Speech enhancement for mobile telephony

The properties of noise fields in automobile interiors are discussed with a view toward speech enhancement for voice-activated mobile telephony. The limitations on performance of adaptive noise cancellation are explained in the context of the spatial correlation properties of the noise field. A simple delay-equalized near-field array of directional microphones is analyzed and found to be effective for increasing the signal-to-noise density ratio (SNR) and reducing the reverberant distortion of the speech, without introducing any further distortion. An array of N microphones, each with a delay and weight chosen according to its distance from the speech source, is a viable solution. Such an array gives gains on the order of N over the speech band, reduces reverberation, and does not introduce waveform distortion. Experimental results confirming the predicted performance are presented