CS 224S / LINGUIST 285Spoken Language Processing

AndrewMaasStanfordUniversity

Spring2017

Lecture3:ASR:HMMs,Forward,ViterbiOriginalslidesbyDanJurafsky

Fun informative read on phoneticsTheArtofLanguageInvention.DavidJ.Peterson.2015.http://www.artoflanguageinvention.com/books/

Outline for Today� ASRArchitecture� DecodingwithHMMs

� Forward� ViterbiDecoding

� HowthisfitsintotheASRcomponentofcourse� Onyourown:N-gramsandLanguageModeling� Apr12:Training,AdvancedDecoding� Apr17:FeatureExtraction,GMMAcousticModeling� Apr24:NeuralNetworkAcousticModels� May1:Endtoendneuralnetworkspeechrecognition

The Noisy Channel Model

� Searchthroughspaceofallpossiblesentences.� Picktheonethatismostprobablegiventhewaveform.

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The Noisy Channel Model (II)� WhatisthemostlikelysentenceoutofallsentencesinthelanguageLgivensomeacousticinputO?

� TreatacousticinputOassequenceofindividualobservations�O=o1,o2,o3,…,ot

� Defineasentenceasasequenceofwords:�W=w1,w2,w3,…,wn

Noisy Channel Model (III)� Probabilisticimplication:PickthehighestprobS:

� WecanuseBayesruletorewritethis:

� SincedenominatoristhesameforeachcandidatesentenceW,wecanignoreitfortheargmax:

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ˆ W = argmaxW ∈L

P(W | O)

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ˆ W = argmaxW ∈L

P(O |W )P(W )€

ˆ W = argmaxW ∈L

P(O |W )P(W )P(O)

Speech Recognition Architecture

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ˆ W = argmaxW ∈L

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likelihood prior

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The noisy channel modelIgnoringthedenominatorleavesuswithtwofactors:P(Source)andP(Signal|Source)

Speech Architecture meets Noisy Channel

Decoding Architecture: five easy pieces

� FeatureExtraction:� 39“MFCC” features

� AcousticModel:� Gaussiansforcomputingp(o|q)

� Lexicon/PronunciationModel� HMM:whatphonescanfolloweachother

� LanguageModel� N-gramsforcomputingp(wi|wi-1)

� Decoder� Viterbialgorithm:dynamicprogrammingforcombiningallthesetogetwordsequencefromspeech

11

Lexicon� Alistofwords� Eachonewithapronunciationintermsofphones� Wegetthesefromon-linepronunciationdictionary

� CMUdictionary:127Kwords� http://www.speech.cs.cmu.edu/cgi-bin/cmudict

� We’llrepresentthelexiconasanHMM

12

HMMs for speech

Phones are not homogeneous!

Time (s)0.48152 0.937203

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Each phone has 3 subphones

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Resulting HMM word model for “six”

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HMM for the digit recognition task

Markov chain for weather

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Markov chain = First-order observable Markov Model

� asetofstates� Q=q1,q2…qN; thestateattimetisqt

� Transitionprobabilities:� asetofprobabilitiesA=a01a02…an1…ann.� Eachaijrepresentstheprobabilityoftransitioningfromstateitostatej

� ThesetoftheseisthetransitionprobabilitymatrixA

� Distinguishedstartandendstates€

aij = P(qt = j |qt−1 = i) 1≤ i, j ≤ N

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aij =1; 1≤ i ≤ Nj=1

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Markov chain = First-order observable Markov Model

Currentstateonlydependsonpreviousstate

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Markov Assumption : P(qi |q1!qi−1) = P(qi |qi−1)

Another representation for start state

� Insteadofstartstate�Specialinitialprobabilityvectorp�Aninitialdistributionoverprobabilityofstartstates

�Constraints:

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π i = P(q1 = i) 1≤ i ≤ N

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π j =1j=1

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The weather figure using pi

The weather figure: specific example

Markov chain for weather�Whatistheprobabilityof4consecutivewarmdays?

� Sequenceiswarm-warm-warm-warm� I.e.,statesequenceis3-3-3-3� P(3,3,3,3)=�p3a33a33a33a33 =0.2x(0.6)3 =0.0432

How about?

� Hothothothot� Coldhotcoldhot

�Whatdoesthedifferenceintheseprobabilitiestellyouabouttherealworldweatherinfoencodedinthefigure?

HMM for Ice Cream

� Youareaclimatologistintheyear2799� Studyingglobalwarming� Youcan’tfindanyrecordsoftheweatherinBaltimore,MDforsummerof2008

� ButyoufindJasonEisner’sdiary� Whichlistshowmanyice-creamsJasonateeverydatethatsummer

� Ourjob:figureouthowhotitwas

Hidden Markov Model

� ForMarkovchains,outputsymbols=statesymbols� Seehot weather:we’reinstatehot

� Butnotinspeechrecognition� Outputsymbols:vectorsofacoustics(cepstral features)� Hiddenstates:phones

� Soweneedanextension!� AHiddenMarkovModel isanextensionofaMarkovchaininwhichtheinputsymbolsarenotthesameasthestates.

� Thismeanswedon’tknowwhichstatewearein.

