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Stress-Accent and Vowel Quality in The Switchboard Corpus Steven Greenberg and Leah Hitchcock International Computer Sci PowerPoint Presentation
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Stress-Accent and Vowel Quality in The Switchboard Corpus Steven Greenberg and Leah Hitchcock International Computer Science Institute 1947 Center Street, Berkeley, CA 94704 http://www.icsi.berkeley.edu/~steveng NIST Workshop on Large Vocabulary Continuous Speech Recognition

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slide1

Stress-Accent and Vowel Quality

in

The Switchboard Corpus

Steven Greenberg and Leah Hitchcock

International Computer Science Institute

1947 Center Street, Berkeley, CA 94704

http://www.icsi.berkeley.edu/~steveng

NIST Workshop on Large Vocabulary Continuous Speech Recognition

Maritime Institute of Technology, May 4, 2001

take home messages
There is an intimate relationship between vocalic identity, nucleic duration and stress accent in spontaneous dialogue (at least in the Switchboard corpus)

Stressed syllables tend to have significantly longer nuclei than their unstressed counterparts, consistent with the findings reported by Silipo and Greenberg in previous years’ meetings regarding the OGI Stories corpus (telephone monologues)

Certain vocalic classes exhibit a far greater dynamic range in duration than others

Diphthongs tend to be longer than monophthongs, BUT ….

The low monophthongs ([ae], [aa], [ay], [aw], [ao]) exhibit patterns of duration and dynamic range under stress (accent) similar to diphtongs

The statistical patterns are consistent with the hypothesis that duration serves under many conditions as either a primary or secondary cue for vowel height (normally associated with the frequency of the first formant)

Take Home Messages
take home messages3
Moreover, the stress-accent system in spontaneous (American) English appears to be closely associated with vocalic identity

Low vowels are far more likely to be fully stressed than high vowels (with the mid vowels exhibiting an intermediate probability of being stressed)

Thus, the identity of a vowel can not be considered independently of stress-accent

The two parameters are likely to be flip sides of the same Koine

Although English is not generally considered to be a vowel-quantity language (as is Finnish), given the close relationship between stress-accent and duration, and between duration and vowel quality, there is some sense in which English (and perhaps other stress-accent languages) manifest certain properties of a “quantity” system

Thus, vowel duration may be an important factor in disambiguating spoken language and therefore should be of interest to the speech recognition community

Take Home Messages
what is usually meant by prosodic stress
Prosody is supposed to pertain to extra-phonetic cues in the acoustic signal

The pattern of variation over a sequence of SYLLABLES pertaining to: syllabic DURATION, AMPLITUDE and PITCH (fo) variation over time (but the plot thickens, as we shall see)

What is (usually) Meant by Prosodic Stress?
why is prosodic stress important
It supposedly provides important information about:

Focus of the speaker’s attention and emphasis for the listener

What is “new” and “important” information

Emotional context of the utterance - surprise, sarcasm, shock, delight anger impatience, etc.

Syntactic disambiguation, particularly at the clausal/sentential level e.g., interrogative, declarative forms

Perceptual processing - parsing the utterance into “chunks” for reliable understanding

Prosody provides a window onto the higher levels of language

Can be useful for developing semantic-oriented models for speech understanding (“Information spotting”)

Prosody affects pronunciation (and vice versa)

Can be useful for modeling pronunciation variation in ASR

Phonetic properties may be correlated with prosodic stress -

THIS IS THE TOPIC FOR TODAY’S PRESENTATION

Why is Prosodic Stress Important?
the nitty gritty a k a the corpus material
SWITCHBOARD PHONETIC TRANSCRIPTION CORPUS (same as Phoneval-2000)

Switchboard contains informal telephone dialogues

54 minutes of material that had previously been phonetically transcribed (by highly trained phonetics students from UC- Berkeley)

45.5 minutes of “pure” speech (filled pauses, junctures filtered out), consisting of:

9,991 words, 13,446 syllables, 33,370 phonetic segments

All of this material had been hand-segmented at either the phonetic- segment or syllabic level by the transcribers

The syllabic-segmented material was subsequently segmented at the phonetic-segment level by a special-purpose neural network trained on 72-minutes of hand-segmented Switchboard material. This automatic segmentation was manually verified

The Nitty Gritty (a.k.a. the Corpus Material)
evaluation material details
AN EQUAL BALANCE OF MALE AND FEMALE SPEAKERS

BROAD DISTRIBUTION OF UTTERANCE DURATIONS

2-4 sec - 40%, 4-8 sec - 50%, 8-17 sec - 10% (mean = 4.75 s)

COVERAGE OF ALL (7) U.S. DIALECT REGIONS IN SWITCHBOARD

A WIDE RANGE OF DISCUSSION TOPICS

VARIABILITY IN DIFFICULTY (VERY EASY TO VERY HARD)

Evaluation Material Details

By Dialect Region

By Subjective Difficulty

Number of Utterances

Subjective Difficulty

Dialect Region

manual transcription of stress accent
2 UC-Berkeley Linguistics students each transcribed the full 45 minutes of material (i.e., there is 100% overlap between the 2)

Three levels of stress-accent were marked for each syllabic nucleus

Fully stressed (78% concordance between transcribers)

Completely unstressed (85% interlabeler agreement)

An intermediate level of accent (neither fully stressed, nor completely unstressed (ca. 60% concordance)

Hence, 95% concordance in terms of some level of stress

The labels of the two transcribers were averaged

In those instances where there was disagreement, the magnitude of disparity was almost always (ca. 90%) one step. Usually, disagreement signaled a genuine ambiguity in stress accent

The illustrations in this presentation are based solely on those data in which both transcribers concurred (i.e., fully stressed or completely unstressed)

A table containing the complete set of data is in a paper submitted to Eurospeech (in the workshop notebook)

Manual Transcription of Stress Accent
the conventional wisdom on stress accent
"Pitch is widely regarded, at least in English, as the most salient determinant of prominence. In other words, when a syllable or word is perceived as 'stressed' or 'emphasized,' it is pitch height or a change in pitch, more than length or loudness that is likely to be mainly responsible (see, for example, Fry 1958, Grimson 1980, pp. 222-226, Lehiste 1976, Fudge, 1984, ch. 1)"

Clark, J. and Yallop, C. (1990) An Introduction to Phonetics and Phonology. Oxford, Blackwell, p. 280.

