VC Transitions Research Articles

Investigation of deep electronic levels in n‐type and p‐type 3C‐SiC using photoluminescence

Among the various SiC polytypes, cubic 3C‐SiC is much more difficult to grow in high crystalline quality than the commercially introduced hexagonal 6H‐SiC and 4H‐SiC counterparts. Besides some benefits of 3C‐SiC for transistor applications related to a greater electron mobility and a lower metal‐oxide‐semiconductor interface trap density compared to 4H‐SiC, new potential optoelectronic applications have been introduced very recently. Boron doped 3C‐SiC may act as an ideal candidate for an intermediate band (IB) solar cell material. Aluminum doped p‐type 3C‐SiC could lead to the development of efficient optoelectrochemical water splitting cells. Finally, 3C‐SiC with its various intrinsic point defects has been considered as a suitable candidate for future spintronic‐applications. All these applications will critically depend on further understanding defect behaviour on atomic level. In our study we investigated free standing n‐type and p‐type 3C‐SiC material grown in our lab. Temperature dependent photoluminescence measurements revealed the presence of carbon vacancy related VC and VC-CSi defect transitions in the p‐type materials but not in the n‐type materials. This observation present in as grown 3C-SiC is believed to have significant impact on the optoelectronic applications. Copyright © 2017 VBRI Press.

Hearing tongue loops: Perceptual sensitivity to acoustic signatures of articulatory dynamics

Previous work has shown that velar stops are produced with a forward movement during closure, forming a forward (anterior) loop for a VCV sequence, when the preceding vowels are back or mid. Are listeners aware of this aspect of articulatory dynamics? The current study used articulatory synthesis to examine how such kinematic patterns are reflected in the acoustics, and whether those acoustic patterns elicit different goodness ratings. In Experiment I, the size and direction of loops was modulated in articulatory synthesis. The resulting stimuli were presented to listeners for a naturalness judgment. Results show that listeners rate forward loops as more natural than backward loops, in agreement with typical productions. Acoustic analysis of the synthetic stimuli shows that forward loops exhibit shorter and shallower VC transitions than CV transitions. In Experiment II, three acoustic parameters were employed incorporating F3-F2 distance, transition slope, and transition length to systematically modulate the magnitude of VC and CV transitions. Listeners rated the naturalness in accord with those of Experiment I. This study reveals that there is sufficient information in the acoustic signature of "velar loops" to affect perceptual preference. Similarity to typical productions seemed to determine preferences, not acoustic distinctiveness.

Coarticulation and contrast in static and dynamic models of second formant trajectories

Real (Stevens et al., 1966) and virtual F2 locus (Sussman et al., 1991) measures are presented for Malayalam lingual plosives. We show that in distinguishing voiceless coronals (dental, alveolar, and retroflex) in VCːV sequences, F2 onsets derived from first-order locus equations (LEs) show only partial delineation of the contrast. The dental-alveolar contrast is effectively maintained, but retroflex and alveolar stops show no significant difference in F2 onset. Following Lindblom and Sussman’s (2012) examination of LEs as a measure of relative coarticulatory resistance, we report F2 slopes for the three coronal stops in VC and CV transitions to assess the implications of this metric in Malayalam. Our findings on the ordering of slope values from steepest to flattest did not follow predictions based on expectations of relative articulatory complexity; namely, alveolars generated a flatter slope than retroflexes, despite Dart and Nihalani’s (1999) demonstration that the retroflex gesture is more complex within the coronals. These results, when compared with temporal measures from exponential models of formant trajectories at consonant implosion and release (i.e., transition velocity and projected F2 locus), suggest a necessary distinction between coarticulation-based place of articulation categorization and formant transition cues utilized in maintaining stop place contrasts.

