10fst-handout

10fst-handout - Massachusetts Institute of Technology...

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Unformatted text preview: Massachusetts Institute of Technology Department of Electrical Engineering & Computer Science 6.345/HST.728 Automatic Speech Recognition Spring, 2010 4/15/10 Lecture Handouts Finite-State Transducers (FSTs) Reading: Garner, A weighted finite state transducer tutorial Mohri, Finite-state transducers in language and speech processing (op- tional) Homework: Assignment 5: Finite-State Transducers MIT Finite-State Transducers for Speech Recognition Introduction Basics Finite-state acceptor (FSA) Finite-state transducer (FST) Deterministic FSA/FST Weighted FSA/FST FSA/FST operations Closure , union , concatenation, intersection, epsilon removal, determinization, minimization, composition Speech Recognizers Construction via composition of FSTs Phone/word graphs & confusion networks Successive Search 6.345/HST.728 Automatic Speech Recognition (2010) Finite-State Transducers 1 MIT Introduction Many ASR components can be represented as finite-state machines n-gram language models Dictionaries Phonological rules (dictionary expansion) N-best lists ... By representing all components as finite-state machines, we can Take advantage of powerful finite-state algorithms Implement algorithms once, use them everywhere Maintain consistency and flexibility to combine components in new ways Use of FSTs in ASR was pioneered by AT&T researchers [Pereira, Riley, Ljolje, Mohri, et al.] 6.345/HST.728 Automatic Speech Recognition (2010) Finite-State Transducers 2 MIT Finite-State Acceptor (FSA) a b 1 2 a 3 b accepts ( a | b ) ab Finite set of states; one initial, at least one final. Arc labels are drawn from alphabet . is the null label. Can be viewed as an acceptor or generator of strings. Accepts/generates a regular language. 6.345/HST.728 Automatic Speech Recognition (2010) Finite-State Transducers 3 MIT Finite-State Transducer (FST) a:b b:a 1 : 2 a: 3 :a e.g., ( aba ) a ( bab ) a Like FSA, but arc labels are pairs of input:output symbols. on input consumes no input. on output generates no output. Can be viewed as generating pairs of strings, accepting pairs of strings, or transducing from input strings to output strings. An FST with labels x : x is equivalent to an FSA. 6.345/HST.728 Automatic Speech Recognition (2010) Finite-State Transducers 4 MIT FSAs/FSTs with Weights 1 a/0.6 a/0.4 2/0.5 b/0.7 b/0.3 Arcs and final states can have weights (costs or scores) Behavior of weights is defined by a semiring ( , , , 1 ) defines how parallel path costs are combined....
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10fst-handout - Massachusetts Institute of Technology...

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