Real-time linguistic analysis for continuous speech understanding

Real-time linguistic analysis for continuous speech understanding* Paolo Baggia Elisabetta Gerbino Egidio Giachin Claudio Rullent CSELT - Centro Studi e Laboratori Telecomunicazioni Via Reiss Romoli, 274 - 10148 Torino, Italy Abstract This paper describes the approach followed in the development of the linguistic proces- sor of the continuous speech dialog system im- plemented at our labs. The application sce- nario (voice-based information retrieval ser- vice over the telephone) poses severe specifi- cations to the system: it has to be speaker- independent, to deal with noisy and corrupted speech, and to work in real time. To cope with these types of applications requires to improve both efficiency and accuracy. At present, the system accepts telephone-quality speech (utterances referring to an electronic mailbox access, recorded through a PABX) and, in the speaker-independent configuration, it correctly understands 72% of the utterances in about twice real time. Experimental results are discussed, as obtained from an implemen- tation of the system on a Sun SparcStation 1 using the C language. 1 Introduction We call continuous speech, as opposed to isolated- word speech, any utterance emitted without interposing pauses between words. This is the way humans natu- rally speak, but to enable a machine to deal with this form of communication constitutes a difficult task be- cause, in addition to the usual speech processing and natural language issues, there is no hint on where the single words of the utterance begin and end. Given the current state-of-the-art, speech understanding pro- totypes concern the comprehension of utterances refer- ring to a well defined semantic domain on a dictionary of almost one thousand words. Our research group is interested in developing au- tomated voice-based information retrieval services over the telephone network. This has some precise impli- cations. First, we are committed to accept continuous- speech sentences expressed in relatively free syntax, oth- erwise the interaction with the service would be too unnatural. Second, the service should be public and ac- cessible from every telephone; this means that the un- *This research has been partially supported by EEC ES- PRIT project no. 2218 SUNDIAL. derstanding system has to be speaker independent and able to process noisy and distorted speech. Third, the response time must be confined within a few seconds; that is, the system has to work in real time. The evolution of the research is gradual. In the present status of work the developed system is speaker independent and it is based on a telephone-line qual- ity speech (a telephone connected through a PABX). It has a vocabulary of 787 words and the processing time is less than 5 seconds (about twice real time). The sen- tence correct understanding rate is nearly 72%. The application task refers to voice access to an electronic mailbox. In its first version the system has been applied to access a geographical data base, and is now adapted (with a slightly bigger vocabulary) to information re- trieval from a train timetable data base. In the follow- ing we present a thorough description of …

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