ABSTRACT
Echo cancellation, which is a key ingredient towards high-quality
communications, appears to be a very challenging filtering task
due to a number of inherent peculiarities. Figure 6.1 depicts a
schematic representation of the echo cancellation problem. In an
ideal communication setting, the loudspeaker at the far-end would
either exclusively transmit the voice of the person at the near-end or
nothing if this person is silent. This should be the case irrespective
of whether the person at the far-end is speaking or not. However,
in realistic conditions, this is not true; the far-end speech signal
travels through the communication channel and it is transmitted via
the loudspeaker in the near-end room. Due to the room impulse
response and delays in the communication line, the microphone
at the near-end captures a filtered version of the far-end speech,
which is transmitted back to the far-end in the form of annoying
echo. In wired communications, even when a room is not involved in
echo generation, known as acoustic echo, electrically induced echo
appears due to unbalanced coupling between the 2-wire and 4-wire
circuits. The echo cancellation task aims at estimating the impulse
response corresponding to the echo path. Doing so, the echo can
be reproduced at the far-end and be subtracted from the received
signal. In this way, the received signal is cleansed from the echo
before its transmission through the loudspeaker.
