. There are two types of echo: acoustic and electric (hybrid).
There are three types of audio conferencing systems: distributed, local and integrated.
Acoustic Echo: The acoustic echo appears due to regenerative coupling. That happens when the sound from the handsfree speaker enters the microphone in the same room.
There are two types of regenerative coupling:
» Mechanical connection between a microphone and a speaker;
» Reflection from the elements surrounding a device.
The following suppression algorithm is applied to eliminate acoustic echo:
» A signal is sent from a distant room;
» The nearest speaker reproduces this signal;
» The signal is received by a microphone at the nearest room;
» At the same time, the signal received from a distant room is being filtered and delayed in order to look like the same signal, that got into a microphone at the nearest room;
» The signal from the near side is subtracted from the filtered distant signal;
» As a result, the given audio signal does not contain echoes anymore and can be transmitted further.
Electric (Hybrid) Echo: This type of echo appears when an electric signal from the data transmission line interferes with the data reception line. This kind of echo exists in analog communication lines and absent in digital lines.
Author: TrueConf Team