ALGONQUIN: iterating laplace's method to remove multiple types of acoustic distortion for robust speech recognition
One approach to robust speech recognition is to use a simple speech model to remove the distortion, before applying the speech recognizer. Previous attempts at this approach have relied on unimodal or point estimates of the noise for each utterance. In challenging acoustic environments, e.g., an airport, the spectrum of the noise changes rapidly during an utterance, making a point estimate a poor representation. We show how an iterative form of Laplace’s method can be used to estimate the clean speech, using a time-varying probability model of the log-spectra of the clean speech, noise and channel distortion. We use this method, called ALGONQUIN, to denoise speech features and then feed these features into a large vocabulary speech recognizer whose WER on the clean Wall Street Journal data is 4.9%. When 10 dB of noise consisting of an airplane engine shutting down is added to the data, the recognizer obtains a WER of 28.8%. ALGONQUIN reduces the WER to 12.6%, well below the WER of 25.0% obtained by our spectral subtraction algorithm, and close to the WER of 9.7% obtained by the slow procedure of retraining the recognizer on training data corrupted by the exact same noise. In fact, if ALGONQUIN is used to denoise the noisy training data before the recognizer is retrained, the WER is improved to 8.5%. For 10 dB of additive uniform white noise, our spectral subtraction algorithm reduces the WER from 55.1% to 33.8%. ALGONQUIN reduces the WER to 14.2%. The recognizer trained on noisy data obtains a WER of 14%, whereas the recognizer trained on noisy data denoised by ALGONQUIN obtains a WER of 9.9%.