".on a - 
September 1994 _ Volume 20, Number 3 
Computat in guistics 
Special Issue on Com 
Introduction to Computational Phonology 
Articles 
Regular Models of Phonological Rule Systems 
Commentary on Kaplan and Kay 
Commentary on Kaplan and Kay 
The Reconstruction Engine: A Computer 
Implementation of the Comparative Method 
Commentary on Lowe and Mazaudon 
Commentary on Lowe and Mazaudon 
The Acquisition of Stress: A Data-Oriented 
Approach 
Commentary on Daelemans, GiUis, 
and Durieux 
Commentary on Daelemans, Gillis, 
and Durieux 
Phonological Analysis in Typed Feature 
Systems 
Commentary on Bird and Klein 
Commentary on Bird and Klein 
Book Reviews 
English Verb Classes and Alternations: A 
Preliminary~ Investigation 
Beth Levin 
Statistically-Driven Computer Grammars of 
English: The IBM/Lancaster Approach 
Ezra Black, Roger Garside, and Geoffrey Leech 
(editors) 
Intelligent Multimedia Interfaces 
Mark T. Maybury (editor) 
~utational Phonology 
Steven Bird iii 
Ronald M. Kaplan and Martin Kay 331 
Mark Liberman 379 
Graeme Ritchie 380 
John B. Lowe and Martine Mazaudon 381 
Steven Lee Hartman 418 
John Hewson 419 
Walter Daelemans, Steven Gillis, and 421 
Gert Durieux 
Prahlad Gupta 452 
Jonathan Kaye 453 
Steven Bird and Ewan Klein 455 
John Coleman 492 
Richard Sproat 493 
Reviewed by Ha~:old Somers 
Reviewed by Dekai Wu 
Reviewed by Kent Wittenburg 
495 
498 
501 
Published Quarterly by the MIT Press for the Association for Computational Linguistics 
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Introduction to Computational Phonology 
Steven Bird* 
University of Edinburgh 
1. Overview 
Despite being the oldest discipline in linguistics, phonology remains largely unex- 
plored from a computational standpoint. While phonology gave us such innovations 
as the 'distinctive feature,' now heavily used in computational linguistics, phonology 
itself has yet to reap the benefits of the formal and technological developments it gave 
rise to. 
Recently, however, computational phonology has been rapidly gaining recognition 
as an independent area of inquiry within computational linguistics. The ACL Special 
Interest Group in Computational Phonology (SIGPHON) was formed in 1991 and has 
served as a focus for ongoing work in the area. In June of that year I proposed that there 
be a special issue of Computational Linguistics dedicated to computational phonology, 
since there were many good-quality papers in circulation that had no obvious venue 
for publication. The resulting collection, which you have before you, is a representative 
sample of this work; some submissions not ready in time for this volume will appear 
in subsequent regular issues. Other work in this area is to be found in the Proceedings 
of the First Meeting of the ACL Special Interest Group in Computational Phonology, published 
by the ACL in 1994, and in two edited collections (Bird 1991; Ellison and Scobbie 
1993). 
The purpose of this short piece is to introduce computational phonology and the 
special issue. I shall begin by presenting some background to the field, followed by 
a survey of the research themes currently under investigation. Next, an overview of 
the papers in this collection is given, concluding with an explanation of the one- 
page commentaries that follow each paper. So, what is phonology, and why should 
computational linguists care about it? 
2. Background 
Phonology is the study of the systems of sounds that are manifested by natural lan- 
guages, the significant contrasts between sounds that are relevant to meaning. As such, 
phonology stands at the interface between grammar, broadly construed, and speech. 
Much of the richness and complexity of phonology derives from the place it occu- 
pies between categorical symbolic systems and parametric physical behavior. Several 
excellent textbooks are available for readers who wish to learn more about phonology. 
Now, why should computational linguists care about phonology? First, phonology 
is an equally valid area of study for a computational linguist as syntax or semantics. 
Solutions in one area may generalize to other areas, as we see, for example, where 
strings of segments are parsed using the same machinery that is used for syntactic 
* University of Edinburgh, Centre for Cognitive Science, 2 Buccleuch Place, Edinburgh EH8 9LW, U.K. 
