Coupling between ASR and MT in SpeechtoSpeech Translation

1
Coupling between ASR and MT in Speech-to-Speech
Translation

• Arthur Chan
• Prepared for
• Advanced Machine Translation Seminar

2
This Seminar

• Introduction (6 slides)
• Ringgers categorization of Coupling between ASR
  and NLU (7 slides)
• Interfaces in Loose Coupling
• 1 best and N-best (5 slides)
• Lattices/Confusion Network/Confidence Estimation
  (12 slides)
• Results from literature
• Tight Coupling
• Neys Theory and 2 methods of Implementation (14
  slides)
• ? Sorry, without FST approaches.
• Some As Is Ideas on This Topic

3
6 papers on Coupling of Speech-to-Speech
Translation

• H. Ney, Speech translation Coupling of
  recognition and translation, in Proc. ICASSP,
  1999.
• Casacuberta et al., Architectures for
  speech-to-speech translation using finite-state
  models, in Proc. Workshop on Speech-to-Speech
  Translation, 2002.
• E. Matusov, S.Kanthak, and H. Ney, On the
  integration of speech recognition and statistical
  machine translation, in Proc. InterSpeech,
  2005.
• S.Saleem, S. C. Jou, S. Vogel, and T. Schultz,
  Using word lattice information for a tighter
  coupling in speech translation systems, in Proc.
  ICSLP, 2004.
• V.H. Quan et al., Integrated N-best re-ranking
  for spoken language translation, in In
  EuroSpeech, 2005.
• N. Bertoldi and M. Federico, A new decoder for
  spoken language translation based on confusion
  networks, in IEEE ASRU Workshop, 2005.

4
A Conceptual Model of Speech-to-Speech Translation
Speech Recognizer
Machine Translator
Speech Synthesizer
Decoding Result(s)
Translation
waveforms
waveforms
5
Motivation of Tight Coupling between ASR and MT

• One best of ASR could be wrong
• MT could be benefited from wide range of
  supplementary information provided by ASR
• N-best list
• Lattice
• Sentenced/Word-based Confidence Scores
• E.g. Word posterior probability
• Confusion network
• Or consensus decoding (Mangu 1999)
• MT quality may depend on WER of ASR (?)

6
Scope of this talk.
Speech Recognizer
Machine Translator
Speech Synthesizer
1-best?
Translation
N-best?
waveforms
waveforms
Lattice?
Confusion network?
7
Topics Covered Today

• The concept of Coupling
• Tightness of coupling between ASR and
  Technology X. (Ringger 95)
• Two questions
• What could ASR provide in loose coupling?
• Discussion of interfaces between ASR and MT in
  loose coupling
• What is the status of tight coupling?
• Neys Formulation

8
Topics not covered

• Direct Modeling
• Use both features in ASR and MT
• Some referred as ASR and MT unification
• Implication of the MT search algorithms on the
  coupling
• Generation of speech from text.
• Presenter doesnt know enough.

9
The Concept of Coupling
10
Classification of Coupling of ASR and Natural
Language Understanding (NLU)

• Proposed in Ringger 95, Harper 94
• 3 Dimensions of ASR/NLU
• Complexity of the search algorithm
• Simple N-gram?
• Incrementality of the coupling
• On-line? Left-to-right?
• Tightness of the coupling
• Tight? Loose? Semi-tight?

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Tightness of Coupling
Tight
Semi-Tight
Loose
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Notes

• Semi-tight coupling could appear as
• Feedback loop between ASR and Technology X for
  the whole utterance of speech
• Or Feedback loop between ASR and Technology X for
  every frame.
• The Ringger system
• A good way to understand how speech-based system
  is developed

13
Example 1 LM

• Someone asserts that ASR has to be used with
  13-grams.
• In tight-coupling,
• A search will be devised to search for the best
  word sequence with best acoustic score 13 gram
  likelihood
• In loose coupling
• A simple search will be used to generate some
  outputs (N-best list, lattice etc.),
• 13-gram will then use to rescore the output.
• In semi-tight coupling
• 1, A simple search will be used to generate
  results
• 2, 13 gram will be applied at the word-end only
  (but exact history will not be stored)

14
Example 2 Higher order AM

• Segmental model assume obs. probability is not
  conditionally independent.
• Someone assert that segmental model is better
  than just HMM.
• Tight coupling Direct search of the best word
  sequence using segmental model.
• Loose coupling Use segmental model to rescore
• Semi-tight coupling Hybrid HMM-Segmental model
  algorithm?

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Summary of Coupling between ASR and NLU
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Implication on ASR/MT coupling

• Generalize many systems
• Loose coupling
• Any system which uses 1-best, n-best, lattice, or
  other inputs for 1-way module communication
• (Bertoldi 2005)
• CMU System (Saleem 2004)
• (Matusov 2005)
• Tight coupling
• (Ney 1999)
• (Casacuberta 2002)
• Semi-tight coupling
• (Quan 2005)

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Interfaces in Loose Coupling1-best and N-best
18
Perspectives

• ASR outputs
• 1-best results
• N-best results
• Lattice
• Consensus network.
• Confidence scores
• How ASR generate these outputs?
• Why they are generated?
• What if there are multiple ASRs?
• (and what if their results are combined?)

