University:
Northwestern State University of Louisiana
Course:
COMM 1010 | Fundamentals Of Speech
Academic year:

2021
Views:

486

Pages:

13
Author:

gewoekerdodx

Source-Filter Theory Source and Filter • The periodic vibration of the vocal folds provides the input signal to the vocal tract resonator • The vocal folds are the source of the signal • The vocal tract is the filter Glottal Source Signal • Top diagram: source • Bottom diagram: Fourier analysis of this • First harmonic: fundamental frequency • All other harmonics: whole number multiples of the fundamental frequency Glottal Source Spectrum Characteristics • There are three sources of variability in glottal source waveform 1. Periodic nature 2. Shape 3. Ratio of open and closed phases Variability in the Periodic Nature of Glottal Wave Forms • No movement in nature will be ever be exactly the same • There will always be some differences • Despite these, for normal vocal fold vibration, we'll say for simplicity that the waveforms are periodic • The nature of these vibrations depend on gender and age Average F0 by Gender and Age Different Vocal signals • Two vocal signals of same duration • Upper waveform • • Period = 8 ms F0 = ----- Hz • • Period = 5 ms F0 = ----- Hz • Lower waveform • Upper waveform has fewer cycles in this time period • Lower waveform has fewer harmonics in same frequency band Harmonics with Different Spectral Slopes • Waveforms can also differ in how steeply they return to atmospheric pressure • Difference has consequences for spectra • Shallower closing  greater tilt • Steeper closing  shallower tilt • This is spectral slope: steepness of decrease in amplitude across frequencies • • Left: fast closure, shallow slope Right: slow closure, steep slope The change in shape of waveform and tilt degree of harmonics in spectrum may be a sign of voice disorders The Vocal Tract: A Closed Tube Length of the vocal tract will vary with size of talker Average size of vocal tract is about 15cm Inputs and Responses • Top waveform: glottal waveform • Bottom waveform: resulting vocal tract responses • Closure in vocal folds  vocal tract response Resonation in the Vocal Tract • Vocal tract resonance property is “excited” every time the vocal folds achieve glottal closure • Given typical vibration frequencies, vocal tract is often resonating • This then goes through vocal tract filter Source + Filter • We have source that is vocal fold vibration • We have filter that acts on it • Filter amplifies certain frequencies and cuts out others • This results in an output spectrum Formants • Schematic representation of vowel /i/ • Output spectrum shows emphasized harmonics • Peaks in output spectrum for vowels and similar consonants are formants • There are three main ones • Acts like a band pass filter Properties of Output • Output = input × filter • In input, harmonics decrease with frequency • In filter, all peaks have similar amplitude • In output, therefore… • Filter plays up some higher harmonics • Some peaks higher than others (because there is less amplitude available in higher frequencies)

Source-Filter Theory

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