Speech enhancement by nonlinear multiband envelope filtering

Starting from a known relation between modulation transfer function (of a room) and speech intelligibility, we tried to enhance speech corrupted by reverberation or noise by suitably filtering the envelope (power) signals in critical frequency bands. Two methods based on FFT-overlap-adding and onlinear prediction, respectively, have been developed. Linear envelope filtering was not successful for neither pre- nor postprocessing, even though the objective SNR was improved. However, inserting a logarithmic nonlinearity or, better, a logarithmic one becoming linear at high levels, did raise intelligibility in the presence of noise when the filter effected strong compression at very low and no change or expansion at higher modulation frequencies. Since noise masking can serve as a model for recruitment in sensorineural hearing impairment, the method might be used in future hearing aids.