19
Origin of the 1-best.

• Decoding of HMM-based ASR
• Searching the best path in a huge HMM-state
  lattice.
• 1-best ASR result
• The best path one could find from backtracking.
• State Lattice (Next page)

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Note on 1-best

• Most of the time 1-best Word Sequence
• Why?
• In LVCSR, storing the backtracking pointer table
  for state sequence takes a lot of memory (even
  nowadays)
• Compare this with the number of frames of score
  one need to be stored
• Usually a backtrack pointer storing
• The previous words before the current word
• Clever structure dynamically allocate
  back-tracking pointer table.

22
What is N-best list?

• Traceback not only from the 1st -best, also from
  the 2nd best and 3rd best, etc.
• Pathway
• Directly from search backtrack pointer table
• Exact N-best algorithm (Chow 90)
• Word pair N-best algorithm (Chow 91)
• A search using Viterbi score as heuristic (Chow
  92)
• Generate lattice first, then generate N-best from
  lattice

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Interfaces in Loose CouplingLattice, Consensus
Network and Confidence Estimation
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What is Lattice?

• A compact representation of state-lattice
• Only word node (or link) are involved
• Difference between N-best and Lattice
• Lattice could be compact representation of N-best
  list.

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How lattice is generated?

• From the decoding backtracking pointer table
• Only record all the links between word nodes.
• From N-best list
• Become a compact representation of N-best
• Sometimes spurious link will be introduced

27
How lattice is generated when there are phone
contexts at the word end?

• Very complicated when phonetic context is
  involved
• Not only word-end needs to be stored but also the
  phone contexts.
• Lattice has the word identity as well as contexts
• Lattice can become very large.

28
How this is resolved?

• Some used only approximate triphone to generate
  lattice in first stage (BBN)
• Some generate lattice even with full CD-phones
  but convert it back to no-context lattices (RWTH)
• Use the lattice with full CD phone contexts
  (RWTH)

29
What ASR folks do when lattice is still too large?

• Use some criteria to prune the lattice.
• Example Criteria
• Word posterior probability
• Application of another LM or AM, then filtering.
• General confidence score
• Maximum lattice density
• (number of words in lattice/number of words)
• Or generate an even more compact representation
  than lattices
• E.g. consensus network.

30
Conclusions on lattices

• Lattice generation itself could be a complicated
  issue
• Sometimes, what post-processing stage (e.g. MT)
  will get is pre-filtered, pre-processed results.

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Confusion Network and Consensus Hypothesis

• Confusion Network
• Or Sausage Network.
• Or Consensus Network

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Special Properties (?)

• More local than lattice
• One can apply simple criteria to find the best
  results
• E.g. consensus decoding is to apply
  word-posterior probability on confusion network.
• More tractable
• In terms of size
• Found to be useful in
• ?
• ?

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How to generate consensus network?

• From the lattice
• Summary of Mangus algorithm
• Intra-word clustering
• Inter-word clustering

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Note on Consensus Network

• Note
• Time information might not be preserved in
  confusion network
• The similarity function directly affect the final
  output of the consensus network.

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Other ways to generate confusion network

• From the N-best list
• Using Rover.
• A mixture of voting and adding confidence of word

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Confidence Measure

• Anything other than likelihood which could tell
  whether the answer is useful
• E.g.
• Word posterior probability
• P(WA)
• Usually compute using lattices
• Language model backoff mode
• Other posterior probabilities (frame, sentence)

37
Interfaces in Loose CouplingResults from the
Literature
38
General word

• Coupling in SST is still pretty new
• Papers are chosen according to whether some
  outputs have been used
• Other techniques such as direct modeling might be
  mixed into the papers.

39
N-best list (Quan 2005)

• Using N-best list for reranking
• Interpolation weights of AM and TM are then
  optimized.
• Summary
• Reranking gives improvements.

40
Lattices CMU results (Saleem 2004)

• Summary of results
• Lattice word error rate improved when lattice
  density improves
• Lattice density and Weight on Acoustic scores
  turns out to be an important parameter to tune
• Too large and small could hurt.

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LWER against Lattice Density
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Modified Bleu scores against lattice density
43
Optimal density and score weight based on
Utterance Length.
44
Consensus Network

• Bertoldi 2005 is probably the only work on
  confusion-network based method
• Summary of results
• When direct modeling is applied
• Consensus Network doesnt beat N-best method.
• Author argues for speed and simplicity of the
  algorithm

45
Confidence Does it help?

• According to Zhang 2006, Yes.
• Confidence Measure (CM) filtering is used to
  filter out unnecessary results in N-best
• Note The approaches used is quite different.