(~) 1994 Association for Computational Linguistics 
Computational Linguistics Volume 20, Number 3 
parsing (e.g. chart parsing \[Church 1987\]), or where a formal system developed for 
semantic representations of tense is applied to the temporal structure of phonology 
(Bird and Klein 1990), or where complex arrangements of phonological features are 
represented in the familiar notation of attribute-value matrices (Wiese 1990). Thus, 
phonology provides a fresh source of applications for the techniques and technologies 
of computational linguistics. 
However, this only demonstrates a flow of information from computational lin- 
guistics to phonology. Can we hope for payoffs in the other direction resulting from a 
wholesale integration of phonology into computational linguistics? It is instructive to 
consider The Sound Pattern of English (Chomsky and Halle 1968) in this regard. Although 
it was intended as a contribution to phonological theory, SPE was also directly im- 
plementable on computer (e.g. Bobrow and Fraser's 'phonological rule tester,' \[1968\]), 
and it was an important foundation for work in speech technology (e.g. Allen, Hunni- 
cutt, and Klatt 1987). Via the work of Johnson (1972), Koskenniemi (1984), and Kaplan 
and Kay--the latter in circulation since the early eighties but appearing in published 
form for the first time in the present collection--one could reasonably argue that SPE 
gave rise to finite-state morphology (Antworth 1990; Ritchie et al. 1992; Sproat 1992). 
The formal framework of SPE was a good deal more explicit and rigorous than 
most of what came after, and so the prospects for a repeat performance coming from 
phonology have never been particularly bright. However, I feel it is now time for 
computational linguists to take another look at phonology. A quarter of a century has 
gone by since SPE, and there is much of interest to be found in the pages of Phonology 
and similar publications. A good place to start is the literature on computational 
phonology itself, since it interprets the theoretical proposals of phonology in a way that 
is more accessible to computational linguists. The stakes are high, since it would not be 
surprising if phonology is still to play an important role in bridging natural language 
technology and speech technology. For even though there is a methodological and 
sociological divide, there remains an imperative to develop fully integrated language 
and speech systems and an enduring need for fresh sources of creative ideas to relate 
the discrete to the continuous. 
Undoubtedly, there will always remain sceptics who think that natural language 
systems that deal just with the written word can afford to ignore phonology. In a 
limited sense they are correct. However, in the longer term, I am convinced that the 
interest in multilingual and multimodal systems will require a more enlightened view 
of phonology. Many languages have genuinely phonological phenomena evident in 
the orthography, such as Finnish (Koskenniemi 1984). Even in English we find cases 
where a spelling rule needs to be sensitive to phonological information. For example, 
the orthographic rule that selects a vs. an breaks down when a following word begins 
with a written vowel but a spoken consonant or vice versa, as in a___u_uranium compound 
and an ytterbium compound. Although it largely works for English and a handful of 
other languages, the assumption that phonology can be ignored by natural language 
systems will collapse for many of the world's languages (e.g. Finnish, Turkish, and 
Arabic). 
One reason why computational phonology has not had a high profile is that work 
in this area has often been dealt with under the heading of computational morphology. 
However, much of what passes as finite-state morphology is actually morphophonol- 
ogy--the phonological factors that influence the appearance of morphemes----or even 
phonology proper. Moreover, the central computational device in finite-state morphol- 
ogy, the finite-state transducer, is not used for specifying the distribution of morphemes 
(i.e. morphotactics), the other main task of morphology. Therefore, that part of finite- 
state morphology that is expressed in terms of finite-state transducers, namely mor- 
iv 
Steven Bird Introduction to Computational Phonology 
phophonology and phonology, is largely coextensive with the domain of SPE. Perhaps 
the appearance of Kaplan and Kay's paper in this collection is symbolic of the recogni- 
tion that there is a close interplay between computational morphology and phonology. 
A second reason that attention to phonology is warranted is that much of phonol- 
ogy is actually not subsumed by computational morphology and speech technology. 
