Selection of word islands in the Hearsay-II speech understanding system

In Hearsay-II, a word recognizer hypothesizes words bottom-up from acoustic data. Usually many competing words are hypothesized for each time interval of speech, with the correct word rarely top-ranked. Due to the unreliable ratings of words and the limited syntactic constraint supplied by single words, the use of single-word islands would cause the recognition system to explore many blind alleys before abandoning an incorrect island. In addition, the multiplicity of words makes the parsing of all possible word sequences extremely time-consuming. The Hearsay-II island selection strategy uses (1) knowledge of what word adjacencies are allowed by the grammar, (2) anaiysis of acoustic data at the junctures between word hypotheses, and (3) heuristics based on the number of competing word hypotheses, to form multi-word islands which the syntax-level knowledge source first checks for grammatically and then attempts to extend to form a complete recognition.