46
Conclusion on Loose Coupling

• SR could give a rich sets of output.
• It is still an unknown what type of output should
  be used in pipeline.
• Currently, it seem to lack of comprehensive
  experimental studies on which method is the best.
  
• Usage of confusion network and confidence
  estimation seem to be under-explored.

47
Tight Coupling Theory and Practice
48
Theory (Ney 1999)
Bayes Rule
Introduce f as hidden var.
Bayes Rule
Assume x doesnt depend on target lang.
Sum to Max
49
Layman point of view

• Three factors
• Pr(e) target language model
• Pr(fe) translation model
• Pr(xf) acoustic model
• Note assumption has been made only the best
  matching f for e is used.

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Comparison with SR

• In SR
• Pr(f) Source language model
• In Tight coupling
• Pr(fe), Pr(e) Translation model and Target
  language model

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Algorithmic Point of View

• Brute Force Method Instead of incorporating LM
  into standard Viterbi algorithm
• Incoporating P(e) and P(fe)
• gt Very complicated

52
Assumptions in Modeling

• Alignment Models (HMM)
• Acoustic Modeling
• Speech Recognizer will produce a word graph.
• Each link with word hypothesis covers the portion
  of acoustic scores. (notation is confusing in
  paper)

53
Lexicon Modeling

• Further assumption from standard IBM models
• Target word is assumed to be dependent on
  previous word
• So, in fact, source LM is actually there.

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First Implementation Local Average Assumptions

• Local Average Assumptions
• P(xe) is used to capture the local
  characteristic of the acoustic.

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Justification of Using Average Local Assumption

• Rephrased from Author (p.3 para 2)
• Lexicon modeling and language modeling will cause
  f_j-1, f_j, f_j1 appear in the math.
• In another words
• It is too complicated to carry out
• Computation advantage the local score could be
  obtained just from the word graph but before
  translation
• gt Full translation strategy could still be
  carried out

56
Computation of P(xe)

• Make use of best source sequence
• Also refer to Wessel 98,
• A commonly used word posterior probability
  algorithm for lattice
• A forward-backward like procedure is used

57
Second Method Monotone Alignment Assumption -
Network
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Monotone Alignment Assumption Formula for Text
Input

• Close-formed solution exist form DP O(JE2)

59
Monotone Alignment Assumption Formula for
Speech Input

• DP
• O(JE2F2)

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How to make Monotone Assumptions work?

• Words needs to be reordered
• As part of search strategy.
• Does acoustic model assumption used?
• i.e. Are we talking about word lattice or still
  state lattice?
• Dont know, seems like we are actually talking
  about word lattice.
• Supported by Matusov 2005

61
Experimental Results in Matusov, Kanthak and Ney
2005

• Summary of the results
• Translation quality is only improved by tight
  coupling when the lattice density is not high.
• Same as Saleem 2004, incorporation of acoustic
  scores help.

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Conclusion Possible Issues of tight coupling

• Possibilities
• In SR, source n-gram LM is very closed to the
  best configuration.
• The complexity of the algorithm is too high,
  approximation is still necessary to make it work.
• When the criterion in tight coupling is used. It
  is possible that the LM and the TM need to be
  jointly estimated.
• The current approaches still havent really
  implement tight-coupling
• There might be bugs in the programs.

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Conclusion

• Two major issues in coupling of SST is discussed
• In loose coupling
• Consensus network and Confidence scoring is still
  not fully utilized
• In tight coupling
• The approach seem to be haunted by very high
  complexity of search algorithm construction

64
Discussion
65
The End. Thanks.
66
Literature

• 2006 Ruiqiang Zhang, Genichiro Kikui. Integration
  of Speech Recognition and Machine Translation
  Speech Recognition Word Lattice Translation.
  Speech Communication. Vol.48, Issues 3-4
• H. Ney, Speech translation Coupling of
  recognition and translation, in Proc. ICASSP,
  1999.
• E. Matusov, S.Kanthak, and H. Ney, On the
  integration of speech recognition and statistical
  machine translation, in Proc. InterSpeech, 2005.
• S.Saleem, S. C. Jou, S. Vogel, and T. Schultz,
  Using word lattice information for a tighter
  coupling in speech translation systems, in Proc.
  ICSLP, 2004.
• V.H. Quan et al., Integrated N-best re-ranking
  for spoken language translation, in In
  EuroSpeech, 2005.
• N. Bertoldi and M. Federico, A new decoder for
  spoken language translation based on confusion
  networks, in IEEE ASRU Workshop, 2005.
• L. Mangu, E. Brill, A. Stolcke, Finding
  consensus in speech recognition word error
  minimization and other applications of confusion
  networks, Computer Speech and Language 14(4),
  373-400., (2000)
• E. Ringger, A Robust Loose Coupling for Speech
  Recognition and Natural Language Understanding,
  1995