Parametric Time-Frequency Domain Spatial Audio

11 Cross-Spectrum-Based Post-Filter Utilizing Noisy and Robust Beamformers

Symeon Delikaris-Manias and Ville Pulkki

Department of Signal Processing and Acoustics, Aalto University, Finland

11.1 Introduction

In this chapter we review a post-filtering technique that can provide additional attenuation of spatial noise, interferers, and diffuse sound to the output of a beamformer with minimal spectral artifacts. Contrary to conventional post-filters such as the ones discussed in Chapter 10, the post-filter shown here employs the cross-spectrum of signals originating from coincident beamformers. The cross-spectrum aims to estimate the target signal energy originating from a specific look direction by using two beamformers with the same phase and unity gain towards that direction. This concept is demonstrated in Figure 11.1, where the spectrograms and beam patterns of two different beamformers are shown. The beamformers in the example are of first (dipole) and second order (quadrupole), where the order refers to the directional selectivity of the beamformers. The higher the order, the more directionally selective the beamformer is. Their polar patterns are shown on the left of Figure 11.1, and the black and white colors indicate the polarity. Both beamforming signals are generated from a spherical compact microphone array of 16 sensors and 0.15 cm radius by applying a set of complex weights. For compact microphone arrays, the higher the order of the beamformer, the more the low-frequency ...