Papers

This is an overview of scientific publications I authored or co-authored so far. See also the full paper list of the Cognitive Systems Lab (CSL) at KIT, the publications of the Arabic Language Technologies group at QCRI, and the SMT group at the University of Cambridge.

Unfolding and Shrinking Neural Machine Translation Ensembles (2017)

Abstract Ensembling is a well-known technique in neural machine translation (NMT) to improve system performance. Instead of a single neural net, multiple neural nets with the same topology are trained separately, and the decoder generates predictions by averaging over the individual models. Ensembling often improves the quality of the generated translations drastically. However, it is not suitable for production systems because it is cumbersome and slow. This work aims to reduce the runtime to be on par with a single system without compromising the translation quality. First, we show that the ensemble can be unfolded into a single large neural network which imitates the output of the ensemble system. We show that unfolding can already improve the runtime in practice since more work can be done on the GPU. We proceed by describing a set of techniques to shrink the unfolded network by reducing the dimensionality of layers. On Japanese-English we report that the resulting network has the size and decoding speed of a single NMT network but performs on the level of a 3-ensemble system.

Felix Stahlberg and Bill Byrne
In Proceedings of the Conference on Empirical Methods in Natural Language Processing (EMNLP 2017), Copenhagen, Denmark, 9-11 September 2017.

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SGNMT - A Flexible NMT Decoding Platform for Quick Prototyping of New Models and Search Strategies (2017)

Abstract This paper introduces SGNMT, our experimental platform for machine translation research. SGNMT provides a generic interface to neural and symbolic scoring modules (predictors) with left-to-right semantic such as translation models like NMT, language models, translation lattices, n-best lists or other kinds of scores and constraints. Predictors can be combined with other predictors to form complex decoding tasks. SGNMT implements a number of search strategies for traversing the space spanned by the predictors which are appropriate for different predictor constellations. Adding new predictors or decoding strategies is particularly easy, making it a very efficient tool for prototyping new research ideas. SGNMT is actively being used by students in the MPhil program in Machine Learning, Speech and Language Technology at the University of Cambridge for course work and theses, as well as for most of the research work in our group.

Felix Stahlberg, Eva Hasler, Danielle Saunders, and Bill Byrne
In Proceedings of the Conference on Empirical Methods in Natural Language Processing (EMNLP 2017), Copenhagen, Denmark, 9-11 September 2017.

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Neural Machine Translation by Minimising the Bayes-risk with Respect to Syntactic Translation Lattices (2017)

Abstract We present a novel scheme to combine neural machine translation (NMT) with traditional statistical machine translation (SMT). Our approach borrows ideas from linearised lattice minimum Bayes-risk decoding for SMT. The NMT score is combined with the Bayes-risk of the translation according the SMT lattice. This makes our approach much more flexible than n-best list or lattice rescoring as the neural decoder is not restricted to the SMT search space. We show an efficient and simple way to integrate risk estimation into the NMT decoder which is suitable for word-level as well as subword-unit-level NMT. We test our method on English-German and Japanese-English and report significant gains over lattice rescoring on several data sets for both single and ensembled NMT. The MBR decoder produces entirely new hypotheses far beyond simply rescoring the SMT search space or fixing UNKs in the NMT output.

Felix Stahlberg, Adria de Gispert, Eva Hasler, and Bill Byrne
In Proceedings of the 15th Conference of the European Chapter of the Association for Computational Linguistics (EACL17), Valencia, Spain, 3-7 April 2017.

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The Edit Distance Transducer in Action: The University of Cambridge English-German System at WMT16 (2016)

Abstract This paper presents the University of Cambridge submission to WMT16. Motivated by the complementary nature of syntactical machine translation and neural machine translation (NMT), we exploit the synergies of Hiero and NMT in different combination schemes. Starting out with a simple neural lattice rescoring approach, we show that the Hiero lattices are often too narrow for NMT ensembles. Therefore, instead of a hard restriction of the NMT search space to the lattice, we propose to loosely couple NMT and Hiero by composition with a modified version of the edit distance transducer. The loose combination outperforms lattice rescoring, especially when using multiple NMT systems in an ensemble.

