Proceedings of the 8th International Workshop on Spoken Language Translation: Papers

Marcello Federico, Mei-Yuh Hwang, Margit Rödder, Sebastian Stüker (Editors)

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December 8-9
San Francisco, California
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Lexicon models for hierarchical phrase-based machine translation
Matthias Huck | Saab Mansour | Simon Wiesler | Hermann Ney

In this paper, we investigate lexicon models for hierarchical phrase-based statistical machine translation. We study five types of lexicon models: a model which is extracted from word-aligned training data and—given the word alignment matrix—relies on pure relative frequencies [1]; the IBM model 1 lexicon [2]; a regularized version of IBM model 1; a triplet lexicon model variant [3]; and a discriminatively trained word lexicon model [4]. We explore sourceto-target models with phrase-level as well as sentence-level scoring and target-to-source models with scoring on phrase level only. For the first two types of lexicon models, we compare several scoring variants. All models are used during search, i.e. they are incorporated directly into the log-linear model combination of the decoder. Phrase table smoothing with triplet lexicon models and with discriminative word lexicons are novel contributions. We also propose a new regularization technique for IBM model 1 by means of the Kullback-Leibler divergence with the empirical unigram distribution as regularization term. Experiments are carried out on the large-scale NIST Chinese→English translation task and on the English→French and Arabic→English IWSLT TED tasks. For Chinese→English and English→French, we obtain the best results by using the discriminative word lexicon to smooth our phrase tables.

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The 2011 KIT QUAERO speech-to-text system for Spanish
Kevin Kilgour | Christian Saam | Christian Mohr | Sebastian Stüker | Alex Waibel

This paper describes our current Spanish speech-to-text (STT) system with which we participated in the 2011 Quaero STT evaluation that is being developed within the Quaero program. The system consists of 4 separate subsystems, as well as the standard MFCC and MVDR phoneme based subsystems we included a both a phoneme and grapheme based bottleneck subsystem. We carefully evaluate the performance of each subsystem. After including several new techniques we were able to reduce the WER by over 30% from 20.79% to 14.53%.

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Named entity translation using anchor texts
Wang Ling | Pável Calado | Bruno Martins | Isabel Trancoso | Alan Black | Luísa Coheur

This work describes a process to extract Named Entity (NE) translations from the text available in web links (anchor texts). It translates a NE by retrieving a list of web documents in the target language, extracting the anchor texts from the links to those documents and finding the best translation from the anchor texts, using a combination of features, some of which, are specific to anchor texts. Experiments performed on a manually built corpora, suggest that over 70% of the NEs, ranging from unpopular to popular entities, can be translated correctly using sorely anchor texts. Tests on a Machine Translation task indicate that the system can be used to improve the quality of the translations of state-of-the-art statistical machine translation systems.

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Unsupervised vocabulary selection for simultaneous lecture translation
Paul Maergner | Kevin Kilgour | Ian Lane | Alex Waibel

In this work, we propose a novel method for vocabulary selection which enables simultaneous speech recognition systems for lectures to automatically adapt to the diverse topics that occur in educational and scientific lectures. Utilizing materials that are available before the lecture begins, such as lecture slides, our proposed framework iteratively searches for related documents on the World Wide Web and generates a lecture-specific vocabulary and language model based on the resulting documents. In this paper, we introduce a novel method for vocabulary selection where we rank vocabulary that occurs in the collected documents based on a relevance score which is calculated using a combination of word features. Vocabulary selection is a critical component for topic adaptation that has typically been overlooked in prior works. On the interACT German-English simultaneous lecture translation system our proposed approach significantly improved vocabulary coverage, reducing the out-of-vocabulary rate on average by 57.0% and up to 84.9%, compared to a lecture-independent baseline. Furthermore, our approach reduced the word error rate by up to 25.3% (on average 13.2% across all lectures), compared to a lectureindependent baseline.

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Combining translation and language model scoring for domain-specific data filtering
Saab Mansour | Joern Wuebker | Hermann Ney

The increasing popularity of statistical machine translation (SMT) systems is introducing new domains of translation that need to be tackled. As many resources are already available, domain adaptation methods can be applied to utilize these recourses in the most beneficial way for the new domain. We explore adaptation via filtering, using the crossentropy scores to discard irrelevant sentences. We focus on filtering for two important components of an SMT system, namely the language model (LM) and the translation model (TM). Previous work has already applied LM cross-entropy based scoring for filtering. We argue that LM cross-entropy might be appropriate for LM filtering, but not as much for TM filtering. We develop a novel filtering approach based on a combined TM and LM cross-entropy scores. We experiment with two large-scale translation tasks, the Arabic-to-English and English-to-French IWSLT 2011 TED Talks MT tasks. For LM filtering, we achieve strong perplexity improvements which carry over to the translation quality with improvements up to +0.4% BLEU. For TM filtering, the combined method achieves small but consistent improvements over the standalone methods. As a side effect of adaptation via filtering, the fully fledged SMT system vocabulary size and phrase table size are reduced by a factor of at least 2 while up to +0.6% BLEU improvement is observed.

