Sub-band filtering, adaptive echo cancellation, and residual echo suppression
are selectively and separately applied in hierarchical order under the control
of a circuit that monitors four separate signals to determine n machine states.
The number of machine states is further divided among m levels of noise in determining
which, and how much of, sub-band filtering, adaptive echo cancellation, and residual
echo suppression to use at any given time. A voice activity detector (VAD) is used
to monitor each signal and provides statistical information in addition to whether
or not a voice signal is detected. Sub-band filter, echo cancelling, and non-linear
processing are not permitted to provide maximum reduction simultaneously. Even
so, the invention provides at least 15 dB more echo suppression than systems without
a sub-band filter bank series coupled with a non-linear processor in each channel.
