David J. Mostow

A production system for speech understanding

A production system was developed for syntactic and semantic processing within the Hearsay-II speech understanding system. The major properties of the system are discussed, including (1) conversion of static language descriptions into productions, (2) compilation and data-directed execution of productions, (3) dynamically modifiable thresholds on goodness of pattern matches, (4) partial matching, and (5) representation and integration of bottom-up, top-down and horizontal searches. Several weaknesses of the production system paradigm in this application appeared during the course of this research. These arose from (1) the arbitrariness of canonical decompositions of patterns into subpatterns, (2) insufficient use of contextually confirming evidence for individual hypotheses due to the narrowness (literality) of monitored conditions, and (3) difficulty in evaluating the varying import of the same generic action in different contexts. Thus, while the uniformity and lack of explicit organization of production systems are touted as their most desirable features, attendant difficulties of dynamically organizing and controlling coherent problem solutions must be seriously considered in problem domains requiring careful allocation of computational resources.

Syntax and semantics in a distributed speech understanding system

The Hearsay II speech understanding system being developed at Carnegie-Mellon University has an independent knowledge source module for each type of speech knowledge. Modules communicate by reading, writing, and modifying hypotheses about various constituents of the spoken utterance in a global data structure. The syntax and semantics module uses rules (productions) of four types: (1) recognition rules for generating a phrase hypothesis when its needed constituents have already been hypothesized; (2) prediction rules for inferring the likely presence of a word or phrase from previously recognized portions of the utterance; (3) respelling rules for hypothesizing the constituents of a predicted phrase; and (4) postdiction rules for supporting an existing hypothesis on the basis of additional confirming evidence. The rules are automatically generated from a declarative (Le., non-procedural) description of the grammar and semantics, and are embedded in a parallel recognition network for efficient retrieval of applicable rules. The current grammar uses a 450-word vocabulary and accepts simple English queries for an information retrieval system.

A PRODUCTION SYSTEM FOR SPEECH UNDERSTANDING

A production system was developed for syntactic and semantic processing within the Hearsay-II speech understanding system. The major properties of the system are discussed, including (1) conversion of static language descriptions into productions, (2) compilation and data-directed execution of productions, (3) dynamically modifiable thresholds on goodness of pattern matches, (4) partial matching, and (5) representation and integration of bottom-up, top-down, and horizontal searches. Several weaknesses of the production system paradigm in this application appeared during the course of this research. These arose from (1) the arbitrariness of canonical decompositions of patterns into subpatterns, (2) insufficient use of contextually confirming evidence for individual hypotheses due to the narrowness (literality) of monitored conditions, and (3) difficulty in evaluating the varying import of the same generic action in different contexts. Thus, while the uniformity and lack of explicit organization of production systems are touted as their most desirable features, attendant difficulties of dynamically organizing and controlling coherent problem solutions must be seriously considered in problem domains requiring careful allocation of computational resources.