Mask-Based Beamforming Applied to the End-Fire Microphone Array

Multi-channel speech enhancement techniques are mainly based on optimal multi-channel speech estimators that comprise a minimum variance distortionless response (MVDR) beamformer followed by a single-channel Wiener post-filter. There are two problems in the application of this theoretically optimal solution. The first is the high sensitivity of the MVDR beamformer to errors in the estimated acoustic transfer function (ATF). The second is the accuracy of the time-varying post-filter coefficients estimated from non-stationary speech and noise. Mask-based beamforming developed in the last decade considerably improves the performance of the MVDR beamformer. In addition, the estimated time-frequency mask can be successfully used in post-filter design. In this paper, we propose several improvements to this approach. First, we propose an end-fire microphone array with a better directivity index than the corresponding broadside array. The proposed microphone array is composed of unidirectional microphone capsules that increase the directivity of the microphone array. Second, we propose preprocessing using a delay-and-sum beamformer before estimating the ideal ratio mask (IRM). Next, we propose a simplified generalized sidelobe canceller (S-GSC), which does not need to estimate ATF. We also improved the design of its blocking matrix by scaling the null space eigenvectors of the speech covariance matrix. The proposed computationally efficient multiple iteration method improves the adaptation of the S-GSC parameters. Finally, we improved the previous IRM-based post-filter, considering the SNR improvement at the output of the S-GSC beamformer. The integral speech enhancement procedure was tested on real room recordings using PESK, STOI, and SDR measures.