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Problems and Solutions in African Tone Language Text–To–Speech

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Problems and Solutions in African Tone Language Text–To–Speech ITRW on Multilingual Speech and ISCA Archive Language Processing http://www.isca-speech.org/archive Stellenbosch, South Africa April 9-11, 2006 Problems and solutions in African tone language Text–To–Speech Dafydd Gibbon, Eno–Abasi Urua, Moses Ekpenyong Universitat¨ Bielefeld, Germany; University of Uyo [email protected]; [email protected]; ekpenyong [email protected] Abstract The present contribution outlines the initial developmeng One of the most useful HLT systems for use with non-literate issues involved in preparing an experimental TTS prototype for communities is Text–to–Speech, with typical applications in Ibibio, and presents detailed computational solutions for three health, market and education information dissemination. How- areas: a tone model; a model for non-local morphological de- ever, current TTS development environments are far from be- pendencies; corpus preparation and processing for a tone lan- ing language independent: adaptation of an existing voice in guage with a small and inconsistent text base. one language to a voice in another is a common procedure. During development of a TTS prototype for Ibibio, a Nigerian 2. Overview of issues Niger–Congo, Lower Cross language, in the Local Language Speech Technology Initiative, several levels of infrastructural The development of an experimental TTS prototype for a new and linguistic problems were identified. This contribution out- language, from project design through resource acquisition to lines these problems, concentrating mainly on requirements for system design, implementation and evaluation is a long haul, African tone language TTS, and formulates solution strategies even with older technologies using diphone TTS shells such as for both rule–driven and data–driven TTS development. MBROLA [3] or more modern technologies with unit selection shells such as Festival [4] or BOSS [5]. 1. Introduction 2.1. Linguistic issues: language typology One of the most useful HLT systems for use with non- On the one hand, much depends on the typology of the language literate communities is Text–to–Speech, with typical applica- concerned, both from the perspective of the creation of a unit tions in health, market and educational information dissemi- database resource for the TTS system, and from the perspective nation. However, current TTS development environments are of the processing of text input to the TTS system. Key issues far from being language independent: adaptation of an exist- include: ing voice in one language to a voice in another is a common procedure. 1. Syllable phonotactics. The complexity of syllable In the LLSTI project [1], the adaptation procedure was ap- phonotactics is a major determinant of the size of the plied to a Nigerian tone language, Ibibio (ISO 693-2: nic; Eth- unit database resource: whether a language is fundamen- nologue: IBB), the official language of Akwa Ibom State in the tally CCCVVCC, like many Indo-European languages, Nigerian Federation. Adaptation is plausible when languages or CV, CCV, CCVC, like many West African languages, are prosodically and phonemically similar but severe problems leads to a significant difference in combinatoric op- arise when languages are very dissimilar (e.g. ‘intonation lan- tions, and therefor in resource size. Syllable phonotac- guages’, for which TTS systems are typically developed, vs. tics includes prosody: syllables may be long or short, ‘tone languages’, e.g. Ibibio). open or closed, strong or weak, stressed/accented or un- These problems can be generalised to other African tone stressed/unaccented, tonal or non–tonal, depending on languages, which have lexical (phonemic) tone but, unlike the language. Asian tone languages, also morphemic tone: e.g. in Ibibio near/far tense is marked by LH/HL tones on tense morphemes. 2. Inflectional morphotactics. Inflectional morphotactics Tone-morpheme combinations could be used in unit selection, determines, for example, whether table lookup or rule– but the problem is compounded by the lack of orthographic tone based techniques are most appropriate for handling the marking, making morphological tone assignment effectively an vocabulary and its adaptation to grammatical context: ‘AI complete’ problem. English tends towards the isolating type, with little in- Additionally, ’terraced tone’ patterning generated by auto- flection; other Indo-European languages are fusional, matic and non-automatic downstep requires a clean interface for tendentially with single inflectional suffixes, each with signal processing of pitch: pitch has a ’hard’ semantic function complex grammatical meanings; other languages, such rather than a ’soft’ pragmatic function in intonation