Hidden Markov Models

Assumptions�Markovassumption:

�Output-independenceassumption

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P(qi |q1!qi−1) = P(qi |qi−1)

P(ot |O1t−1,q1

t ) = P(ot | q t )

Eisner task

GivenObservedIceCreamSequence:

1,2,3,2,2,2,3…

Produce:HiddenWeatherSequence:

H,C,H,H,H,C…

HMM for ice cream

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Different types of HMM structure

Bakis =left-to-right Ergodic =fully-connected

The Three Basic Problems for HMMs

Problem1(Evaluation):GiventheobservationsequenceO=(o1o2…oT),andanHMMmodelF =(A,B),howdoweefficientlycomputeP(O|F),theprobabilityoftheobservationsequence,giventhemodel?

Problem2(Decoding):GiventheobservationsequenceO=(o1o2…oT),andanHMMmodelF =(A,B),howdowechooseacorrespondingstatesequenceQ=(q1q2…qT) thatisoptimalinsomesense(i.e.,bestexplainstheobservations)?

Problem3(Learning):HowdoweadjustthemodelparametersF =(A,B) tomaximizeP(O|F )?

Jack Ferguson at IDA in the 1960s

Problem 1: computing the observation likelihood

GiventhefollowingHMM:

Howlikelyisthesequence313?

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How to compute likelihood

� ForaMarkovchain,wejustfollowthestates313andmultiplytheprobabilities

� ButforanHMM,wedon’tknowwhatthestatesare!

� Solet’sstartwithasimplersituation.� Computingtheobservationlikelihoodforagivenhiddenstatesequence� SupposeweknewtheweatherandwantedtopredicthowmuchicecreamJasonwouldeat.

� i.e.,P(313|HHC)

Computing likelihood of 3 1 3 given hidden state sequence

Computing joint probability of observation and state sequence

Computing total likelihood of 3 1 3� Wewouldneedtosumover

� Hothotcold� Hothothot� Hotcoldhot� ….

� Howmanypossiblehiddenstatesequencesarethereforthissequence?

� HowaboutingeneralforanHMMwithNhiddenstatesandasequenceofTobservations?� NT

� Sowecan’tjustdoseparatecomputationforeachhiddenstatesequence.

Instead: the Forward algorithm� Adynamicprogrammingalgorithm

� JustlikeMinimumEditDistanceorCKYParsing�Usesatabletostoreintermediatevalues

� Idea:� Computethelikelihoodoftheobservationsequence

� Bysummingoverallpossiblehiddenstatesequences

� Butdoingthisefficiently� Byfoldingallthesequencesintoasingletrellis

The forward algorithm� Thegoaloftheforwardalgorithmistocompute

�We’lldothisbyrecursion

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The forward algorithm� Eachcelloftheforwardalgorithmtrellisalphat(j)

� Representstheprobabilityofbeinginstatej� Afterseeingthefirstt observations�Giventheautomaton

� Eachcellthusexpressesthefollowingprobability

The Forward Recursion

The Forward Trellis

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The Forward Algorithm

Decoding� Givenanobservationsequence

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� AndanHMM� Thetaskofthedecoder

� Tofindthebesthidden statesequence

� GiventheobservationsequenceO=(o1o2…oT),andanHMMmodelF =(A,B),howdowechooseacorrespondingstatesequenceQ=(q1q2…qT) thatisoptimalinsomesense(i.e.,bestexplainstheobservations)

Decoding� Onepossibility:

� ForeachhiddenstatesequenceQ� HHH,HHC,HCH,

� ComputeP(O|Q)� Pickthehighestone

� Whynot?�NT

� Instead:� TheViterbialgorithm� Isagainadynamicprogramming algorithm�UsesasimilartrellistotheForwardalgorithm

Viterbi intuition� Wewanttocomputethejointprobabilityoftheobservationsequencetogetherwiththebeststatesequence

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P(q0,q1,...,qT ,o1,o2,...,oT ,qT = qF | λ)

Viterbi Recursion

The Viterbi trellis

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Viterbi intuition� Processobservationsequencelefttoright� Fillingoutthetrellis� Eachcell:

Viterbi Algorithm

Viterbi backtrace

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HMMs for Speech� Wehaven’tyetshownhowtolearn theAandBmatricesforHMMs;� we’lldothatonThursday� TheBaum-Welch(Forward-Backwardalg)

� Butlet’sreturntothinkaboutspeech

Reminder: a word looks like this:

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The Evaluation (forward) problem for speech� TheobservationsequenceOisaseriesofMFCCvectors

� ThehiddenstatesWarethephonesandwords� Foragivenphone/wordstringW,ourjobistoevaluateP(O|W)

� Intuition:howlikelyistheinputtohavebeengeneratedbyjustthatwordstringW?

Evaluation for speech: Summing over all different paths!� fayayayayvvvv� ffayayayayvvv� ffffayayayayv� ffayayayayayayv� ffayayayayayayayayv� ffayvvvvvvv

The forward lattice for “five”

The forward trellis for “five”

Viterbi trellis for “five”

Viterbi trellis for “five”

Search space with bigrams

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Viterbi trellis

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Viterbi backtrace

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Summary: ASR Architecture� Fiveeasypieces:ASRNoisyChannelarchitecture

� FeatureExtraction:� 39“MFCC” features

� AcousticModel:� Gaussiansforcomputingp(o|q)

� Lexicon/PronunciationModel� HMM:whatphonescanfolloweachother

� LanguageModel� N-gramsforcomputingp(wi|wi-1)

� Decoder� Viterbialgorithm:dynamicprogrammingforcombiningallthesetogetwordsequencefromspeech

67