Phonetically Trained and Untrained Adults' Transcription of Place of Articulation for Intervocalic Lingual Stops With Intermediate Acoustic Cues

In this study, the authors aimed to investigate how listener training and the presence of intermediate acoustic cues influence transcription variability for conflicting cue speech stimuli. Twenty listeners with training in transcribing disordered speech, and 26 untrained listeners, were asked to make forced-choice labeling decisions for synthetic vowel-consonant-vowel (VCV) sequences "a doe" (/do/) and "a go" (/go/). Both the VC and CV transitions in these stimuli ranged through intermediate positions, from appropriate for /d/ to appropriate for /g/. Both trained and untrained listeners gave more weight to the CV transitions than to the VC transitions. However, listener behavior was not uniform: The results showed a high level of inter- and intratranscriber inconsistency, with untrained listeners showing a nonsignificant tendency to be more influenced than trained listeners by CV transitions. Listeners do not assign consistent categorical labels to the type of intermediate, conflicting transitional cues that were present in the stimuli used in the current study and that are also present in disordered articulations. Although listener inconsistency in assigning labels to intermediate productions is not increased as a result of phonetic training, neither is it reduced by such training.

Cochlea-scaled entropy, not consonants, vowels, or time, best predicts speech intelligibility

Speech sounds are traditionally divided into consonants and vowels. When only vowels or only consonants are replaced by noise, listeners are more accurate understanding sentences in which consonants are replaced but vowels remain. From such data, vowels have been suggested to be more important for understanding sentences; however, such conclusions are mitigated by the fact that replaced consonant segments were roughly one-third shorter than vowels. We report two experiments that demonstrate listener performance to be better predicted by simple psychoacoustic measures of cochlea-scaled spectral change across time. First, listeners identified sentences in which portions of consonants (C), vowels (V), CV transitions, or VC transitions were replaced by noise. Relative intelligibility was not well accounted for on the basis of Cs, Vs, or their transitions. In a second experiment, distinctions between Cs and Vs were abandoned. Instead, portions of sentences were replaced on the basis of cochlea-scaled spectral entropy (CSE). Sentence segments having relatively high, medium, or low entropy were replaced with noise. Intelligibility decreased linearly as the amount of replaced CSE increased. Duration of signal replaced and proportion of consonants/vowels replaced fail to account for listener data. CSE corresponds closely with the linguistic construct of sonority (or vowel-likeness) that is useful for describing phonological systematicity, especially syllable composition. Results challenge traditional distinctions between consonants and vowels. Speech intelligibility is better predicted by nonlinguistic sensory measures of uncertainty (potential information) than by orthodox physical acoustic measures or linguistic constructs.

Testing Licensing by Cue: A Case of Russian Palatalized Coronals

The hypothesis ‘licensing by cue’ by Steriade holds that phonological contrastsare maintained in environments that provide better acoustic cues to the contrastsand are neutralized in environments that provide poorer acoustic cues or nocues. This paper tests the hypothesis by examining the distribution of a phonologicalcontrast - the Russian plain/palatalized coronal stops /t/ and /t^j/ in various syllable-final contexts. The results of a series of acoustic and perceptual experimentspresented in this paper provide some support for the hypothesis: the relativesalience of releases in different word boundary contexts (_#k > _#n, _#s) correlatesstrongly with the general patterns of neutralization of the contrast in similarword-internal contexts (_k > _n, _s) in Russian and other related languages. At thesame time, the relative salience of VC transitions in different vowel contexts(a_ > u_ > i_) has apparently little to do with attested patterns of neutralization.The results suggest that some perceptual cues are phonologically more relevantthan others, providing evidence for interactions between phonetics and phonologymore complex than predicted by the hypothesis.

