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 e