Speech and Human-Machine Dialog / The Springer International Series in Engineering and Computer Science Bd.770 (PDF)

false starts, which are common in normal speech, as afternoon flight from from Denver to San Francisco. Only a few research prototype systems, e.g., CMUPHOENIX (Ward, 1994), take these effects into account. The ability to cope with spontaneous speech is crucial for the design of systems in real world applications. The following sections, taken from (Minker et al., 1999), introduce the case grammar formalism, used by the system L’ATIS (Bennacef et al., 1994), as well as the train travel-based systems MASK (Gauvain et al., 1997) and ARISE (Lamel et al., 1998). Another example of a case grammar-based implementation is the CMU-PHOENIX parser (Ward, 1994).

2. Case Grammar Formalism

In the domain of spoken language information retrieval, spontaneous effects in speech are very important. These include false starts, repetitions and ill-formed utterances. Thus, it would be improvident to base the semantic extraction exclusively on a syntactic analysis of the input utterance. Parsing failures due to ungrammatical syntactic constructs may be reduced if those phrases containing important semantic information could be extracted whilst ignoring the non-essential or redundant parts of the input utterance. Restarts and repeats frequently occur between the phrases. Poorly syntactical constructs often consist of well-formed phrases which are semantically correct.

One approach to extracting semantic information is based on case frames. A frame is a data structure, a type of knowledge representation in artificial intelligence (Minsky, 1975). It is a cluster of facts and objects that describe some typical object or situation, together with specific inference strategies for reasoning about the situation (Allen, 1988). A frame may be thought of as a network of nodes and relations. The top levels of a frame are fixed, and represent facts that are always true about the supposed situation. The lower levels have terminals or slots that need to be filled-in by specific instances of data. Each terminal can specify conditions its assignments must meet. The assignments themselves are usually smaller sub-frames. Collections of related frames are linked together into frame systems.

The original concept of a case frame as described by Fillmore (1968) is based on a set of universally applicable cases. They express the relationship between a verb and the related syntactic groups. Fillmore’s cases correspond in fact to the Latin declensions: nominative, accusative and instrumental. Bruce (1975) extended the Fillmore theory to any concept-based system and defined an appropriate semantic grammar, whose formalism is given in Figure 2.1.

The case grammar uses in fact the stereotypical data structure of frames (Minsky, 1975). However, in order to fill in the frame slots, the notion of syntax (Fillmore, 1968) is added in the form of local marker-constraint relations. In the example query