Measures of voiced frication for automatic classification

As an approach to understanding the characteristics of the acoustic sources in voiced fricatives, it seems apt to draw on knowledge of vowels and voiceless fricatives, which have been relatively well studied. However, the presence of both phonation and frication in these mixed-source sounds offers the possibility of mutual interaction effects, with variations across place of articulation. This paper examines the acoustic and articulatory consequences of these interactions and explores automatic techniques for finding parametric and statistical descriptions of these phenomena. A reliable and consistent set of such acoustic cues could be used for phonetic classification or speech recognition. Following work on devoicing of European Portuguese voiced fricatives [Jesus and Shadle, in Mamede et al. (eds.) (Springer-Verlag, Berlin, 2003), pp. 1–8]. and the modulating effect of voicing on frication [Jackson and Shadle, J. Acoust. Soc. Am. 108, 1421–1434 (2000)], the present study focuses on three types of information: (i) sequences and durations of acoustic events in VC transitions, (ii) temporal, spectral and modulation measures from the periodic and aperiodic components of the acoustic signal, and (iii) voicing activity derived from simultaneous EGG data. Analysis of interactions observed in British/American English and European Portuguese speech corpora will be compared, and the principal findings discussed.

Proficient bilinguals require more information for vowel identification than monolinguals

Even proficient bilinguals have been shown to experience more difficulty understanding speech in noise than monolinguals. One potential explanation is that bilinguals require more information than monolinguals for phoneme identification. We tested this hypothesis by presenting gated, silent-center vowels to two groups of listeners: (1) monolingual American English speakers and (2) proficient Spanish–English bilinguals, who spoke unaccented or mildly accented English. To create the stimuli, two American English speakers were recorded as they read the following items: ‘‘beeb, bibb, babe, bebb, babb,’’ and ‘‘bob.’’ Duration-preserved silent-center versions of three tokens of each item were created by retaining varying amounts of the CV and VC transitions (10, 20, 30, or 40 ms) and attenuating the remainder of the vowel center to silence. Duration-neutral versions of silent-center tokens were created by lengthening or shortening the silent portion to match the tokens vowel duration to the average for all the tokens. Listeners identified the unedited (full vowel), duration-preserved, and duration neutral silent-center tokens in a six-alternative forced-choice task. The two groups of listeners identified the unedited tokens with similar accuracy. In the silent-center conditions, however, the bilinguals identified the stimuli less accurately than the monolinguals. [Work supported in part by NIH-NIDCD Grant No. 1R03DC005561-01A1.]

Acoustics and perception of Russian plain-palatalized contrast

Acoustic and perceptual correlates of Russian syllable-final plain and palatalized coronal stops were investigated. The acoustic analysis based on the data from six talkers showed that the stops were consistently released both word-finally and before stops. Both consonants in final position were strongly affricated and aspirated. The VC transitions to palatalized stops extended for more than half of the vowel, substantially affecting its quality. The perception experiment (12 listeners) determined that both bursts and transitions were perceptually relevant to the identification of the two phonemes. The contrast in final and preconsonantal environments differed in the relative importance of these cues: VC transition was more important for the plain-palatalized distinction in final position, while burst was a major cue to the contrast in preconsonantal position. The findings suggest that the presence of the plain-palatalized phonemic contrast in perceptually less favorable syllable coda positions is compensated for by additional acoustic information that assists a listener in the recoverability of the contrast. [Work supported by Social Sciences and Humanities Research Council of Canada.]

Spectral transition in the perception of English segments

There is a long-standing debate on whether speech is perceived through steady states or transitions [D. Kewley-Port and D. Pisoni, J. Acoust. Soc. Am. 73, 1779–1793 (1983)]. Furui [S. Furui, J. Acoust. Soc. Am. 80, 1016–1025 (1986)], gating Japanese monomoraic syllables, found listeners identified syllables correctly when they heard the point of maximal spectral transition (maximum D, a measure based on cepstral coefficients). In this paper, this work is extended to English, and to a wider variety of transitions. 128 English words, representing transitions between all manners of articulation, were gated at 20-ms intervals. VC, CC, and VV transitions were used in addition to Furui’s CV and CjV syllables. The results show that for almost all transitions (diphones), there is a clear point at which listener’s identification of the second segment becomes highly accurate. For many diphones (54%), this point is within 15 ms of the maximum D. However, for other diphones (especially those with far-reaching perceptual cues), the point of recognition is farther from the maximum D. This paper will discuss reasons for the discrepancies between Furui’s experiment and the current one, including language-specific factors, and will address implications for the dynamic theory of speech perception.

