The area of word prosody, and, more generally, the description of other prosodic units which have quasi-morphemic functions, is gradually emerging as an important area for spoken language lexica. For present purposes, prosodic properties are defined as properties of word forms which are larger than phonemes. Further specification in phonetic terms (e.g. F0 patterning ) and in semantic terms (e.g. attitudinal meaning) may also be given but is not essential for present purposes.
One type of lexical information on prosody pertains to phonological or morphological properties of words, such as Swedish pitch accents , or stress positions in words. Some aspects of word prosody are predictable on the basis of the regular phonological and morphological structure of words, but some are idiosyncratic. Examples in English where word stress is significant include the noun-verb alternation type as in export - /eksp:t/ (Noun), /eksp:t/ (Verb). In German, word stress is significant for instance in distinguishing between compound separable particle verbs and derived inseparable prefixed verbs as in übersetzen - /y:bztsn/ (compound) vs. /y:bztsn/ (derivation).
It has been shown [Waibel (1988)] that taking word prosody into account in English can produce a significant improvement in recognition rate.
In addition, there is lexical information associated with prosodic units which occur independently of particular words, and therefore may themselves be regarded as lexical signs and be inventarised in a prosodic lexicon [Aubergé (1992)]. To give a highly simplified example in a basic attribute-value notation, a prosodic lexicon for an intonation language might have the following structure.
<phonetics pitch> = fall
<semantics> = statement or instruction.
<phonetics pitch> = rise
<semantics> = question or polite instruction.
This kind of information, in which prosodic categories function as a kind of morpheme with an identifiable meaning, is generally not regarded as lexical information, but treated as a separate layer of organisation in language. Intonation is being taken increasingly into account for prosodic parsing in two main senses of this term:
Prosodic representation in the lexicon is in general restricted to the prosodic properties of words, such as stress position in English, Dutch, and German words, or tonal accent in Swedish words, or to rhythmically relevant units such as the syllable and the foot . For spoken language processing in which prosody plays a role, it is also necessary to include an inventory of prosodic forms, and their meanings, which play a role at the sentence level, independently of specific words: i.e. a prosodic lexicon .
It should be borne in mind that in linguistics, ``prosody'' currently has a broader meaning, and covers all properties of pronunciation which are not directly concerned with defining consonants and vowels. Prosody in this sense covers, for example, syllable structure and phonological word phonotactics, as well as the more traditional categories of intonation, accent, and phrasing.
The IPA defines symbols for representing lexical and non-lexical types of prosody, and a subset (for word prosody) has been encoded in the SAMPA alphabet. However, the state of knowledge in the area of prosody is less stable than in the area of segmental word structure, and a range of different conventions is available [Bruce (1989)]; in this area, there are SAMPA ``dialects '', for instance replacing SAMPA " and % for primary and secondary stress by the more iconic (single quote) and (two single quotes) or " (double quote).
The ToBI (Tones and Break Indices) transcription , originally developed for American English, has now been applied to several languages (see also Chapter 5).
In oriented spoken language lexicography within the VERBMOBIL project, attribute-based formal representations of prosodic features in the lexicon have been developed using the ILEX (Integrated Lexicon) model and the lexical knowledge representation language DATR [Bleiching (1992), Gibbon (1991)].
There is an increasing tendency no longer to regard prosodic representations as totally exotic and quite unlike anything else.
But there is still insufficient consensus on lexical prosodic features to permit generally valid recommendations to be made for prosodic representations in the lexicon. For most purposes, plain SAMPA or ToBI
style symbols will be adequate.
For covering new ground with extended lexica for use with discourse phenomena at the dialogue level, a lexical knowledge representation language with a more general notation, as illustrated above, may be more appropriate.