Signal Processing to Reduce Multipath Distortion in Small Rooms

Abstract

The transmission between a sound source and a microphone in a small, hard-walled room is characterized by multiple peaks and sharp valleys. Sound received by such a microphone often is judged to be undesirably distorted or spectrally colored. Speakerphones and conference telephony systems are vulnerable to this acoustic distortion. A technique is described for combining the signals from two or more microphones to produce a single output having less multipath distortion than any of the individual microphone signals. The output from each microphone is filtered into a number of bandpass signals occupying contiguous frequency ranges. The microphone receiving greatest average power in a given frequency band contributes that signal band to the output. Each microphone, then, is used only for those frequency bands in which its received power is greater than any other microphone, so that the combined output contains less spectral coloration than the signal from any individual microphone. The processing method is studied by simulation on a digital computer. Broad-band noise and speech signals are used to test the method.