Speech motor coordination in children with phonological disorders.

This study examined the question of whether children with phonological disorders have problems in speech motor coordination. Four children with phonological disorders, four typically developing children matched for age and gender, and four adults were asked to say multiple repetitions of ‘‘bad dog’’ at normal and fast rates of speech. The productions were digitized and formant tracks were obtained using LPC analysis. Standard deviations were calculated over the second‐formant values frame by frame along the four 50‐ms spans of the CV and VC transitions. The F2 tracks of the adults were much less variable than those of both groups of children. The F2 tracks of the phonologically disordered children were more variable than those of their typical age peers for three of the four transitions; however, the F2 tracks of the phonologically disordered children were less variable than their age peers for the initial ‘‘ba’’ transition, the only transition in which the tongue was not involved in both the consonant and the vowel articulation. This result suggests that children with phonological disorders may have difficulty in rapidly sequencing gestures that involve conflicting configurations for the same articulator.

Age differences in the uptake of acoustic cues in spoken word recognition

This study compares the uptake of acoustic‐phonetic information during lexical access in younger (20–30 years) and older (55–70 years) listeners to determine if these groups are equally efficient in utilizing the coarticulatory information in VC transitions. Examined were place contrasts between stops (p/t and p/k) and fricatives (s/sh and z/yog), and manner contrasts between stops and nasals (b/m and d/n) using the gating paradigm [Warren and Marslen‐Wilson, Percept. Psychophys. 41, 262–275 (1987)]. Stimuli consisted of 21 pairs of CVC words differing only in terms of the final consonant (e.g., pot versus pop). These words were heard in their natural or cross‐spliced forms in which the initial CV of one word was appended to the consonant offset of its match. Gated versions of each word were constructed on the basis of this ‘‘splicing’’ point such that the first gate included the initial CV up to 100 msec before end of the vowel and then proceeded in 20‐ms increments (gates that included portions of voiceless stop closure providing no additional acoustic information were eliminated). Results will be discussed in terms of possible age‐related decrements in phonetic processing. [Work supported by NIA.]

Acoustic analysis of Italian [r] and [l

Acoustic analysis of Italian [r] and [l] was carried out. Speech materials consisting of prestressed VCV syllables produced by 3 male and 3 female native speakers of Italian formed the basis of the present study. Each syllable was repeated 3 times by each speaker. The vowel in the syllable was [i,a,u] and the consonant was [r] or [l] in geminated and nongeminated form, leading to 216 syllables. Confirming the results of a previous study on Italian stop consonants [R. Rossetti, 2874 (1994)] results of the present analysis showed that the first vowel duration was systematically shortened in the geminated consonants. The data also showed that in the present case the duration of the consonant as well as, in most cases, of the VC and CV transitions were lengthened in the geminated form. Spectral analysis showed that there is little or no effect on the spectral properties of the consonants due to the presence of gemination. However, [r] in geminated form is in a few cases devoiced. Informal perceptual tests indicate that [r] can be automatically generated from [l] by introducing a silent portion within the stationary portion of [l].

VC and CV transitions and the recognition of Canadian French syllables containing /l/

According to recent works, the formant structure of consonant /l/ is influenced by the adjoining vowels, so that the liquid appears as a modulation of these ones in frequency and intensity. The aim of this paper was to find out to what extent the VC and CV transitions are still needed in order that a syllable containing /l/ be recognizable. To that end, nonsense /V1V/ utterances in which both V’s correspond to the same vowel were recorded by a Canadian French speaking informant and digitalized. In addition, vowels /i, e, eh, a, o, u, y/, recorded in isolation, were also digitalized. Through the splicing and editing of the acoustic signal, the first vowel of each utterance, then the second one, was replaced successively by each of the vowels previously recorded in isolation, thus giving 112 stimuli. Listening tests through a panel comprising of 16 French Canadian subjects were performed. Results showed that a syllable containing /1/ is recognized most when the replacing vowel in the utterance is /i/ or /y/. In most other cases, the syllable is not recognized and the consonant itself might be perceived as a vowel.

