Abstract. This chapter presents robust adaptive beamforming techniques designed specifically for microphone array applications. The basics of adaptive beamformers are first reviewed with the Griffiths-Jim beamformer (GJBF). Its robustness problems caused by steering vector errors are then discussed with some conventionally proposed robust beamformers. As better solutions to the conventional robust beamformers, GJBFs with an adaptive blocking matrix are presented in the form of a microphone array. Simulation results and real-time evaluation data show that a new robust adaptive microphone array achieves improved robustness against steering vector errors. Good sound quality of the output signal is also confirmed by a subjective evaluation.
IntroductionBeamforming is a technique which extracts the desired signal contaminated by interference based on directivity, Le. spatial signal selectivity [1]-[5]. This extraction is performed by processing the signals obtained by multiple sensors such as microphones, antennas, and sonar transducers located at different positions in the space. The principle of beamforming has been known for a long time. Because of tlie vast amount of necessary signal processing, most research and development effort has been focused on geological investigations and sonar, which can afford a higher cost. With the advent of LSI technology, the required amount of signal processing has become relatively small. As a result, a variety of research projects where acoustic beamforming is applied to consumer-oriented applications, have been carried out [6].Applications of beamforming include microphone arrays for speech enhancement. The goal of speech enhancement is to remove undesirable signals such as noise and reverberation. Among research areas in the field of speech enhancement are teleconferencing [7] Beamforming can be considered as multidimensional signal processing in space and time. Ideal conditions assumed in most theoretical discussions are not always maintained. The target DOA (direction of arrival), which is assumed to be stable, does change with the movement of the speaker. The sensor gains, which are assumed uniform, exhibit significant distribution. As a result, the performance obtained by beamforming may not be as good as