Felix Stahlberg, Eva Hasler, and Bill Byrne
In Proceedings of The 1st Conference on Machine Translation (WMT16), Berlin, Germany, 11-12 August 2016.

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Syntactically Guided Neural Machine Translation (2016)

Abstract We investigate the use of hierarchical phrase-based SMT lattices in end-to-end neural machine translation (NMT). Weight pushing transforms the Hiero scores for complete translation hypotheses, with the full translation grammar score and full n-gram language model score, into posteriors compatible with NMT predictive probabilities. With a slightly modified NMT beam-search decoder we find gains over both Hiero and NMT decoding alone, with practical advantages in extending NMT to very large input and output vocabularies.

Felix Stahlberg, Eva Hasler, Aurelien Waite, and Bill Byrne
In Proceedings of The 54th Annual Meeting of the Association for Computational Linguistics (ACL 2016), Berlin, Germany, 7-12 August 2016.

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QATIP - An Optical Character Recognition System for Arabic Heritage Collections in Libraries (2016)

Abstract Nowadays, commercial optical character recognition (OCR) software achieves very high accuracy on high-quality scans of modern Arabic documents. However, a large fraction of Arabic heritage collections in libraries is usually more challenging - e.g. consisting of typewritten documents, early prints, and historical manuscripts. In this paper, we present our end-user oriented QATIP system for OCR in such documents. The recognition is based on the Kaldi toolkit and sophisticated text image normalization. This paper contains two main contributions: First, we describe the QATIP interface for libraries which consists of both a graphical user interface for adding and monitoring jobs and a web API for automated access. Second, we suggest novel approaches for language modelling and ligature modelling for continuous Arabic OCR. We test our QATIP system on an early print and a historical manuscript and report substantial improvements - e.g. 12.6% character error rate with QATIP compared to 51.8% with the best OCR product in our experimental setup (Tesseract).

Felix Stahlberg and Stephan Vogel
In Proceedings of The 12th Document Analysis Systems Workshop (DAS 2016), Santorini, Greece, 11-14 April 2016.

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QAT2 - The QCRI Advanced Transcription and Translation System (2015)

Abstract QAT2 is a multimedia content translation web service developed by QCRI to help content provider to reach audiences and viewers speaking different languages. It is built with establishing open source technologies such as KALDI, Moses and MaryTTS, to provide a complete translation experience for web users. It translates text content in its original format, and produce translated videos with speech-to-speech translation. The result is a complete native language experience for end users on foreign language websites. The system currently supports translation from Arabic to English.

Ahmed Abdelali, Ahmed Ali, Francisco Guzman, Felix Stahlberg, Stephan Vogel, Yifan Zhang
In Proceedings of the 16th Annual Conference of International Speech Communication Association (INTERSPEECH 2015), Dresden, Germany, 6-10 September 2015.

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The QCRI Recognition System for Handwritten Arabic (2015)

Abstract This paper describes our recognition system for handwritten Arabic. We propose novel text line image normalization procedures and a new feature extraction method. Our recognition system is based on the Kaldi recognition toolkit which is widely used in automatic speech recognition (ASR) research. We show that the combination of sophisticated text image normalization and state-of-the art techniques originating from ASR results in a very robust and accurate recognizer. Our system outperforms the best systems in the literature by over 20% relative on the abcde-s configuration of the IFN/ENIT database and achieves comparable performance on other configurations. On the KHATT corpus, we report 11% relative improvement compared to the best system in the literature.

Felix Stahlberg and Stephan Vogel
In Proceedings of The 18th International Conference on Image Analysis and Processing (ICIAP 2015), Genova, Italy, 9-11 September 2015.

