03-07-2013, 01:04 PM
ECHO CANCELLATION
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INTRODUCTION
Echo is the repetition of a waveform due to reflection from points
where the characteristics of the medium through which the wave
propagates changes. Echo is usefully employed in sonar and radar for
detection and exploration purposes. In telecommunication, echo can
degrade the quality of service, and echo cancellation is an important part of
communication systems. The development of echo reduction began in the
late 1950s, and continues today as new integrated landline and wireless
cellular networks put additional requirement on the performance of echo
cancellers. There are two types of echo in communication systems: acoustic
echo and telephone line hybrid echo. Acoustic echo results from a feedback
path set up between the speaker and the microphone in a mobile phone,
hands-free phone, teleconference or hearing aid system. Acoustic echo may
be reflected from a multitude of different surfaces, such as walls, ceilings
and floors, and travels through different paths. Telephone line echoes result
from an impedance mismatch at telephone exchange hybrids where the
subscriber's 2-wire line is connected to a 4-wire line. The perceptual effects
of an echo depend on the time delay between the incident and reflected
waves, the strength of the reflected waves, and the number of paths through
which the waves are reflected. Telephone line echoes, and acoustic
feedback echoes in teleconference and hearing aid systems, are undesirable
and annoying and can be disruptive. In this chapter we study some methods
for removing line echo from telephone and data telecommunication systems,
and acoustic feedback echoes from microphone–loudspeaker systems.
Acoustic and Hybrid Echoes
Echo can severely affect the quality and intelligibility of voice conversation
in a telephone system. The perceived effect of an echo depends on its
amplitude and time delay. In general, echoes with an appreciable amplitude
and a delay of more than 1 ms are noticeable. Provided the round-trip delay
is on the order of a few milliseconds, echo gives a telephone call a sense of
“liveliness”. However, echoes become increasingly annoying and
objectionable with the increasing round-trip delay and amplitude in
particular for delays of more than 20 ms. Hence echo cancellation is an
important aspect of the design of modern telecommunication systems such
as conventional wireline telephones, hands-free phones, cellular mobile
(wireless) phones, or teleconference systems.
Telephone Line Hybrid Echo
Hybrid echo is the main source of echo generated from the public-switched
telephone network (PSTN). Echoes on a telephone line are due to the
reflection of signals at the points of impedance mismatch on the connecting
circuits. Conventionally, telephones in a given geographical area are
connected to an exchange by a 2-wire twisted line, called the subscriber's
lineline, which serves to receive and transmit signals. In a conventional
system a local call is set up by establishing a direct connection, at the
telephone exchange, between two subscribers’ loops.
Hybrid Echo Suppression
The development of echo reduction began in the late 1950s with the advent
of echo suppression systems. Echo suppressors were first employed to
manage the echo generated primarily in satellite circuits. An echo
suppresser (Figure 14.4) is primarily a switch that lets the speech signal
through during the speech-active periods and attenuates the line echo during
the speech-inactive periods. A line echo suppresser is controlled by a
speech/echo detection device. The echo detector monitors the signal levels
on the incoming and outgoing lines, and decides if the signal on a line from,
say, speaker B to speaker A is the speech from the speaker B to the speaker
A, or the echo of speaker A. If the echo detector decides that the signal is an
echo then the signal is heavily attenuated. There is a similar echo
suppression unit from speaker A to speaker B. The performance of an echo
suppresser depends on the accuracy of the echo/speech classification
subsystem.
Adaptive Echo Cancellation
Echo cancellation was developed in the early 1960s by AT&T Bell Labs
and later by COMSAT TeleSystems. The first echo cancellation systems
were experimentally implemented across satellite communication networks
to demonstrate network performance for long-distance calls.
Figure 14.5 illustrates the operation of an adaptive line echo canceller. The
speech signal on the line from speaker A to speaker B is input to the 4/2
wire hybrid B and to the echo canceller. The echo canceller monitors the
signal on line from B to A and attempts to model and synthesis a replica of
the echo of speaker A. This replica is used to subtract and cancel out the
echo of speaker A on the line from B to A. The echo canceller is basically
an adaptive linear filter. The coefficients of the filter are adapted so that the
energy of the signal on the line is minimised. The echo canceller can be an
infinite impulse response (IIR) or a finite impulse response (FIR) filter. The
main advantage of an IIR filter is that a long-delay echo can be synthesised
by a relatively small number of filter coefficients. In practice, echo
cancellers are based on FIR filters. This is mainly due to the practical
difficulties associated with the adaptation and stable operation of adaptive
IIR filters.
Echo Cancellation for Digital Data Transmission
Echo cancellation becomes more complex with the increasing integration of
wireline telephone systems and mobile cellular systems, and the use of
digital transmission methods such as asynchronous transfer mode (ATM)
for integrated transmission of data, image and voice. For example, in ATM
based systems, the voice transmission delay varies depending on the route
taken by the cells that carry the voice signals. This variable delay added to
the delay inherent in digital voice coding complicates the echo cancellation
process.
The 2-wire subscriber telephone lines that were originally intended to
carry relatively low-bandwidth voice signals are now used to provide
telephone users with high-speed digital data links and digital services such
as video-on-demand and internet services using digital transmission
methods such as the asynchronous digital subscriber line (ADSL).