Patent US 5,864,809

A speech recognition apparatus recognizes utterances of unnatural speech having a higher performance of recognition accuracy with a smaller amount of speech learning data. The speech recognition apparatus includes an acoustic-phonetic variability learning unit, a normal speech model memory, a spectrum smoother-modifier and a speech recognizer. An input speech signal is acoustically analyzed and transformed into a time-series feature vector. The acoustic-phonetic variability learning unit learns an acoustic-phonetic change of spectrum caused by unnatural speech and generates a plurality of acoustic-phonetic variability models. The normal speech model memory stores a normal speech model learned based on normal speech data. The spectrum smoother-modifier modifies the normal speech model based on a plurality of the acoustic-phonetic variability model and generates a plurality of spectrum-modified speech models. The speech recognizer recognizes the time-series feature vector based on the normal speech model and the spectrum-modified speech model.