languages as Finno-Ugric languages and many African languages, [2]. are agglutinating, with chains of suffixes, each of which On the language side, agglutinative inflectional morphol- has a simple grammatical meaning. Inflectional morpho- ogy generally makes it infeasible to list all inflected word forms tactics also includes prosody: inflection may affect stress except for small vocabularies and complex subject-verb-object patterns, as in the German singular noun DOKtor, plural person concord makes morphological tone assignment difficult. DokTORen, or tonal patterns, as in Ibibio verbal mor- Finally, specific problems of sparseness and text inconsis- phology where, simplifying slightlym, rising and falling tency in written but non-standardised languages arise. tones on a future or past prefix particle represent distal future or past, and proximal future or past, respectively • For historical reasons there are competing or- (ya`a,´ ya´a;` ma`a,´ ma´a).` thographies: they differ in details. 3. Morphotactics of word formation. Derivation with • Existing documents created by untrained person- one root and affixes, compounding with more than nel tend to be highly inconsistent in orthography, one root, determine the composition of the vocabu- punctuation and format (though this is hardly dif- lary. Prosody also plays a central role: in languages ferent from documents in many other languages such as English, stress patterns (IMport/imPORT; TELE- the world over). phone/teLEphony/telePHONic; BLACKboard) are in- • The body of available texts is small, for IT pur- volved. In West African language, low or high tonal poses the available data are therefore very sparse. interfixes may be involved, as in Ibibio: en` o` (gift) +´ (tonal interfix) + ab` as` `ı (God) = en` o`ab´ as` `ı (personal proper • Available lexicographic material is also very re- name). stricted, and in a legacy paper format. • 4. Sentence structure. The combinatorics of words into Available information on grammar is neither pre- larger units takes place in conjunction with inflectional cise enough nor complete enough to be of much morphology: different orders of subject, object and verb use for automatic processing (e.g. parser develop- in simple sentences; different concord conditions in sim- ment), though the morphology is more complete, ple sentences (e.g. subject–verb, object–verb, ergative); and the phonology is in fact complete. serial verb constructions; different conventions for em- 3. Infrastructural resources. The ubiquity of electricity and bedded sentences. of internet connectivity which academic researchers ex- pect in, for example, European contexts, is either not 2.2. Resource issues given, or very expensive. Power cuts can be frequent, and are sometimes predictable due to known times when Many aspects of TTS development depend on the development demand is likely to outstrip supply. For most hands-on environment: from data resources (corpora, lexica, grammars), computational work either a desktop with backup gen- through hardware and software, to the instutional environment, erator or a laptop with extra batteries is required. The with its personnel structures and economic basis, and in this re- usual solution is a laptop. Unsealed magnetic storage spect conditions vary from region to region, and from continent media (floppy disks) have a very short life, because of to continent. dust, heat and humidity. In relation to Europe, resource conditions differ signifi- cantly in all of these respects in West Africa, and, of course, The last of these points is crucial for local language IT de- the issue of speech synthesis for local African languages is it- velopment: creaming off development work into well–equipped self very heterogeneous. labs in more northerly (or southerly) latitudes is likely to fan the The experimental prototype development project for Ibibio flames of reputation for the labs concerned, but is not likely to was conducted at the University of Uyo, located in Uyo, the cap- lead to significant value for or acceptance by users of the local ital of the Akwa Ibom State in the Nigerian Federation, in the language unless local developers are responsible for develop- South East of Nigeria, between the Cross and Calabar rivers. ment on an equal footing; an optimal solution would be part- Universitat¨ Bielefeld was coordinating cooperation partner in nerships between well–equipped labs and regional, less well- the project. The official language of Akwa Ibom State is Ibibio. equipped labs. The 2 million speakers of Ibibio constitute a rather large ap- plication pool for speech synthesis applications in education, 3. A note on tonal typology health–care, agricultural and geographical information systems. The Department of Linguistics and Nigerian Languages at Uyo In this and the following sections, attention is focussed on the University
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