Multilevel Semantic Analysis in an Automatic Speech Understanding and Dialog System

MULTILEVEL SEMANTIC ANALYSIS IN AN AU'I~MATIC SPEECH UNDERSTANDING AND DIALOG SYSTEM Ute Ehrlich Lehrstuhl f[ir Inforrmtik 5 (Mustererkeunung) Universitat Erlangen-Nfirnberg Martensstr. 3, 8520 Erlangen, F. IL Germany ABSTRACT At our institute a speech understanding and dialog system is developed. As an example we model an information system for timetables and other information about intercity trains. In understanding spoken utterances, additional problems arise due to pronunciation variabilities and vagueness of the word recognition process. Experiments so far have also shown that the syntactical analysis produces a lot more hypotheses instead of reducing the number of word hypotheses. The reason for that is the possibility o! combining nearly every group of word hypotheses which are adjacent with respect to the speech signal to a syntactically correct constituent. Also, the domain independent semantic analysis cannot be used for filtering, because a syntactic sentence hypothesis normally can be interp.reted in several different ways, respectively a set of syntactic hypotheses for constituents can be combined to a lot of semantically interpretible sentences. Because of this combinatorial explcaiun it seems to be reasonable to introduce domain dependent and contextual knowledge as early as possible, also for the semantic analysis. On the other hand it would be more efficient prior to the whole semantic interpretation of each syntactic hypothesis or combination of syntactic hypotheses to find possible candidates with less effort and interpret only the more probable Ones. 1. Introduction In the speech understanding and dialog system EVAR (Niemann et al. 1985) developed at our institute there are four different modules for understanding an utterance of the user (Brietzmaun 1984): the syntactic analysis, the task-independent semantic analysis, the domain-dependent pcagmatic analysis, and another module for dialog-specific aspects. The semantic module disregards nearly all of the thematic and situational context. Only isolated utterances are analyzed. So the main points of interests are the semantic consistency of words and the underlying relational structure of the sentence. The analysis of the functional relations is based on the valency and case theory (Tesniero 1966, Fillmore 1967). In this theory the head verb of the sentence determines how many noun groups or prel:csitional groups are needed for building up a syntactically correct and semantically consistent sentence. For these slots in a verb frame further syntactic and semantic restrictions can also be given. 2. Semtntic and Progmstic Consistency Semantic Consistency The semantic knowledge of the module consists of lexical meanings of words and selectional restrictior~ between them. These restrictions are possible for a special word, fur example the preposition 'nach' ('to Hamburg') requires a noun with the meamng LOCation. In the case of a frame they are for a whole constituent; for example, the verb 'wchnen' ('to live in Hamburg') needs a preposition~l group also with the me'~ning LOCation. The selectional restrictions are expressed in the dictionary by the feature SELECTION. The semantic classes (features) are hierarchically organized in a way, so that all subclasses of a class also are accepted as compatible. For example, if a word with the semantic class CONcrete …

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