Speech Enhancement - Digital Speech: Coding for Low Bit Rate Communication Systems - page 406

near-end signal and the echo are added together at the output of the hybrid.
Since the far-end signal is available as a reference for the echo canceller, the
replica of the echo r(i) is estimated by matching the signals on both paths of

the four-wire section. The estimated echo is then subtracted from the total of
the returned echo and the near-end signal,

ˆ

− ˆ u(i) = x(i) + r(i) r(i) (11.52)

The difference between r(i) , the returned echo, and r(i) , the estimated echo,

should be as small as possible for good echo cancellation performance. The
echo canceller estimates the echo by using the far-end reference signal in a
transversal filter such as the one shown in Figure 11.23. This filter basically
acts as a tapped delay line. If the impulse response of the filter is same as the
echo path response, then the estimated echo and the returned echo become
identical, resulting in perfect echo cancellation. Since the echo path response
is not known in advance and may vary slowly with time, the coefficients of
the transversal filter are adapted. In order to produce no distortion on the
near-end talker signal, the filter coefficients are only updated when there is
no near-end activity.
The number of filter coefficients, which may be very significant from a
complexity point of view, is usually determined by the length of the echo
path impulse response, which typically lasts 2 to 4ms, requiring 32 taps
(4

ˆ

10 − 6 ) approximately. However, the impulse response of the
echo path may be delayed by some time depending on the distance between
the position of the echo canceller and the hybrid in the system. Moreover
more than a few taps may be needed to accurately model the response of the
hybrid. Therefore, the use of 64 or 128-tap filters are typical.

10 − 3 / 125 × ×

From far-end

Reference Signal

Z −1 Z −1 Z −1 Z −1

y(i)

a 1

a 2

a0 ap

Echo

^

r(i)

−

To far-end

+

+

^

x(i) + r(i) − r(i) x(i) + r(i)

Figure 11.23 Block diagram of a transversal filter used in echo cancellation