Dependencies

1
Dependencies

• Vasileios Hatzivassiloglou
• University of Texas at Dallas

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Sample input and lattice
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Sample random and default generation
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Sample constrained MC generation
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Text-to-speech

• Another type of generation application, which
  also includes elements of translation
• In addition to phonemes (basic sound units),
  prosodic features must be generated
• Many applications
• Automated telephone systems
• Assist users with disabilities
• Other innovative uses

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Modeling dependencies

• Local n-gram models cannot capture certain
  dependencies and additional information
• Syntactic constraints
• The velocity of the seismic waves rises to
• Semantic constraints
• Discourse constraints (sentence order and
  cohesion)

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Syntactic constraints

• Often represented with a context-free grammar
  that models phrase structure
• Non-terminal nodes correspond to constituents,
  building blocks such as different types of
  phrases
• Terminal nodes correspond to words
• The grammar imposes a hierarchical, tree
  structure on sequences of words

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Example grammar

• S ? NP VP
• NP ? DET NN DET NNS NP PP
• VP ? V V NP
• NN ? dog homework
• V ? ate
• DET ? the my
• Note the recursive structure

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Verb subcategorization

• Additional constraints can be represented with a
  division into subcategories. We say that a verb
  subcategorizes for a particular complement
• Object (eat)
• Prepositional phrase (put)
• Predicative adjective (make)
• Infinitive (like)
• That-clause (think)
• Wh-clause (ask)

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Probabilistic context-free grammars

• Associate a probability with each production rule
• All alternatives for a non-terminal must sum to
  one
• Robust
• Allow for comparison of alternative parses
• Do not capture local dependencies

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Semantic constraints

• Start with predicates corresponding to verbs (and
  some nouns)
• Each predicate has arguments that need to fulfill
  specific semantic roles
• Example eat
• Alternative Semantic grammars directly capture
  semantic distinctions by differentiating the
  non-terminals

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Reading

• Introduction to Section 3.2, Sections 3.2.1
  3.2.2
• Introduction to Chapter 11, Section 11.1