Combination of robust adaptive beam-forming with acoustic echo cancellation for acoustic human/machine front-ends (2003)

With a continuously increasing desire for natural and comfortable human/machine interaction, the acoustic interface of any terminal for multimedia or telecommunication services is challenged to allow seamless and hands-free audio communication in such diverse acoustic environments as passenger cabins of cars or office and home environments. Typical applications include audio-/video-conferencing, dialog systems, computer games, command-and-control interfaces, dictation systems, or high-quality audio recordings. Compared to speech and audio capture using a microphone next to the desired source, seamless audio interfaces cause the desired source signal to be impaired by reverberation due to reflective acoustic environments, local interference and noise, and acoustic echoes from loudspeakers. All these interferers are not only annoying to human listeners but, more importantly, they are detrimental for example when speech recognition is involved. Acoustic echo cancellation is desirable whenever a reference of the loudspeaker signals is available, since acoustic echo cancellation allows maximum suppression of these interferers. Techniques for acoustic echo cancellation evolved over the last two decades, and led to the recent presentation of a five-channel acoustic echo canceller (AEC) for real-time operation on a personal computer. For suppression of local interferers and noise, beamforming microphone arrays are very effective since they suppress interference and noise by spatio-temporal filtering without distorting the desired signal -- in contrast to single-channel speech enhancement based on temporal filtering. Here, adaptive data-dependent beamformers seem to be the optimum choice since they account for the characteristics of the desired signal and of the interferers in order to maximize the suppression of interference and noise. However, adaptive data-dependent beamformers are particularly challenging for speech and audio signals in realistic acoustic conditions for several reasons: - Reverberation of the acoustic environment with respect to (w.r.t.) the desired signal, moving desired sources, or array imperfections as position errors or gain and phase mismatch of the microphones may lead to distortion of the desired signal by the adaptive beamformer. - The strong time-variance of typical interferers, as, e.g., competing talkers, makes the estimation of the characteristics of the interferers difficult, since it is generally not sufficient to estimate them during pauses of the desired signal. - The aperture of the array and the number of sensors are generally limited due to product design constraints. This reduces robustness against distortion of the desired signal and interference rejection at low frequencies. - Typical acoustic environments for hands-free acoustic human/machine interfaces are highly diversified, reaching from passenger cabins of cars with low reverberation times and with mostly slowly time-varying diffuse interference and noise, but with low signal-to-interference-plus-noise ratios (SINRs), to office and home environments with strong reverberation and mostly strongly time-varying directional interference. An adaptive beamformer which addresses the problem of reverberation, moving sources, and array imperfections is presented in (Hoshuyama, 1999). However, adaptation of this version of the 'generalized sidelobe canceller' (GSC) is difficult, and often leads to cancellation of the desired signal or to transient interference and noise when the desired speaker and interference and noise are active simultaneously ('double-talk'). Therefore, we propose in this work a robust version of this GSC (RGSC) with low computational complexity, which effectively resolves the adaptation problems, which may be used down to low frequencies, and which may be applied to such diversified acoustic conditions as described above. For practical multimedia terminals, a combination of a beamforming microphone array with acoustic echo cancellation is desirable in order to optimally suppress acoustic echoes, interference, and noise. Here, we generally have to deal with the problem of maximally exploiting positive synergies between acoustic echo cancellation and beamforming, while minimizing the computational complexity of the combined system. Therefore, we compare in this work various combinations of beamforming and acoustic echo cancellation, each of them having advantages for different situations. Especially, a new structure is presented, which uses the RGSC as a basis, and which outperforms the other combinations for highly transient echo paths with low computational complexity.