An Introduction to Machine Translation

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2 An Introduction to Machine Translation W. John Hutchins The Library, University of East Anglia, Norwich, UK and Harold L. Somers Centre for Computational Linguistics, University of Manchester Institute of Science and Technology, Manchester, UK ACADEMIC PRESS Harcourt Brace Jovanovich, Publishers LONDON SAN DIEGO NEW YORK BOSTON SYDNEY TOKYO TORONTO

3 This book is printed on acid-free paper ACADEMIC PRESS LIMITED Oval Road LONDON NW1 7DX United States Edition published by ACADEMIC PRESS INC. San Diego, CA Copyright 1992 by ACADEMIC PRESS LIMITED All Flights Reserved No part of this book may be reproduced in any form by photostat, microfilm, or by any other means, without written permission from the publishers A catalogue record for this book is available from the British Library ISBN X Printed in Great Britain at the University Press, Cambridge

4 Contents Foreword by Martin Kay... xi Preface... xv List of abbreviations and symbols... xix 1. General introduction and brief history The aims of MT Some preliminary definitions Brief history of MT Further reading Linguistic background The study of language Grammar Phonology and orthography Morphology and the lexicon Syntax Syntactic features and functions Deep and surface structure Predicate-argument structure Semantics Text relations Representations Dependency Phrase structure Feature-based representations Canonical and logical form... 27

5 vi Contents 2.9. Formal grammar and linguistic theory Context free grammars and rewrite rules Transformational rules Traces and gaps X theory Valency grammar Case grammar Unification grammar Influential theories and formalisms Lexical Functional Grammar Categorial grammar Government and Binding Generalized Phrase Structure Grammar Semantic compositionality Further reading Computational aspects Data and programs Separation of algorithms and data Modularity System design Problems of input and output Interactive systems 'Foreign' languages Lexical databases Computational techniques in early systems Parsing Top-down and bottom-up Backtracking Feature notations Trees Charts Production systems Unification Further reading Basic strategies Multilingual versus bilingual systems Direct systems, transfer systems and interlinguas Non-intervention vs. on-line interactive systems Lexical data Further reading Analysis Morphology problems Lexical ambiguity Category ambiguity Homography and polysemy Transfer ambiguity... 87

6 Contents vii 5.3. Structural ambiguity Types of structural ambiguity Real structural ambiguity Accidental structural ambiguity Resolution of structural ambiguity Use of linguistic knowledge Contextual knowledge Real world knowledge Other strategies Anaphora resolution Quantifier scope ambiguity Further reading Problems of transfer and interlingua Lexical differences Structural differences Levels of transfer representation Morphological transfer Transfer-based systems Lexical transfer Structural transfer Transfer with a structural interlingua Interlingua-based systems Structural representation in interlingua systems Lexical representation in interlingua systems 'Restricted' interlinguas Knowledge-based methods Example-based methods Summary: Comparison of transfer-based and interlingua-based systems Further reading Generation Generation in direct systems Generation in indirect systems Generation in transfer-based systems Generation in interlingua systems Pre-determined choices and structure preservation Stylistic improvements in generation Generation as the 'reverse' of analysis Further reading The practical use of MT systems Fully automatic high quality translation (FAHQT) Machine-aided human translation (MAHT) Human-aided machine translation (HAMT) Pre-editing Post-editing Interactive MT

7 viii Contents Interactive systems for monolingual users Sublanguage systems Use of low quality MT Further reading Evaluation of MT systems Types and stages of evaluation Linguistic evaluation of 'raw' output Quality assessment Error analysis Evaluation by researchers Evaluation by developers Evaluation by potential users Linguistic assessment Evaluation of limitations, improvability, and extendibility Technical assessment Assessment of personnel requirements and ease of use Evaluation of costs and benefits Evaluation by translators Evaluation by recipients Further reading Systran Historical background The basic system Dictionaries Computational aspects Translation processes Characteristics of the system Improvability Evaluations, users, and performance Sources and further reading SUSY Background Basic system design Data structure Pre-editing and the fail-soft RESCUE operator Analysis Text input and dictionary look-up Morphological analysis Homograph disambiguation Phrasal analysis Structural analysis Semantic disambiguation Transfer and synthesis Conclusion Sources and further reading

8 Contents ix 12. Météo Historical background The translation environment: input, pre-processing and post-editing The translation processes Dictionary look-up Syntactic analysis Syntactic and morphological generation The computational processes The data structure Rule formalism Rule application Summary and discussion Sources and further reading Ariane (GETA) Historical background General description Multi-level representation Linguistic processes Morphological analysis Multi-level analysis Transfer and generation Rule-writing formalisms ATEF ROBRA TRANSF and SYGMOR Concluding remarks Sources and further reading Eurotra Background Organisation and system design Computational approach Objects and structures Translators and generators Implementation Linguistic aspects Research on linguistic topics An illustrative example Conclusions Sources and further reading METAL Historical background... ; The basic system The linguistic databases Dictionaries Grammatical rules

9 x Contents The translation programs Characteristics of the German-English system Recent developments towards a multilingual system Evaluations, users and performance Sources and further reading Rosetta Background Montague grammar Reversibility and isomorphism Translation processes Structural correspondences Subgrammars Rule classes Lexical transfer Comments and conclusions Sources and further reading DLT Background The interlingua System design Dependency parsing Metataxis Interlingual data and SWESIL English to Esperanto semantic processing Esperanto to French semantic processing Evaluation of SWESIL Conclusions Sources and further reading Some other systems and directions of research AI and Knowledge-based MT at CMU Example-based MT at BSO Statistics-based MT at IBM Sublanguage translation: TITUS MT for monolingual users Speech translation: British Telecom and ATR Reversible grammars Computational developments Concluding comments Sources and further reading Bibliography Index...351

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