In addition domain-specific constituents and word classes have been defined. As mentioned above the syntactic grammar in the system serves two purposes. Besides being used for making a structural analysis as a first step towards a semantic interpretation of the utterance, it also forms the basis for the automatically generated word-pair grammars applied for reducing the number of word models during the acoustic signal processing. The latter function requires strict formulation of the grammar for syntactic analysis in order to make the linguistic knowledge used in speech recognition as precise and constrained as possible. For example on the basis of an analysis of the main verbs with focus on their contextual word patterns domain-specific selectional restrictions and categories can be determined and exploited in the linguistic processing.
Selectional restrictions express constraints on combinations of lexical units in a given context.
By focussing on the argument structure for lexical units, interrelated semantic concepts can be identified. How domain-specific knowledge is expressed in selectional restrictions can be illustrated by looking at following sample utterances from the collected corpus. As a first step towards solving this problem, all the domain-specific main verbs and their contextual patterns were examined in concordances. Based on this analysis the domain-specific concepts which link lexical units together were identified. The knowledge of domain-specific selectional restrictions is expressed in the lexicon and grammar of the NLP module.