  In this paper, a multi-channel time-varying covariance matrix model for late
reverberation reduction is proposed. Reflecting that variance of the late
reverberation is time-varying and it depends on past speech source variance,
the proposed model is defined as convolution of a speech source variance with a
multi-channel time-invariant covariance matrix of late reverberation. The
multi-channel time-invariant covariance matrix can be interpreted as a
covariance matrix of a multi-channel acoustic transfer function (ATF). An
advantageous point of the covariance matrix model against a deterministic ATF
model is that the covariance matrix model is robust against fluctuation of the
ATF. We propose two covariance matrix models. The first model is a covariance
matrix model of late reverberation in the original microphone input signal. The
second one is a covariance matrix model of late reverberation in an extended
microphone input signal which includes not only current microphone input signal
but also past microphone input signal. The second one considers correlation
between the current microphone input signal and the past microphone input
signal. Experimental results show that the proposed method effectively reduces
reverberation especially in a time-varying ATF scenario and the second model is
shown to be more effective than the first model.
