Deconvolution of Reverberant Signals – Frank K. Soong (Bell Labs, Lucent Technologies)
A highly reverberant acoustic environment can have negative effects on the audio quality of a hands-free communication device like teleconferencing system, automatic speech recognizer (ASR), etc. In this talk, first the nature of reverberation of a room is characterized. The temporal “shadowing effects” and non-minimum phase characteristics of a typical impulse response of a large room, which can last a quarter second to half a second, are analyzed. Due to the non-minimum phase nature of such a long impulse response, an LPC-based, inverse filter can only provide a rather limited dereverberation. As a result, significant residual reverberation still exist and it can deteriorate the audio quality of a teleconference and the recognition performance of an ASR. We further investigate the dereverberation problem using single- or multi-channel approach. For a single-channel setup, an inverse filter with an optimal delay is chosen to maximally deconvolve a nonminimum phase impulse response and up to 10 dB dereverberation improvement can be achieved. For a multi(two)- channel setup, a least-squares solution is devised and very clean dereverberation (~35 dB) can be obtained. We will demonstrate the original reverberant speech along with various dereverberation algorithms that we tried. However, in the above single- and multi-channel approaches, the requirement that the room impulse response needs to be identified first renders the solution not readily applicable to a real situation. For example, the fact that the impulse response from a sound source to a microphone pickup can change instantaneously when the sound source is switched from one speaker to another or gradually when a speaker moves his/her head position demands it to be identified continously . Currently we are investigating blind deconvolution techniques which use available signals like speech directly rather than chirp-like artificial probing signals and identify a room impulse response quickly. Fast adaptation schemes for tracking a changing room impulse response are also under current study and related issues will be discussed in this talk.