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Speech Recognition

And

Text-to-Speech Systems

Phonology

• Phonetic alphabets

• Phonological rules

• Computational Phonology

• Phonological Learning

• Optimality Theory

Phonetics

• Study of the pronunciation of words

• Words are strings of symbols which represent phones

• Can also include prosody

Phonetic Alphabets

• International Phonetic Alphabet (IPA)– Evolving standard since 1888– Goal is to be able to transcribe the sounds of

all human languages

Phonetic SymbolsIPA

• International Phonetic Alphabet

• Evolving standard since 1988

• Goal is to be able to transcribe sounds of all human languages

ARPAbet

• Specifically for American English

• can be used where non-ASCII fonts are inconvenient (such as in online pronunciation dictionaries)

Phonological Rules

• Not all [t]s are created equally

• Phones are pronounced differently in different contexts (phoneme vs. allophone)

• e.g. [t] in tunafish is aspirated

• e.g. [t] in starfish (following initial s) is unaspirated

• Broad transcription vs. narrow transcription

Phonological Rules

• ladder

• lotus

t

d{ } [ ] / V__V

Two-Level Morphology

• Koskenniemi (1983)• Most phonological rules are independent• Feeding and bleeding relations are rare• Explicitly code when rule is obligatory or optional

Rule type Interpretationa:b c ___ d a is always realized as b in the context c ___ d

a:b c ___ d a may be realize as b only in the context c ___ d

a:b c ___ d

a must be realized as b in the context c ___ d and nowhere else

a:b / c ___ d a is never realized as b in the context c ___ d

Optimality Theory (OT)

• Prince and Smolensky, 1993

• Is a Connectionist theory of language

• Views phonological derivation based on:– Two functions (GEN and EVAL) and– A set of ranked violable constraints (CON)

• Assumed to be cross-linguistic generalizatoins

Optimality Theory (OT)

• Given underlying form:– GEN function produces all imaginable surface

forms – EVAL function then applies each constraint in

CON to these surface forms in order of constraint rank

Optimality Theory (OT)

• Constraints– Faithfulness (checks how faithful the surface

form is to the underlying form)• e.g. FaithV—says “Don’t delete or insert vowels”• e.g. FaithC—says “Don’t delete or insert

consonants”

– Markedness (imposes requirements on the structural well-formedness of the output)

• e.g. *Complex –says “no complex onsets or codas”

http://en.wikipedia.org/wiki/Optimality_Theory

Optimality Theory (OT)

• Uses constraints to filter out unneeded surface forms

• Some constraints are more important than others

Optimality Theory (OT)

• Can OT be implemented by finite-state transducers?

• Is essential to enforce constraint only if does not reduce possibilities to zero

Optimality Theory (OT)

• Ordinal OT grammars– Tesar & Smolensky (1998) – No absolute ranking values

• i.e. they accepted only an ordinal relation between the constraint rankings

– learning algorithm (Error-Driven Constraint Demotion, EDCD)

• changes the ranking order whenever the form produced is different from the adult form

– Fast and convergent, but extremely sensitive to errors in the learning data

http://www.fon.hum.uva.nl/praat/manual/OT_learning_1__Kinds_of_OT_grammars.html

Optimality Theory (OT)

• Stochastic OT grammars– Boersma (1997b) / Boersma (1998) / Boersma (2000) – every constraint has a ranking value along a

continuous ranking scale – a small amount of noise is added to this ranking

value at evaluation time – associated error-driven learning algorithm (Gradual

Learning Algorithm, GLA) effects small changes in the ranking values of the constraints with every learning step

– can learn languages with optionality and variation

http://www.fon.hum.uva.nl/praat/manual/OT_learning_1__Kinds_of_OT_grammars.html

SIGMORPHON

• ACL Special Interest Group on Computational Morphology and Phonology (SIGMORPHON)

• formerly known as the ACL Special Interest Group on Computational Phonology (SIGPHON )

• Recent research developments• Matters of interest in computational

phonology and morphology

http://salad.cs.swarthmore.edu/sigphon/systems.shtml