Sean A. Fulop

Algorithms for computing the time-corrected instantaneous frequency (reassigned) spectrogram, with applications.

A modification of the spectrogram (log magnitude of the short-time Fourier transform) to more accurately show the instantaneous frequencies of signal components was first proposed in 1976 [Kodera et al., Phys. Earth Planet. Inter. 12, 142-150 (1976)], and has been considered or reinvented a few times since but never widely adopted. This paper presents a unified theoretical picture of this time-frequency analysis method, the time-corrected instantaneous frequency spectrogram, together with detailed implementable algorithms comparing three published techniques for its computation. The new representation is evaluated against the conventional spectrogram for its superior ability to track signal components. The lack of a uniform framework for either mathematics or implementation details which has characterized the disparate literature on the schemes has been remedied here. Fruitful application of the method is shown in the realms of speech phonation analysis, whale song pitch tracking, and additive sound modeling.

Separation of components from impulses in reassigned spectrograms

Two computational methods for pruning a reassigned spectrogram to show only quasisinusoidal components, or only impulses, or both, are presented mathematically and provided with step-by-step algorithms. Both methods compute the second-order mixed partial derivative of the short-time Fourier transform phase, and rely on the conditions that components and impulses are each well-represented by reassigned spectrographic points possessing particular values of this derivative. This use of the mixed second-order derivative was introduced by Nelson [J. Acoust. Soc. Am. 110, 2575-2592 (2001)] but here our goals are to completely describe the computation of this derivative in a way that highlights the relations to the two most influential methods of computing a reassigned spectrogram, and also to demonstrate the utility of this technique for plotting spectrograms showing line components or impulses while excluding most other points. When applied to speech signals, vocal tract resonances (formants) or glottal pulsations can be effectively isolated in expanded views of the phonation process.

Yeyi Clicks: Acoustic Description and Analysis

Yeyi has the largest known inventory of click sounds in the Bantu language family. It is now entering a moribund state, and this paper documents a variety of acoustic and distributional details of the clicks found in the speech of 13 Yeyi speakers by presenting sound inventories, spectrograms, palatograms, and related acoustic data. The durations of the closure and release phases of the clicks were measured, and an analysis demonstrates that the two duration measures together are statistically able to distinguish the dental, alveolar, palatal, and lateral clicks from one another. A second quantitative study examines the discriminability of the four click places using solely the anterior burst power spectra, as parametrized using the first four spectral moments. The places of articulation are found to be moderately well classified by this means. The patterns of interspeaker variation affecting the clicks are also documented, and these are found to accord rather well with the classification errors made by the optimal classifier using the anterior burst spectra.

A Spectrogram for the Twenty-First Century

Just as World War II was breaking out in Europe in 1939, a prototype of a remarkable electrical device was being completed at Bell Telephone Laboratories, under the direction of Ralph Potter. This device was able to provide, on a strip of paper, a continuous running document of the Fourier spectrum of a sound signal as it changed through time. Because of the war it was kept under wraps, but its detailed construction and numerous applications were revealed to the scientific community in a series of papers published in the Journal of the Acoustical Society of America (JASA) in 1946, wherein it was called the Sound Spectrograph. The running spectral analysis that it output was termed a spectrogram. The spectrograph has been recorded in history as one of the most useful and influential instruments for acoustic signal processing. In particular, the fields of phonetics and speech communication, which motivated the development of the machine, have been completely transformed by its widespread adoption. Over the decades, the cumbersome and delicate analog spectrograph hardware was transformed into more robust digital hardware at first, and then as computers became generally more powerful, into the digital software incarnations most of us use today. The underlying principle of the spectrogram has never changed; most applied acousticians who do time-frequency analysis are content to use software that in essence simulates the output that appeared 60 years ago in JASA (Fig. 1). Of what else in acoustics can the same be said? Do we use 60-year old microphones? Tape recorders? Loudspeakers? Well, in truth, some of us have not been so content, but a more useful analytical process has never been generally recognized. The rich area of signal proA SPECTROGRAM FOR THE TWENTY-FIRST CENTURY

Accuracy of formant measurement for synthesized vowels using the reassigned spectrogram and comparison with linear prediction.

This brief report describes a small study which was undertaken with nine synthetic vowel tokens, in an effort to demonstrate the validity of the reassigned spectrogram as a formant measurement tool. The reassigned spectrograms performance is also compared with that of a typical pitch-asynchronous linear predictive analysis and is found to be superior. In this study, reassigned spectrograms were further processed to highlight the formants and then were used to measure these synthetic vowel formants generally to within 0.5% of their known true values, far surpassing the accuracy of a typical linear predictive analysis procedure which was inaccurate by as much as 17%. The overall accuracy of reassigned spectrographic formant measurement is thus demonstrated in these cases.

Breathy and whispery voicing in White Hmong

The White dialect of Hmong uses breathy voice as a tonal feature, and also a distinctive whispery voice as a stop consonant feature. In this paper, acoustic measurements are shown to validate the apparent differences between these two similar phonation types. In particular, relative harmonic intensity and harmonicity were found to be, in general, three ways distinct among Hmong modal, breathy, and whispery phonation. The discovery of distinctly pronounced breathy and whispery phonation in a single language has implications for the representational theory which is used to specify the phonetic grammar.

Phonetic applications of the time-corrected instantaneous frequency spectrogram.

A reassigned or time-corrected instantaneous frequency spectrogram has been developed in the work of a number of practitioners. Here we present a general description of this imaging technique and explore its manifold applications to acoustic phonetics. The TCIF spectrogram shows the locations of signal components with unrivalled precision, eliminating the blurring and smearing of components that hamper the readability of the conventional spectrogram. Formants of vowels and other resonants are shown with great accuracy by observing glottal pulsations at very short time scales with a wideband analysis. A further post-processing technique is also described, by which signal components such as formants, as well as impulsive events, can be effectively isolated to the exclusion of other signal information. When the phonation process is examined this closely, a variety of evidence surfaces which supports recent developments in the theory and computational simulation of aeroacoustic phenomena in speech. Narrowband analysis is also demonstrated to permit pitch tracking with relative ease.

Acoustic correlates of the fortis/lenis contrast in Swiss German plosives

Features of articulatory tension in plosives, such as increased closure duration, have not been shown to be perceptually salient in the time following the plosion (with the exception of increased F0) but are thought to influence perception by their appearance in the closure phase preceding plosion [K. Kohler, Phonetica 36, 332–343 (1979)]. Kohler [Phonetica 41, 150–174 (1984)] has claimed that the two plosive classes in Swiss German cannot contrast initially (where closure features are not audible), as neither voicing nor aspiration supports the contrast. This paper reports preliminary acoustic data (spectrograms) obtained from the natural production of Swiss German plosives by a native speaker. The data indicate that sonarants following fortis consonants exhibit increased high formant clarity and intensity in relation to the first spectral peak. Kohler and W. A. van Dommelen [J. Phon. 15, 365–381 (1987)] show a similar acoustic feature to be associated with global ‘‘tense voice’’ manner in the vowels sur...

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Articulatory modeling of so‐called advanced tongue root vowels

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