Tile procedure to construct a word predictor in a speech understanding system from a task-specific grammar defined in a CFG or a DCG

The procedure to construct a word predictor in a speech understanding system from a task-specific grammar defined ill a CFG or a DCG Yasuhisa Nilmi, Shigeru Uzuhara and Yutaka Kobayashi Department of Computer Science Kyoto Institute of Technology Matsugasaki, Sakyo-ku, Kyoto 606, Japan Abstract This paper describes a method for converting a task-dependent grammar into a word predictor of a speech understanding system, Since tile word prediction is a top-down operation, left recursive rules induces an infinite looping. We have solved this problem by applying an algorithm for bottom-up parsing, 1. Introduction which tile ends terminate at different portions on the phonetic sequence, and the other represents the se- quences of syntactic categories (called category sequences), each of which is associated with one of the word strings, ln this situation, the controller chooses the word string with tile highest score, sends tile associated category sequence to the word predictor and asks it to predict those syntactic categories which can syntactically follow the se- lected sequence. In this paper we present a method for converting a task-specific gravmnar into a word predictor, an im- portant component of a speech understanding system. A context free grammar (CFG) or an augmented transi- tion network grammar (ATNG) have been used to des- cribe task-speclfic constraint. When a CFG is used, Early's algorithm[l], one of the most efficient top- down parsing algorithms, has been used to make word prediction[2]. When an ATNG is used, word prediction is simply made by tentatively traveling along arcs going out freln a state in an ATNG[3],[4],[5]. Since the word prediction is a top-down operation, it is difficult to avoid falling into an infinite loop if the task-specific grammar includes a left recurslve rule. F. Perelra and D. Warren have developed a definite clause grammar (DCG)[6]. The rules described in a DCG are directly converted into a set of Pro]og clauses, which works as a parser with an aid of tile powerful pattern matching mechanism of Prolog. Thus syntactic analysis can be done without writing a specia] parser working on the rules of the grammar. Since tile syn- tactic analysis based on a DCG parser also works in top-down fashion, it shares the same difficulty as the top-down parsers have. ¥. Matsumoto et at. have developed a method for converting a set of rules described in a DCG into a bottom-up parser which has overcome thls difficulty without any loss of the advantages of a DCG[7]. We discuss an application of this method to a word predictor, that is, the method for transforming task- specific linguistic constraint defined in a CFG or a DCG into a Prolog program which acts as a left-to- right word predictor. 2. Word predictlon in a ~eech understandlnq~sj(stem Fig.l shows a typical configuration of a speech understanding system based on a hierarchical model. An acoustic-phonetic processor analyzes of an input uttereance and transforms it into a sequence of pho- netically labeled segments. Provided that a part of an utterance has been dealt with, the controller …

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