The assumption that the ERL is a constant of value 6 dB is usually incorrect in most
applications. Even if the ERL is 6 dB, p
n can still be greater than the threshold
without near-end speech because of the dispersive characteristics of the echo path and/
or far-end speech. If the ERL is higher than 6 dB, it will take longer to detect the
presence of near-end speech. On the other hand, if the ERL is below 6 dB, most far-end
speech will be falsely detected as near-end speech. For practical applications, it is better
to dynamically estimate the time-varying threshold r by observing the signal level of
x(n) and d(n) when the near-end speech u(n) is absent.
9.4.5 Residual Echo Suppressor
Nonlinearities in the echo path of the telephone circuit and uncorrelated near-end
speech limit the amount of achievable cancellation in a typical adaptive echo canceler
from 30 to 35 dB. The residual echo suppressor shown in Figure 9.15 is used to remove
the last vestiges of remaining echo. This device also effectively removes echo during the
initial convergence of the echo canceler if off-line training stage is prohibited.
The most widely used residual echo suppressor is a center clipper with an input±
output characteristic illustrated in Figure 9.16. The center clipper is used to remove the
low-level echo signal caused by circuit noises, finite-precision errors, etc., which cannot
be canceled by the echo canceler. This nonlinear operation is expressed as
xn, jxnj > ,
where b is the clipping level. This center clipper completely eliminates signals below the
clipping level, but leaves instantaneous signal values greater than the clipping level
unaffected. Thus large signals go through unchanged, but small signals are eliminated.
Since small signals are consistent with echo, the device achieves the function of residual
echo suppression. The clipping threshold b determines how `choppy' the speech will
sound with respect to the echo level. A large value of b suppresses all the residual echoes
but also deteriorates the quality of the near-end speech. Usually the threshold is set so as
to equal or exceed the return echo peak amplitude.
Figure 9.16 Input±output relationship of center clipper
ADAPTIVE ECHO CANCELLATION