In general, work in these two fields has focused on SPE-style phonology alone and 
has not, by and large, connected with current phonological theory or addressed purely 
phonological concerns. Again, computational phonology should provide usable imple- 
mentations of more recent models so that they can be incorporated into computational 
work on morphology and speech. 
Finally, one might reasonably ask why a phonologist ought to be interested in 
computational phonology. At the most obvious level, computational phonology should 
provide support for developing theories and testing them against data, removing some 
of the hackwork involved in achieving formal and empirical adequacy. Additionally, 
computational phonology may be able to provide formal devices that are useful in 
phonology proper, as in the case of the information-theoretic evaluation metric (Ellison 
1993) that is intended to replace the naive symbol-counting version. One can also ob- 
serve that phonology has its own divide between theoreticians who work on abstract 
models supported by small collections of data drawn from a wide variety of languages, 
and investigators working on large scale analyses of individual languages (such as the 
work of the Summer Institute of Linguistics on the orthographies of approximately 
1,100 languages \[D. Crozier, personal communication\]). To this observer, it seems like 
there could be more communication of new data from the field phonologist to the the- 
oretical phonologist and, in the reverse direction, communication of new hypotheses 
and useful theoretical devices that would play an active part in the search for inter- 
esting new data. It seems plausible that computational systems that let phonologists 
experiment with large amounts of data and a variety of theoretical models have an 
important part to play in bridging the gap between the 'theory people' and the 'data 
people.' 
3. Research Themes 
I have attempted to identify four strands of work and cite a representative sample of 
work within each. Unfortunately, much valuable, relevant work has had to be omitted 
from the citation lists below for reasons of space. 
Formal reconstruction and language-theoretic results. Work in this area seeks 
to provide coherent and well-understood formal frameworks in which phonological 
theories can be expressed. Some work takes an existing theory as its starting point 
and seeks to refine it and express it in increasing levels of formality, while other work 
begins from an existing formalism and tries to adapt its expressive capabilities to the 
needs of phonology. Since most work contains a mixture of both, I shall not attempt 
a classification. Rather, I shall loosely classify a selection of the work based on the 
formal method used: unification (Carson 1988; Chung 1990; Coleman 1991; Scobbie 
1991; Broe 1993; Walther 1993), predicate logic (Bird 1990; Bouma 1991; Russell 1993), 
modal logic (Bird and Blackburn 1991; Calder and Bird 1991), type theory (Klein 
1991; Mastroianni 1993), categorial grammar/logic (Wheeler 1981; Dogil 1984; van der 
Linden 1991; Oehrle 1991; Steedman 1991; Moortgat and Morrill to appear), finite-state 
devices (Kay 1987; Kornai 1991; Wiebe 1992; Bird and Ellison 1994), electrical circuitry 
(Gilbers 1992), and formal language theory (Ristad 1990; Kornai 1991; Ritchie 1992; 
Wiebe 1992). 
V 
Computational Linguistics Volume 20, Number 3 
This work addresses phonological theories such as autosegmental, metrical, under- 
specification, and government phonology. The paper by Kaplan and Kay in this col- 
lection is another example of work in this general vein. 
Implementations. Work in this area is directed at producing computer programs 
that can be used by phonologists to develop and test theories. A variety of SPE im- 
plementations exist (independently of the finite-state transducer model) starting from 
Bobrow and Fraser (1968) and including models for applying rules in reverse (Bear 
1990; Maxwell 1991). Other theoretical frameworks that have been implemented to 
a greater or lesser extent include lexical phonology (Williams 1991), autosegmen- 
tal phonology (Bird 1990; Albro 1994; Bird and Ellison 1994), diachronic phonology 
(Hewson 1974; Eastlack 1977; Lowe and Mazaudon 1989), inheritance-based models 
(Daelemans 1987; Reinhard and Gibbon 1991) and connectionist models (see the next 
paragraph on learning). The paper by Lowe and Mazaudon in this collection is an 
example of other work under the heading of implementations. 
