  The estimation of the time- and frequency-dependent coherent-to-diffuse power
ratio (CDR) from the measured spatial coherence between two omnidirectional
microphones is investigated. Known CDR estimators are formulated in a common
framework, illustrated using a geometric interpretation in the complex plane,
and investigated with respect to bias and robustness towards model errors.
Several novel unbiased CDR estimators are proposed, and it is shown that
knowledge of either the direction of arrival (DOA) of the target source or the
coherence of the noise field is sufficient for unbiased CDR estimation. The
validity of the model for the application of CDR estimates to dereverberation
is investigated using measured and simulated impulse responses. A CDR-based
dereverberation system is presented and evaluated using signal-based quality
measures as well as automatic speech recognition accuracy. The results show
that the proposed unbiased estimators have a practical advantage over existing
estimators, and that the proposed DOA-independent estimator can be used for
effective blind dereverberation.

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