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<Paper uid="C90-3004">
  <Title>Phonological Processing of Speech Variants</Title>
  <Section position="3" start_page="0" end_page="0" type="metho">
    <SectionTitle>
2. Phonological Processing
</SectionTitle>
    <Paragraph position="0"> The phonological parser, PhoPa, described In Carson (1988) uses as Its linguistic knowledge base a network representation of the phonotactics of a particular language in order to parse the phonetic sequence into phonemic syllables. The phonotactio network is feature-based and serves as a phonological word template consisting of nonreduced and reduced syllables.</Paragraph>
    <Paragraph position="1"> Following Church (1987), allophonic information is considered to be important for distinguishing syllable boundaries and thus a canonlcalisation step is necessary in order to filter out the variant information which is not relevant atthe phonemic level. In PhoPa a feature-based transduction relation is responsible for translating between the allophonio and the phonemic domains. A transition label in the network consists of two feature bundles, an input transition bundle and an output transition bundle each containing C-features (Carson, 1988). C-features are those features which are needed to characterise sound classes which participate in a particular phonetic process.</Paragraph>
    <Paragraph position="3"> Using a restricted form of unification, an acceptance function tests whether a particular allophonic input string of feature bundles may be processed. Since the input feature bundles may be underspecified, a redundancy component consisting of feature cooccurrence restrictions tests for feature-value consistency and attempts to optimise the the information in the feature bundles.</Paragraph>
    <Paragraph position="4"> The processing strategy used in PhoPa is thus phonetic data driven or variant-invariant. With the core of PhoPa only wordforms in the standard language can be analysed.</Paragraph>
    <Paragraph position="5"> variants of canonical phonemic forms. If we allow a standard phonemic form to have more than one variant then an exhaustive synthesis process would generate all possible variants of the standard wordform.</Paragraph>
    <Paragraph position="6"> It is in fact the case that the network context provides exacty the defined search space which is necessary for the automatic speech variant learning extension and furthermore it allows for structurally based heuristics which reduce this search space during synthesis. After synthesis, speech variants are integrated into the lexicon for efficient later recognition; this is clearly expensive on storage, however.</Paragraph>
    <Paragraph position="7">  For the purpose of the discussion which follows, it is important to note that the phonotactic transduction network used in PhoPa is In theory non-directional, that is to say, the transduction interpreter can be applied to either the allophonic (phonetic or variant) or the phonemic (invariant) level. Thus, a processor which uses the variant-invariant strategy performs analysis and a processor which uses the invariant-variant strategy performs synthesis. The synthesis process therefore generates</Paragraph>
  </Section>
  <Section position="4" start_page="0" end_page="0" type="metho">
    <SectionTitle>
3. Cla,tslficatlon of Speech Variants
</SectionTitle>
    <Paragraph position="0"> Speech variants can occur either as a result of phonological processes, for example elision, epenthesls or assimilation, or they can arise in line with a regional or dialectal sound change. Speech variants can, however, be classifed according to three abstract processes based on segments: deletion, insertion and substitution. Each of these abstract processes has a corresponding abstract rule type. A deletion rule deletes a whole segment and can also modify feature values in neighbouring segments.</Paragraph>
    <Paragraph position="1"> An Insertion rule inserts a whole segment and a substitution rule is applied to single features in particular segments having the effect of substituting one segment for another. All rule types require a context consisting of directly neighbouring segments. However, this context can in some cases be empty.</Paragraph>
    <Paragraph position="2"> Each of the abstract processes can only occur within a particular range of the syllable. On the basis of German data, the following are the &amp;quot;most probable&amp;quot; ranges for the three processes. Deletion and insertion occur only in the rhyme (peak and/or coda) of the syllable and substitution has the whole syllable as its range (i.e. it can occur in the onset or peak or coda). These facts allow for a structure-based heuristic which defines the application range for each process type and thus limits the search space required for the extension. Since the syllable structure is directly represented in the phonotactio network (see Fig.</Paragraph>