Acoustic and perceptual properties of intervocalic consonants produced by talkers with cerebral palsy

Speech produced by ten talkers with cerebral palsy was studied. Talkers recorded semantically anomalous sentences of the form ‘‘The N1 is Ving the N2’’ where N1, N2, and V are single-syllable nouns and verbs (in the bisyllabic infinitive form) selected from lists of noun and verb target words. Listeners heard sentences over headphones and identified the target words in a closed response set task in which each target word had an associated set of four to six minimally different response alternatives (e.g., {boat, moat, vote, oat}). For the present analyses, only verb targets that differed in the final consonant of the root word (e.g., waiting, wading, waning, waving, weighing) have been examined. Perceptual data were analyzed for percentage of incorrect responses and proportions of various types of errors (e.g., voicing/place/manner) per token. Acoustic analyses included measures of vowel and consonant durations, extent of VC and CV formant transitions, categorical measures of a variety of acoustic features such as presence/absence of voicing in the consonant constriction interval, nasalization, and laryngealization. Analyses completed on six of the ten talkers’ data suggest that inappropriate presence/absence of voicing and laryngelization, and restricted F1 VC (but not CV) transitions are most predictive of perceptual errors. [Work supported by the Nemours Foundation.]

On the sufficiency of compound target specification of isolated vowels and vowels in /bVb/ syllables

It has been suggested [e.g., Strange et al., J. Acoust. Soc. Am. 74, 695-705 (1983); Verbrugge and Rakerd, Language Speech 29, 39-57 (1986)] that the temporal margins of vowels in consonantal contexts, consisting mainly of the rapid CV and VC transitions of CVC's, contain dynamic cues to vowel identity that are not available in isolated vowels and that may be perceptually superior in some circumstances to cues which are inherent to the vowels proper. However, this study shows that vowel-inherent formant targets and cues to vowel-inherent spectral change (measured from nucleus to offglide sections of the vowel itself) persist in the margins of /bVb/ syllables, confirming a hypothesis of Nearey and Assmann [J. Acoust. Soc. Am. 80, 1297-1308 (1986)]. Experiments were conducted to test whether listeners might be using such vowel-inherent, rather than coarticulatory information to identify the vowels. In the first experiment, perceptual tests using "hybrid silent center" syllables (i.e., syllables which contain only brief initial and final portions of the original syllable, and in which speaker identity changes from the initial to the final portion) show that listeners' error rates and confusion matrices for vowels in /bVb/ syllables are very similar to those for isolated vowels. These results suggest that listeners are using essentially the same type of information in essentially the same way to identify both kinds of stimuli. Statistical pattern recognition models confirm the relative robustness of nucleus and vocalic offglide cues and can predict reasonably well listeners' error patterns in all experimental conditions, though performance for /bVb/ syllables is somewhat worse than for isolated vowels. The second experiment involves the use of simplified synthetic stimuli, lacking consonantal transitions, which are shown to provide information that is nearly equivalent phonetically to that of the natural silent center /bVb/ syllables (from which the target measurements were extracted). Although no conclusions are drawn about other contexts, for speakers of Western Canadian English coarticulatory cues appear to play at best a minor role in the perception of vowels in /bVb/ context, while vowel-inherent factors dominate listeners' perception.

