The Use of Relative Duration in Syntactic Disambiguation

The Use of Relative Duration in Syntactic Disambiguation M. Ostendorf t P. J. Prices J. Bear: C.W. Wightmant t Boston University 44 Cummington St. Boston, MA 02215 Sl:tI International 333 Ravenswood Ave. Menlo Park, CA 94025 Abstract We describe the modification of a grammar to take ad- vantage of prosodic information automatically extracted from speech. The work includes (1) the development of an integer "break index" representation of prosodic phrase boundary information, (2) the automatic detec- tion of prosodic phrase breaks using a hidden Markov model on relative duration of phonetic segments, and (3) the integration of the prosodic phrase break informa- tion in SRI's Spoken Language System to rule out alter- native parses in otherwise syntactically ambiguous sen- tences. Initial experiments using ambiguous sentences read by radio announcers achieved good results in both detection and parsing. Automatically detected phrase break indices had a correlation greater than 0.86 with hand-labeled data for speaker-dependent models; and, in a subset of sentences with preposition ambiguities, the number of parses was reduced by 25% with a simple grammar modification. Introduction "Prosody," the suprasegmental information in speech, i.e., information that cannot be localized to a specific sound segment, can mark lexical stress, identify phrasing breaks and provide information useful for semantic inter- pretation. Although all of these aspects may be useful in spoken language systems, particularly important are prosodic phrase breaks which can provide cues to syntac- tic structure to help select among competing hypotheses, and thus help to disambiguate otherwise ambiguous sen- tences. In speech understanding applications, informa- tion such as prosody that aids disambiguation, is par- ticularly important, since speech input, as opposed to text, introduces a vast increase in the amount of am- biguity a parser must face. For example, Harrington and Johnstone [7] found that even when all phonemes are correctly identified, the indeterminancy of phoneme sequences when word boundaries are unknown yields in excess of 1000 word string parses for many of their 4 to 10 word sentences. Moreover, these estimates rise dramati- cally as indeterminancy is introduced in the phoneme se- quences: only 2% of the same sentences had fewer than 1,000 parses when phonetically similar phonemes were clustered together (e.g., voiced stops). This indetermi- nancy vastly increases the work for a parser. The work reported here focuses on the use of rela- tive duration of phonetic segments in the assignment of syntactic structure, assuming a known word sequence. Specifically, relative duration of phonemes is estimated by a speech recognizer constrained to recognize the cor- rect string of words. These duration values are then used to compute phrase break indices, which are in turn used to rule out alternative parses in otherwise syntactically ambiguous sentences. In this paper, we begin by pro- viding some theoretical background on prosodic phrase breaks, and by describing a numerical representation of phrase breaks for use in speech understanding. Next we describe algorithms for automatic recognition of these break indices from speech for known text, and the mod- ification of a grammar to …

Content truncated to 500 words