For many decades, hybrid devices have been used to connect 2-wire local
circuits and 4-wire long distance circuits in telephone lines. This
leads to a weIl known problem, whereby echoes are generated. The delay
introduced by telecommunication satellites exacerbated this problem and
the need for new methods of echo control soon became obvious. The best
solution to date for solving this problem was invented in the 1960s at
Bell Labs by Kelly, Logan, and Sondhi, and consists of identifying the
echo path generated by the hybrid by means of an adaptive filter, a
technique that became known as an echo canceler. The echo canceler
allowed full-duplex communication which was not possible with older echo
suppression techniques. Later, with the development of hands-free
teleconferencing systems, an- other echo problem appeared; but this time
the echo was due to the coupling between the loudspeaker and microphone.
It is not surprising that the same solution was proposed to solve this
problem, and most of today's telecon- ferencing systems have an acoustic
echo canceler. More recently, attention has been given to the very
interesting problem of multichannel acoustic echo cancellation, which
leads to more exciting applications that take advantage of our binaural
auditory system.