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Detecting Dense Foreground Stripes in Arabic Handwriting for Accurate Baseline Positioning (2015)

Abstract Since Arabic script has a strong baseline, many state-of-the-art recognition systems for handwritten Arabic make use of baseline-dependent features. For printed Arabic, the baseline can be detected reliably by finding the maximum in the horizontal projection profile or the Hough transformed image. However, the performance of these methods drops significantly on handwritten Arabic. In this work, we present a novel approach to baseline detection in handwritten Arabic which is based on the detection of stripes in the image with dense foreground. Such a stripe usually corresponds to the area between lower and upper baseline. Our method outperforms a previous method by 22.4% relative for the task of finding acceptable baselines in Tunisian town names in the IFN/ENIT database.

Felix Stahlberg and Stephan Vogel
In Proceedings of The 13th International Conference on Document Analysis and Recognition (ICDAR 2015), Nancy, France, 23-26 August 2015.

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Document Skew Detection Based on Hough Space Derivatives (2015)

Abstract One of the basic challenges in page layout analysis of scanned document images is the estimation of the document skew. Precise skew correction is particularly important when the document is to be passed to an optical character recognition system. In this paper, we propose a very generic and robust method which can cope with a wide variety of document types and writing systems. It uses derivatives in the Hough space to identify directions with sudden changes in their projection profiles. We show that this criterion is useful to identify the horizontal and vertical direction with respect to the document. We test our method on the DISEC’13 data set for document skew detection. Our results are comparable to the best systems in the literature.

Felix Stahlberg and Stephan Vogel
In Proceedings of The 13th International Conference on Document Analysis and Recognition (ICDAR 2015), Nancy, France, 23-26 August 2015.

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Cross-lingual Lexical Language Discovery from Audio Data Using Multiple Translations (2015)

Abstract Zero-resource Automatic Speech Recognition (ZR ASR) addresses target languages without given pronunciation dictionary, transcribed speech, and language model. Lexical discovery for ZR ASR aims to extract word-like chunks from speech. Lexical discovery benefits from the availability of written translations in another source language. In this paper, we improve lexical discovery even more by combining multiple source languages. We present a novel method for combining noisy word segmentations resulting in up to 11.2% relative F-score gain. When we extract word pronunciations from the combined segmentations to bootstrap an ASR system, we improve accuracy by 9.1% relative compared to the best system with only one translation, and by 50.1% compared to monolingual lexical discovery.

Felix Stahlberg, Tim Schlippe, Stephan Vogel, and Tanja Schultz
In Proceedings of The 40th International Conference on Acoustics, Speech, and Signal Processing (ICASSP 2015), Brisbane, Australia, 19-24 April 2015.

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Journal: Word Segmentation and Pronunciation Extraction from Phoneme Sequences through Cross-Lingual Word-to-Phoneme Alignment (2014)

Abstract In this paper, we study methods to discover words and extract their pronunciations from audio data for non-written and under-resourced languages. We examine the potential and the challenges of pronunciation extraction from phoneme sequences through cross-lingual word-to-phoneme alignment. In our scenario a human translator produces utterances in the (non-written) target language from prompts in a resource-rich source language. We add the resource-rich source language prompts to help the word discovery and pronunciation extraction process. By aligning the source language words to the target language phonemes, we segment the phoneme sequences into word-like chunks. The resulting chunks are interpreted as putative word pronunciations but are very prone to alignment and phoneme recognition errors. Thus we suggest our alignment model Model 3P that is particularly designed for cross-lingual word-to-phoneme alignment. We present two different methods (source word dependent and independent clustering) that extract word pronunciations from word-to-phoneme alignments and compare them. We show that both methods compensate for phoneme recognition and alignment errors. We also extract a parallel corpus consisting of 15 different translations in 10 languages from the Christian Bible to evaluate our alignment model and error recovery methods. For example, based on noisy target language phoneme sequences with 45.1% errors, we build a dictionary for an English Bible with a Spanish Bible translation with 4.5% OOV rate, where 64% of the extracted pronunciations contain no more than one wrong phoneme. Finally, we use the extracted pronunciations in an automatic speech recognition system for the target language and report promising word error rates – given that pronunciation dictionary and language model are learned completely unsupervised and no written form for the target language is required for our approach.

