Speech Enhancement - Digital Speech: Coding for Low Bit Rate Communication Systems - page 405

Four-wire
Trunk

Two-wire
Subscriber loop

Two-wire
Subscriber loop

Hybrid Hybrid

Echo Echo

S1 S2

Figure 11.21 Block diagram of duplex connection between two subscribers

a round trip delay of approximately 540ms (270ms each way) is possible; this
makes the echo very disturbing and may in fact make it impossible to carry
out a conversation. In such cases, it is essential to control or remove the echo.
Since the subjective disruption of echo is proportional to the round trip delay
as well as the echo energy level, the echo control techniques usually depend
on the circuit length.
Some international connections use a switch (called an echo suppressor)
operated by the voice activity, which attempts to impose an open circuit on
the return path from listener to talker when the listener is silent. However,
an echo suppressor cannot operate during double-talk and hence produces
choppy echo. For this reason, echo suppressors are now being replaced by
echo cancellers that are based on adaptive filtering techniques.

11.4.1 DigitalEchoCancellerSet-up
A block diagram of an echo canceller for one direction of transmission is
shown in Figure 11.22, where the far-end talker signal is denoted by y(i) ,
the unwanted echo signal by r(i) , and the near-end talker signal by x(i) .The

y(i)

From far-end
talker

Near-end
talker

Echo
Canceller

Hybrid

x(i)

^

To far-end
listener

r(i)

− + x(i) + r(i)

+

^

u(i) = x(i) + r(i) − r(i)

Figure 11.22 Block diagram of an echo canceller