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Using Wikipedia to translate domain-specific terms in SMT
Jan Niehues | Alex Waibel

When building a university lecture translation system, one important step is to adapt it to the target domain. One problem in this adaptation task is to acquire translations for domain specific terms. In this approach we tried to get these translations from Wikipedia, which provides articles on very specific topics in many different languages. To extract translations for the domain specific terms, we used the interlanguage links of Wikipedia . We analyzed different methods to integrate this corpus into our system and explored methods to disambiguate between different translations by using the text of the articles. In addition, we developed methods to handle different morphological forms of the specific terms in morphologically rich input languages like German. The results show that the number of out-of-vocabulary (OOV) words could be reduced by 50% on computer science lectures and the translation quality could be improved by more than 1 BLEU point.

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Modeling punctuation prediction as machine translation
Stephan Peitz | Markus Freitag | Arne Mauser | Hermann Ney

Punctuation prediction is an important task in Spoken Language Translation. The output of speech recognition systems does not typically contain punctuation marks. In this paper we analyze different methods for punctuation prediction and show improvements in the quality of the final translation output. In our experiments we compare the different approaches and show improvements of up to 0.8 BLEU points on the IWSLT 2011 English French Speech Translation of Talks task using a translation system to translate from unpunctuated to punctuated text instead of a language model based punctuation prediction method. Furthermore, we do a system combination of the hypotheses of all our different approaches and get an additional improvement of 0.4 points in BLEU.

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Soft string-to-dependency hierarchical machine translation
Jan-Thorsten Peter | Matthias Huck | Hermann Ney | Daniel Stein

In this paper, we dissect the influence of several target-side dependency-based extensions to hierarchical machine translation, including a dependency language model (LM). We pursue a non-restrictive approach that does not prohibit the production of hypotheses with malformed dependency structures. Since many questions remained open from previous and related work, we offer in-depth analysis of the influence of the language model order, the impact of dependency-based restrictions on the search space, and the information to be gained from dependency tree building during decoding. The application of a non-restrictive approach together with an integrated dependency LM scoring is a novel contribution which yields significant improvements for two large-scale translation tasks for the language pairs Chinese–English and German–French.

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Speaker alignment in synthesised, machine translation communication
Anne H. Schneider | Saturnino Luz

The effect of mistranslations on the verbal behaviour of users of speech-to-speech translation is investigated through a question answering experiment in which users were presented with machine translated questions through synthesized speech. Results show that people are likely to align their verbal behaviour to the output of a system that combines machine translation, speech recognition and speech synthesis in an interactive dialogue context, even when the system produces erroneous output. The alignment phenomenon has been previously considered by dialogue system designers from the perspective of the benefits it might bring to the interaction (e.g. by making the user more likely to employ terms contained in the system’s vocabulary). In contrast, our results reveal that in speech-to-speech translation systems alignment can in fact be detrimental to the interaction (e.g. by priming the user to align with non-existing lexical items produced by mistranslation). The implications of these findings are discussed with respect to the design of such systems.

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How good are your phrases? Assessing phrase quality with single class classification
Nadi Tomeh | Marco Turchi | Guillaume Wisinewski | Alexandre Allauzen | François Yvon

We present a novel translation quality informed procedure for both extraction and scoring of phrase pairs in PBSMT systems. We reformulate the extraction problem in the supervised learning framework. Our goal is twofold. First, We attempt to take the translation quality into account; and second we incorporating arbitrary features in order to circumvent alignment errors. One-Class SVMs and the Mapping Convergence algorithm permit training a single-class classifier to discriminate between useful and useless phrase pairs. Such classifier can be learned from a training corpus that comprises only useful instances. The confidence score, produced by the classifier for each phrase pairs, is employed as a selection criteria. The smoothness of these scores allow a fine control over the size of the resulting translation model. Finally, confidence scores provide a new accuracy-based feature to score phrase pairs. Experimental evaluation of the method shows accurate assessments of phrase pairs quality even for regions in the space of possible phrase pairs that are ignored by other approaches. This enhanced evaluation of phrase pairs leads to improvements in the translation performance as measured by BLEU.

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Annotating data selection for improving machine translation
Keiji Yasuda | Hideo Okuma | Masao Utiyama | Eiichiro Sumita

In order to efficiently improve machine translation systems, we propose a method which selects data to be annotated (manually translated) from speech-to-speech translation field data. For the selection experiments, we used data from field experiments conducted during the 2009 fiscal year in five areas of Japan. For the selection experiments, we used data sets from two areas: one data set giving the lowest baseline speech translation performance for its test set, and another data set giving the highest. In the experiments, we compare two methods for selecting data to be manually translated from the field data. Both of them use source side language models for data selection, but in different manners. According to the experimental results, either or both of the methods show larger improvements compared to a random data selection.