"In fact, although it is clear that stressed syllables often have greater overall acoustic intensity than weakly stressed ones, loudness seems to be the least salient and least consistent of the three parameters of pitch, duration and loudness - at least for purposes such as signaling stress" (ibid, p. 282)

“Thus, acording to the ‘general consensus’ the important parameters are (in order) - PITCH, DURATION, LOUDNESS”

(the latter most closely correlated with TOTAL ENERGY (i.e., duration x amplitude, cf. further on)

The “Conventional Wisdom” on Stress-Accent
ogi stories pitch doesn t cut the mustard
Although pitch range is the most important of the fo-related cues, it is not as good a predictor of stress as DURATION

Amplitude

Pitch Range

Duration

Av. Pitch

OGI Stories - Pitch Doesn’t Cut the Mustard
total energy is the best predictor of stress
Duration x Amplitude is superior to all other combination pairs of acoustic parameters. Pitch appears redundant with duration.

Duration x Amplitude

Dur x Pitch Range

Pitch Range x Average

Dur x Pitch Av

Pitch Av x Amp

Pitch Range x Amp

Duration

Total Energy is the Best Predictor of Stress
a brief primer on vocalic acoustics
Vowel quality is generally thought to be a function primarily of two articulatory properties - both related to the motion of the tongue

The front-back plane is most closely associated with the second formant frequency (or more precisely F2 - F1) and the volume of the front-cavity resonance

The height parameter is closely linked to the frequency of F1

In the classic vowel “triangle” segments are positioned in terms of the tongue positions associated with their production, as follows:

A Brief Primer on Vocalic Acoustics
slide13

Duration/Amplitude/Int. Energy - Which?

  • There are supposed to be large differences in the “intrinsic” amplitude and duration of vowels
  • Could such differences be compensated for in terms of stress?
  • Let’s take a closer look!
slide14

Amplitude Differences - Stressed/Unstressed

  • There are very small differences in amplitude between stressed and unstressed nuclei
  • The lax monophthongs tend to be have a slightly larger dynamic range than diphthongs
slide15

Durational Differences - Stressed/Unstressed

  • There is a large dynamic range in duration between stressed and unstressed nuclei
  • Diphthongs and tense, low monophthongs tend to have a larger range than the lax monophthongs
slide16

Int. Energy Differences - Stressed/Unstressed

  • There is a large dynamic range in integrated energy between stressed and unstressed nuclei
  • Diphthongs and tense, low monophthongs tend to have a larger range than the lax monophthongs
spatial patterning of duration and amplitude
Let’s return to the vowel triangle and see if it can shed light on certain patterns in the vocalic data

The duration, amplitude (and their product, integrated energy, will be plotted on a 2-D grid , where the x-axis will always be in terms of hypothetical front-back tongue position (and hence remain a constant throughout the plots to follow)

The y-axis will serve as the dependent measure, sometimes expressed in terms of duration, or amplitude, or their product

Spatial Patterning of Duration and Amplitude
amplitude monophthongs vs diphthongs
Amplitude - Monophthongs vs. Diphthongs

Monophthongs

Diphthongs

All nuclei

amplitude monophthongs vs diphthongs31
Amplitude - Monophthongs vs. Diphthongs

Monophthongs

Diphthongs

Stressed

Unstressed

duration monophthongs vs diphthongs36
Duration - Monophthongs vs. Diphthongs

Monophthongs

Diphthongs

Stressed

Unstressed

int energy monophthongs vs diphthongs41
Int. Energy - Monophthongs vs. Diphthongs

Monophthongs

Diphthongs

Stressed

Unstressed

slide42

Mystery Parameter

  • There is one other parameter which when plotted in a vowel triangle plot shows an interesting pattern
  • This is - proportion of stressed an unstressed nuclei
amplitude monophthongs vs diphthongs44
Amplitude - Monophthongs vs. Diphthongs

Monophthongs

Diphthongs

All nuclei

summary and conclusions
There is an intimate relationship between vocalic identity, nucleic duration and stress accent in spontaneous dialogue (at least in the Switchboard corpus)

Stressed syllables tend to have significantly longer nuclei than their unstressed counterparts, consistent with the findings reported by Silipo and Greenberg in previous years’ meetings regarding the OGI Stories corpus (telephone monologues)

Certain vocalic classes exhibit a far greater dynamic range in duration than others

Diphthongs tend to be longer than monophthongs, BUT ….

The low monophthongs ([ae], [aa], [ay], [aw], [ao]) exhibit patterns of duration and dynamic range under stress (accent) similar to diphtongs

The statistical patterns are consistent with the hypothesis that duration serves under many conditions as either a primary or secondary cue for vowel height (normally associated with the frequency of the first formant)

Summary and Conclusions
summary and conclusions48
Moreover, the stress-accent system in spontaneous (American) English appears to be closely associated with vocalic identity

Low vowels are far more likely to be fully stressed than high vowels (with the mid vowels exhibiting an intermediate probability of being stressed)

Thus, the identity of a vowel can not be considered independently of stress-accent

Thus, vowel duration may be an important factor in disambiguating spoken language and therefore should be of interest to the speech recognition community

Summary and Conclusions