The results of performance in speech enhancement are presented in chapter 4. The proposed beamformer null-constraint could be used to improve the OutputSINR both in HC-KF and SOE-KF. Using the wideband selection of steering vector bound in constrained Kalman filter works in solving signal mismatch problem. In this thesis, the fact can be observed that the proposed method is capable of noise reduction and maintaining the signal distortionless at the same time. The robustness of beamformer against array steering error compared to other algorithms is shown in Fig. 42-44;48-50.
The relationship between the noise reduction and the dereverberation is a tradeoff of a beamformer. Because the existence of desired source from different direction is unknown and the environment reverberation is non-stationary, the better noise reduction performance may suppress the desired source, particularly in low frequency-bands when the number of arrays is small. To make sure the directivity of steered response is in low frequency-bands, the regulation of steering vector bound is needed according to the characteristic as in Fig. 17-19.
There are several areas for improvement in selecting the appropriate steering vector bound and tracking the beamformer nulls for interference suppression. The inequality nonlinear beam constraint would amplify the desired source and diffused noise at meanwhile; and the null constraint would make a distortion to the desired source in low frequency-bands. Therefore, it would be interesting in noise post-filtering using broadband beamforming similar to [29][30]. This is left as a future research topic. As a conclusion, a more robust adaptive post-filter for diffused noise and more solid sources tracking (similar to superdirective beamformer [34] in low frequency-bands) will be helpful to overcome the signal mismatch problem under such environment further.
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