26-07-2014, 03:43 PM
ECHO CANCELLATION
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Introduction:
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. There are two types of echo in
a telephone system (Figure 14.1):
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. For a local call, there
is usually no noticeable echo either because there is not a significant
impedance mismatch on the connecting 2-wire local lines or because the
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
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
Summary
Telephone line echo and acoustic feedback echo affect the functioning of
telecommunication and teleconferencing systems. In general, line echo
cancellation, is a relatively less complex problem than acoustic echo
cancellation because acoustic cancellers need to model the more complex
environment of the space of a room.
We began this chapter with a study of the telephone line echoes arising
from the mismatch at the 2/4-wire hybrid bridge. In Section 14.2, line echo
suppression and adaptive line echo cancellation were considered. For
adaptation of an echo canceller, the LMS or the RLS adaptation methods
can be used. The RLS methods provides a faster convergence rate and better
overall performance at the cost of higher computational complexity.
In Section 14.3, we considered the acoustic coupling between a
loudspeaker and a microphone system. Acoustic feedback echo can result in
howling, and can disrupt the performance of teleconference, hands-free
telephones, and hearing aid systems. The main problems in implementation
of acoustic echo cancellation systems are the requirement for a large filter to
model the relatively long echo, and the adaptation problems associated with
the eigenvalue spread of the signal. The sub-band echo canceller introduced
in Section 14.4 alleviates these problems.
[attachment=66283]
Introduction:
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. There are two types of echo in
a telephone system (Figure 14.1):
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. For a local call, there
is usually no noticeable echo either because there is not a significant
impedance mismatch on the connecting 2-wire local lines or because the
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
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
Summary
Telephone line echo and acoustic feedback echo affect the functioning of
telecommunication and teleconferencing systems. In general, line echo
cancellation, is a relatively less complex problem than acoustic echo
cancellation because acoustic cancellers need to model the more complex
environment of the space of a room.
We began this chapter with a study of the telephone line echoes arising
from the mismatch at the 2/4-wire hybrid bridge. In Section 14.2, line echo
suppression and adaptive line echo cancellation were considered. For
adaptation of an echo canceller, the LMS or the RLS adaptation methods
can be used. The RLS methods provides a faster convergence rate and better
overall performance at the cost of higher computational complexity.
In Section 14.3, we considered the acoustic coupling between a
loudspeaker and a microphone system. Acoustic feedback echo can result in
howling, and can disrupt the performance of teleconference, hands-free
telephones, and hearing aid systems. The main problems in implementation
of acoustic echo cancellation systems are the requirement for a large filter to
model the relatively long echo, and the adaptation problems associated with
the eigenvalue spread of the signal. The sub-band echo canceller introduced
in Section 14.4 alleviates these problems.