Introduction to the Special Issue on Multichannel Signal Processing for Audio and Acoustics Applications

HE IEEE Signal Processing Society has its roots in an area where acoustics, speech, and signal processing converge, as was reflected in the former name of the society when it was founded in 1974. The interface between acoustics, speech, and signal processing is still an area of great interest to the society, with many fundamental problems still unsolved. Research is driven by applications where acoustic signals have to be captured, transmitted, and/or reproduced in an acoustic environment that includes echoes, noise, and reverberation Considering human/machine interfaces as a major area of applications, it is obvious that signal processing becomes more challenging as the distance between humans and the machines increases, as the signal bandwidth increases, and as the acoustic environment becomes more complex and hostile. Increasingly sophisticated algorithms have been developed since the mid-1970s and along with the availability of greatly increased and affordable computational power, multichannel signal processing algorithms naturally evolved for exploiting the spatial dimension of acoustic signals. The importance and popularity of this field was well reflected by the large number of submissions to this special issue. The volume of high-quality papers could not be fitted into the page budget allotted to us. Thus, we regrettably had to decide to publish some of them in a second section of this special issue as part of a regular issue of the TRANSACTIONS in early 2005. For sound reproduction, where we want to provide a pair of desired signals at the listeners’ ear drums, seamless human/machine interfaces based on multichannel techniques have been implemented since the invention of stereo systems. However, providing the true spatial sound experience in large listening spaces became possible only with new multichannel signal processing techniques, such as wavefield synthesis. Still, major challenges remain, especially phase-true equalization of listening room acoustics and the cancellation of local noise sources and interferers. On the other hand, acquisition of audio and speech signals has been a research topic since the invention of the microphone and still today presents major challenges for the signal processing community. Structurally the simplest problem, the acoustic feedback from loudspeakers into microphones is addressed by acoustic echo cancellation: From the single-channel case which has been investigated since the 1970s, research has moved on to stereo and multichannel reproduction, recently culminating in a new wave-domain adaptive filtering concept which has been presented for the first time at ICASSP 2004. For removing unwanted interference and noise from desired signals, multichannel techniques utilize spatial diversity to discriminate between desired and undesired components, either by exploiting different spatial coherence properties or by beamforming, which directs a beam of increased sensitivity towards the desired source. For traditional beamforming, source localization is necessary if the location of the source is not known a priori. Reflecting current research emphasis in these areas, the paper by Cohen addresses filtering of the