(Semi‐)automatic pitch‐synchronous computation of glottal flow

Large‐scale investigations of the aerodynamics of voice production are most easily performed using noninvasive measurement methods. Glottal airflow can be estimated by inverse filtering the mouth flow, measured with a pneumotachograph mask. A semi‐automatic pitch‐synchronous inverse filtering method was developed that combines the advantages of interactive inverse filtering and inverse filtering with fixed‐filter settings: Our method yields accurate and calibrated estimates of glottal flow during VC and CV transitions, which are amenable for parametrization and can be used for bulk processing. Using glottal dosing and opening moments derived from the electroglottogram, covariance LPC analysis on the closed glottis interval is employed to compute the optimal inverse filter for each period. For male speakers, the method yields quite stable waveforms. These will be parametrized in order to develop rules for a physiologically interpretable, time‐varying synthetic voice source. The control parameters used are F0, duty cycle, top‐top flow amplitude, glottal leak area, and vertical phasing. A number of these parameters can be derived from both the EGG and the glottal flow. Comparison of parameter values from the two signals will give an indication of their reliability. The results will be discussed.

Plosive consonant identification in ambiguous sentences

The hypothesis tested in this paper is that the acoustic–phonetic information at the sentence level (irrespective of the meaning of the sentence) makes it easier to interpret those cues to plosive consonant identity which are contained in vocalic transitions. Listeners were given a two-way choice in meaningful Dutch sentences which allowed two different intervocalic plosive consonants in the same position, and from which various parts of the VCV segments (plosive burst, vocalic transitions) had been deleted. The results were compared with results from a condition in which the VCV segments were presented without the sentence context. The hypothesis was confirmed for CV transitions, except for /k/, but disconfirmed for nearly all VC transitions. This could mean that in experiments involving the perception of isolated VCV or CVC utterances, whether spoken in isolation or excised from longer utterances, the contribution of the CV transition to plosive consonant perception may be underestimated.

The perception of plosive consonants by hearing‐impaired listeners

The reduced speech perception by hearing‐impaired subjects has often been studied. In order to investigate the role of coarticulation we did some experiments on a group of 20 hearing‐impaired and 10 normal‐hearing subjects. From 45 embedded CVCVC words we deleted either the plosive bursts, or the vocalic CV or VC transitions, or the stationary vowel portions, or any combination of these four. The listeners were asked to identify the three plosive consonants in the resulting stimuli. The results for the normal‐hearing subjects agree with results of Schouten and Pols [Proc. 10th ICPS, Utrecht (1983), p. 464]: identification is affected more by deletion of plosive bursts than of vocalic transitions, and in medial consonants the CV transition contains more information about the consonant than does the VC transition. For the hearing‐impaired listeners the vocalic transitions play a more important role: identification with the plosive bursts only results in guessing; in contrast with the results for the normal‐hearing subjects more errors occur for the final consonant than for the initial or the medial consonant. We also found some relations with other auditory functions measured. [Work supported by ZWO.]

Perception of intervocalic stop consonants: the contributions of closure duration and formant transitions.

Acoustic analyses of vowel-consonant-vowel (VCV) utterances indicate that they generally include formant transitions from the first vowel into a period of closure (VC transitions), and transitions out of the closure into the second vowel (CV transitions). Three experiments investigated the perceptual importance of the VC transitions, the CV transitions, and the closure period in identification of medial stop consonants varying in place of articulation. Experiment 1 compared identification of members of synthetic VC and CV continua with those from VCV series made by concatenating corresponding VC and CV stimuli using various closure durations. Experiment 2 examined identification of VCV stimuli constructed with only VC, only CV, or both VC and CV transitions; again closure duration was systematically varied. Experiment 3 correlated CV and VC identification with identification of VCV stimuli. Neither closure duration nor formant transition structure (i.e., only VC, only CV, or both) had an independent effect on identification. Instead, the formant structure and closure duration together strongly affected stop identification. When both VC and CV transitions were present, the CV transitions contributed somewhat more to identification of medial stops with short closures, than the VC transitions did. With longer closure durations, neither set of transitions appeared to determine perceived place of articulation in any simple way. Overall, the data indicate that the perception of a medial consonant is more than simply a (weighted) sum of its parts.