Non-traditional Prosodic Features for Automated Phrase-Break Prediction Brierley, Claire

The goal of automatic phrase break prediction
is to emulate human performance in
terms of naturalness and intelligibility when
assigning prosodic-syntactic boundaries to
input text. Techniques can be deterministic
or probabilistic; in either case, the problem
is treated as a classification task and outputs
from the model are evaluated against 'gold
standard' phrase break annotations in the
reference dataset or corpus. These annotations
may represent intentions (of the speaker or
writer) or perceptions (of the listener or reader)
about alternating chunks and boundaries in the
speech stream or in text, where the chunking
bears some relationship to syntactic phrase
structure but is thought to be simpler, shallower
and flatter.
In this paper, we begin by reviewing
methodologies and feature sets used in phrase
break prediction. For example, a tried and
tested
rule-based
method is to employ some
form of 'chink-chunk' algorithm (Liberman and
Church, 1992) which inserts a boundary after
punctuation and whenever the input string
matches the sequence: open-class or content
word (chunk) immediately followed by closed-
class or function word (chink), based on the
principle that chinks initiate new prosodic
phrases.
We discuss the limitations of using traditional
features in the form of syntactic and text-based
cues as boundary correlates, with illustrative
experimental predictions from a shallow parser
and evidence from the corpus. We then discuss
the limitations of evaluating any phrase break
model against a "gold standard" which itself only
represents one phrasing variant for an utterance
or text.
There is an emerging trend of leveraging
real-world knowledge to improve performance
in machine learning, including speech and
language applications. Nevertheless, we have
diagnosed a deficiency of
a priori
knowledge
of
prosody
in the feature sets used for the
phrase break prediction task. In contrast, a
competent human reader is able to project
holistic linguistic insights, including projected
prosody, onto text and to treat them as part
of the input (Fodor, 2002). In this respect,
multiple prosodic annotation tiers in the Aix-
MARSEC corpus (Auran
et al.
, 2004) have
been revelatory, since they capture the prosody
implicit in text and currently absent in learning
paradigms for phrase break models.
Insights such as: (i) the
transferability
of
the chinks and chunks rule; plus (ii) the
possibility of encoding a variety of prosodic
phenomena (including rhythm and beats) in
categorical labels (
cf.
the Aix-MARSEC corpus);
plus (iii) an appreciation of prosodic variance
gleaned from corpus evidence of alternative
parsing and phrasing strategies, have informed
the creation of ProPOSEL (Brierley and
Atwell, 2008a; 2008b), a domain-independent
prosodic annotation tool.
ProPOSEL is a
pro
sody and
p
art-
o
f-
s
peech
E
nglish
l
exicon of 104,049 entry groups, which
merges information from several widely-used
lexical resources for corpus-based research
in speech synthesis and speech recognition.
Its record structure supplements word-form
entries with syntactic annotations from four
rival POS-tagging schemes, mapped to fields
for: default open and closed-class word
categories; syllable counts; two different
phonetic transcription schemes; and lexical
stress patterns, namely abstract representations
of rhythmic structure (as in
201
for
disappear
,
with secondary stress on the first syllable and
primary stress on the final syllable).
We then contend that native English speakers
may use certain sound patterns as
linguistic
signs
for phrase breaks, having observed these
same patterns at rhythmic junctures in poetry.
We also contend that such signs can be
extracted from canonical forms in the lexicon
and presented as input features for the phrase

2
break classifier in the same way that real-
world knowledge of syntax is represented in
POS tags; and that like content-function word
status or punctuation, such features are domain-
independent and can be projected onto
any
corpus. One such sound pattern is the subset of
complex vowels, which we define as the eight
diphthongs, plus the triphthongs, of Received
Pronunciation (Roach, 2000: 21-24).
Finally, we test the correlation between pre-
boundary lexical items bearing complex vowels
and gold-standard phrase break annotations on
different kinds of speech via the chi-squared
statistic, to determine whether the perceived
association is statistically significant or not.
Our findings indicate that this correlation is
extremely statistically significant: it is present
in contemporary, formal, British English speech
(Brierley and Atwell, 2009) and seventeenth
century English verse (Brierley and Atwell,
2010a); and it holds for spontaneous as well as
read speech, and for multiple speakers (Brierley
and Atwell, 2010b). We hypothesise that while
complex vowels seem to constitute phrase break
signifiers
in English, this may translate to a
subset of the vowel system in other languages.
References
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