Maximum kurtosis beamforming with a subspace filter for distant speech recognition

This paper presents a new beamforming method for distant speech recognition (DSR). The dominant mode subspace is considered in order to efficiently estimate the active weight vectors for maximum kurtosis (MK) beamforming with the generalized sidelobe canceler (GSC). We demonstrated in [1], [2], [3] that the beamforming method based on the maximum kurtosis criterion can remove reverberant and noise effects without signal cancellation encountered in the conventional beamforming algorithms. The MK beamforming algorithm, however, required a relatively large amount of data for reliably estimating the active weight vector because it relies on a numerical optimization algorithm. In order to achieve efficient estimation, we propose to cascade the subspace (eigenspace) filter [4, Section 6.8] with the active weight vector. The subspace filter can decompose the output of the blocking matrix into directional signals and ambient noise components. Then, the ambient noise components are averaged and would be subtracted from the beamformer´s output, which leads to reliable estimation as well as significant computational reduction. We show the effectiveness of our method through a set of distant speech recognition experiments on real microphone array data captured in the real environment. Our new beamforming algorithm provided the best recognition performance among conventional beamforming techniques, a word error rate (WER) of 5.3%, which is comparable to the WER of 4.2% obtained with a close-talking microphone. Moreover, it achieved better recognition performance with a fewer amounts of adaptation data than the conventional MK beamformer.