Automatic learning. This work aims to provide models to (i) simulate human 
behavior and test of theories of human language acquisition, and (ii) provide the 
working phonologist with useful generalizations about a certain body of data under 
study. Examples of the first type are (Lathroum 1989; Touretzky and Wheeler 1993; 
Gupta and Touretzky 1992; Hare 1990; Gasser and Lee 1990; Gasser 1992; Shillcock 
et al. 1992; Goldsmith 1993; Larson 1992), and these all use connectionist models. 
Examples of the second type are all symbolic (Johnson 1984; Dresher and Kaye 1990; 
Ellison 1993; Bird 1994). Daelemans, Gillis, and Durieux have contributed a paper to 
the present collection that fits into this category of automatic learning. 
Interfacing to grammar and speech. The final grouping contains work that is 
intended to integrate computational models of phonology with computational models 
of grammar and of speech. Concerning the phonology-grammar interface, all this work 
is covered under the paragraph on formal reconstruction above. The assumption is 
that if phonological models are formalized and if they employ the same computational 
model as is used for computational syntax and semantics, then interfacing to grammar 
ought to be relatively straightforward. Another instance of this work is the contribution 
to the present collection by Bird and Klein. Recent work on integrating phonology with 
speech synthesis includes Hertz (1990), Coleman (1992), and Dirksen (1992), and there 
is also a large literature on the phonology of intonation as it relates to synthesis (e.g. 
Anderson, Pierrehumbert, and Liberman 1984; Ladd 1987). 
This concludes the discussion of the various current research themes in compu- 
tational phonology. As chance would have it, each of these themes is manifested by 
one of the papers in the present collection. We now go on to survey these papers 
briefly. The reader is referred to the commentaries for more detailed overviews of the 
contributions. 
4. Brief Survey of Contributions 
These papers are given in the same order as the categories of the previous section and 
in the order in which they appear in the collection itself. 
Kaplan and Kay: Regular Models of Phonological Rule Systems. Kaplan and Kay 
have finally provided the "widely cited but notoriously unpublished work" (Ritchie 
et al. 1992:20) that establishes the mathematical foundation for finite-state computa- 
tional phonology and morphology. This is without question the flagship paper of this 
collection. 
Lowe and Mazaudon: The Reconstruction Engine: A Computer Implementation of the 
Comparative Method. This paper presents an implementation of a technique from 
vi 
Steven Bird Introduction to Computational Phonology 
diachronic linguistics, known as the comparative method, for comparing word forms 
taken from cognate languages in order to reconstruct aspects of the ancestor language 
from which the languages are derived. The system is applied to data from a group of 
Tibeto-Burman languages spoken in Nepal. 
Daelemans, Gillis, and Durieux: The Acquisition of Stress: A Data-Oriented Approach. 
This paper consists of a rather striking demonstration that an empiricist learning model 
actually performs better than the nativist 'Principles and Parameters' approach, con- 
cerning the task of assigning primary stress to a corpus of around 5,000 Dutch words. 
Bird and Klein: Phonological Analysis in Typed Feature Systems. This contribution 
shows how a model of phonology incorporating complex multi-tiered structures can 
be integrated with a constraint-based grammar of the HPSG variety. Applications to 
nonconcatenative morphology in Sierra Miwok and deletion in French are given. 
5. Commentaries 
The commentaries were conceived as a way of involving more people in the special 
issue, and of identifying, for each paper, the noteworthy achievements and remaining 
areas of contention. I felt that this would add interest and perspective to the collection 
and would enable outsiders to gain a deeper insight into the workings of the field. 
Two commentators were selected for each paper who have an established reputation 
for work in the same area of specialization as the paper in question, and who, in some 
cases, hold contrary views to those being advanced by the authors. 
Acknowledgments 
I am deeply indebted to Julia Hirschberg 
who gave me her full support as I 
undertook the editorial tasks involved in 
preparing this collection. She also took sole 
editorial responsibility for my contribution 
with Ewan Klein. I am also thankful to the 
members of SIGPHON, the CL Editorial 
Board, and many others for the painstaking 
reviewing they willingly undertook, and to 
the authors and commentators for their 
enthusiastic participation in this project and 
for their comments on this introduction. 
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