  The close relationship between the feedforward ANC system and the stereo
acoustic echo cancellation system is revealed in this paper. Accordingly, the
convergence behavior of the ANC system can be analyzed by investigating the
joint auto-correlation matrix of the reference and the filtered reference
signal. It is proved that the straightforward secondary path modeling can be
carried out without the injection of any additive noise as long as the control
filter is of a sufficiently long length. Furthermore, by taking advantage of
the time-varying characteristic of the control filter, effective modeling of
the secondary path can be even achieved without any restriction on the control
filter length.