    <Paragraph position="3"> 3 below), the search through the network can be restricted to a particular sub-structure (onset, rhyme eto. - for both nonreduced and reduced syllables). Considering the syllable structure in terms of a tree, the heuristic defines the optimal starting point for the search and the search proceeds in a depth-first fashion through the syllable tree.</Paragraph>
    <Paragraph position="4"> 22 2 Thus, since the application range for an insertion rule is the rhyme, the optimal starting point for the search is the peak. If the search is unsuccessful in the peak then the coda is searched.</Paragraph>
    <Paragraph position="5">  Speech variants are described by declarative metarules which describe a particular phenomenon. The metarules have a left hand side and a right hand side, each of which consists of feature bundles, and they must belong to one of the abstract rule types mentioned above.</Paragraph>
    <Paragraph position="6"> Thus, epenthesis will be described by a metarule of insertion rule type. The epenthesis of a homorganiC/ voiceless plosive between a nasal and a an apical fricative in German, for example, would be described by the following metarule:</Paragraph>
    <Paragraph position="8"> This caters for the forms \[gAmps\] for/gAins/, German: &lt;Gains&gt;; \[gAnts\] for /gAns/, German: &lt;Gans&gt;; \[ge#zAg&amp;quot;ks\] for/ge#zAg&amp;quot;s/, German: &lt;Gesangs&gt;.</Paragraph>
    <Paragraph position="9"> As can be seen, it is possible to describe what would normally be thought of as two processes, namely an epenthesis process and an assimilation process, in terms of a single metarule.</Paragraph>
  </Section>
  <Section position="5" start_page="0" end_page="0" type="metho">
    <SectionTitle>
4. The Extension Procedure
</SectionTitle>
    <Paragraph position="0"> The extension procedure consists of two stages: phonotactic extension, whereby additional restrictions are added to the phonotaotic network, and specialised word model construction which results in an extended phonological lexicon.</Paragraph>
    <Paragraph position="1"> The phonotactio extension is concerned with the automatic extension of the phonotactic network by introducing new transitions. The extension procedure has two input components, the metarule and the lingistic knowledge base, namely the phonotactic network. Since in its original form the metarule cannot be directly Incorporated into the phonotaotio network, the extension procedure first applies a metarule Interpreter which produces a graph representation for the metarule. The graph representation corresponds to the network representation exactly, that is to say that each transition in the graph consists of two feature bundles: an input specification and an output specification. The output transistion specification is always the phonemic form.</Paragraph>
    <Paragraph position="2"> When the graph representation has been produced, a possible unification is sought for the network and the left hand side of the metarule. This involves a search through the networkwithin the range defined by a heuristic. In the case of a substitution rule, for example, a unification would be sought first among the transitions of the onset and then among the transitions of the rhyme. In fact all possible unifications are sought within the application range of the process type, since a metarule may be underspecified and thus may apply In more than one fully-specified context.</Paragraph>
    <Paragraph position="3">  If the left hand side of the metarule is unifiable with the network, the right hand side of the metarule is inserted into the network at the relevant place, by unifying the first</Paragraph>
  </Section>
  <Section position="6" start_page="0" end_page="0" type="metho">
    <SectionTitle>
3 23
</SectionTitle>
    <Paragraph position="0"> and last states of the rule with the relevant states in the network. This phase of the extension procedure results in additional restrictions being added into the phonotactic network.</Paragraph>
    <Paragraph position="1"> When the phonotactiC/ extension is complete, specialised word models are compiled on the basis of the extended phonotactic network. The task of the compiler is to construct for each lexicon entry (i.e. standard phonemic form) a corresponding word model containing all variants of the entry. The compiler uses an invariant-variant processing strategy, and thus transduotion takes place in reverse, that is to say, a translation between the phonemic and the allophonic domains. The compiler notes the paths which are consulted In the extended phonotactie network and produces a verification net for the input word. Word models are therefore subnets of the complete network. Thus, on the basis of a metarule describing a particular phenomenon, the phonological lexicon can learn new variants of a standard wordform.</Paragraph>
  </Section>
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