Felix Stahlberg, Tim Schlippe, Stephan Vogel, and Tanja Schultz
In Computer Speech & Language, 2014.

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Towards Automatic Speech Recognition Without Pronunciation Dictionary, Transcribed Speech and Text Resources in the Target Language Using Cross-Lingual Word-to-Phoneme Alignment (2014)

Abstract In this paper we tackle the task of bootstrapping an Automatic Speech Recognition system without an a priori given language model, a pronunciation dictionary, or transcribed speech data for the target language Slovene -- only untranscribed speech and translations to other resource-rich source languages of what was said are available. Therefore, our approach is highly relevant for under-resourced and non-written languages. First, we borrow acoustic models from a strongly related language (Croatian) and apply a Croatian phoneme recognizer to the Slovene speech. Second, we segment the recognized phoneme strings into word units using cross-lingual word-to-phoneme alignment. Third, we compensate for phoneme recognition and alignment errors in the segmented phoneme sequences and aggregate the resulting phoneme sequence segments in a pronunciation dictionary for Slovene. Orthographic representations are generated using a Croatian phoneme-to-grapheme model. Finally, we use the resulting dictionary and the Croatian acoustic models to recognize Slovene. Our best recognizer achieves a Character Error Rate of 52% on the BMED corpus.

Felix Stahlberg, Tim Schlippe, Stephan Vogel, and Tanja Schultz
In Proceedings of The 4th International Workshop on Spoken Language Technologies for Under-resourced Languages (SLTU 2014), St. Petersburg, Russia, 14-16 May 2014.

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Pronunciation Extraction from Phoneme Sequences Through Cross-Lingual Word-to-Phoneme Alignment (2013)

Abstract With the help of written translations in a source language, we cross-lingually segment phoneme sequences in a target language into word units using our new alignment model Model 3P. From this, we deduce phonetic transcriptions of target language words, introduce the vocabulary in terms of word IDs, and extract a pronunciation dictionary. Our approach is highly relevant to bootstrap dictionaries from audio data for Automatic Speech Recognition and bypass the written form in Speech-to-Speech Translation, particularly in the context of under-resourced languages, and those which are not written at all. Analyzing 14 translations in 9 languages to build a dictionary for English shows that the quality of the resulting dictionary is better if the vocabularies in source and target language are about the same size, shorter sentences, more word repetitions, and formal equivalent translations.

Felix Stahlberg, Tim Schlippe, Stephan Vogel, and Tanja Schultz
In Proceedings of The 1st International Conference on Statistical Language and Speech Processing (SLSP 2013), Tarragona, Spain, 29-31 July 2013.

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Word Segmentation Through Cross-Lingual Word-To-Phoneme Alignment (2012)

Abstract We present our new alignment model Model 3P for cross-lingual word-to-phoneme alignment, and show that unsupervised learning of word segmentation is more accurate when information of another language is used. Word segmentation with cross-lingual information is highly relevant to bootstrap pronunciation dictionaries from audio data for Automatic Speech Recognition, bypass the written form in Speech-to-Speech Translation or build the vocabulary of an unseen language, particularly in the context of under-resourced languages. Using Model 3P for the alignment between English words and Spanish phonemes outperforms a state-of-the-art monolingual word segmentation approach on the BTEC corpus by up to 42% absolute in F-Score on the phoneme level and a GIZA++ alignment based on IBM Model 3 by up to 17%.

Felix Stahlberg, Tim Schlippe, Stephan Vogel, and Tanja Schultz
In Proceedings of The Fourth IEEE Workshop on Spoken Language Technology (SLT 2012), Miami, Florida, 2-5 December 2012.

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