Recognition of conversational telephone speech using the JANUS speech engine

Recognition of conversational speech is one of the most challenging speech recognition tasks to-date. While recognition error rates of 10% or lower can now be reached on speech dictation tasks over vocabularies in excess of 60,000 words, recognition of conversational speech has persistently resisted most attempts at improvements by way of the proven techniques to date. Difficulties arise from shorter words, telephone channel degradation, and highly disfluent and coarticulated speech. In this paper, we describe the application, adaptation, and performance evaluation of our JANUS speech recognition engine to the Switchboard conversational speech recognition task. Through a number of algorithmic improvements, we have been able to reduce error rates from more than 50% word error to 38%, measured on the offical 1996 NIST evaluation test set. Improvements include vocal tract length normalization, polyphonic modeling, label boosting, speaker adaptation with and without confidence measures, and speaking mode dependent pronunciation modeling