This section contains a detailed description of the EGG signal especially with respect to the shape of the waveform and to the time domain characteristics of the physiological features.
As already mentioned, the EGG signal is regarded as a correlate of the glottal area or the glottal opening width. In an experiment by Gilbert, Potter and Hoodin (1984) an insulating strip was inserted between the vocal folds of an adult male during phonation to prevent electrical contact between them. There was no apparent effect on the production an acoustic wave, but after the removal of the insulator the amplitude of the EGG signal increased by ca. four times. Additionally, the results published by Scherer et al.(1988) enable the researcher to establish a linear relaltionship between the vocal fold contact area and the output of the electroglottograph. However, proper placement of the electrodes is of a special importance since a slight shift might cause spurious effects in the recorded signal.
The relationship between the gross features of the EGG waveform and the major phases of the glottal cycle are given by Baken (1992), Rothenberg and Mahshie (1988), Childers and Krishnamurthy (1985) and presented in Fig.16. Modal phonation with a full glottal closure has been assumed for this particular representation. There is no common agreement as to the direction in which the changes in vocal fold contact are represented. In this entire study however, the increased vocal fold contact is consistently plotted upwards on the y axis.
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Phases of the vocal folds contact:
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The time derivative of the Lx waveform is usually used in the determination of the periodicity of the signal. It can also be used to identify the distinguishable changes in the slopes during the phases of increasing and decreasing impedance which correspond to those of the simplified model of the EGG (see Fig.18).
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The positive peak of the derivative serves as an indication the instant of the glottal closure (CGI). Almost all researchers regard it as a robust and reliable marker of the pitch period for various voice qualities and intensities (Childers & Krishanurthy, 1985; Baken, 1992; Fourcin, 1993; Colton & Conture, 1990; Vieira et al., 1996). The procedure is both simpler than measuring F0 in the acoustic signal (Hess, 1991) and allows a more precise measurement of it. Thus, if a relatively strong EGG is registered, it is recommendable to use it as a reference for the precise measurement of speech fundamental frequency (Askenfeldt et al., 1980; Hess & Indefrey, 1987). Problems occur if the signal-to-noise ratio is poor or if F0 is measured during voice offset and onset, when several EGG pulses can exhibit a modified (or distorted) shape. It should be noted that the distribution of the EGG-measured pitch is even used to evaluate the degree of voice pathology (Fourcin,1993; documentation of the Laryngograph Processor; Fourcin & Abberton, 1977) when the acoustic methods of F0 determination fail.
The negative peak of the EGG derivative is regarded as an indicator of glottal opening. During the opening phase the glottal area increases monotonically until it reaches its maximum. Baer et al. (1983), Childers and Krishnamurthy (1985) state (based on high-speed films of the vocal folds vibration) that the movements of the vocal folds that are reflected in the EGG have two distinct phases. First, the EGG decreases monotonically, reflecting the decreasing in lateral contact between the vocal folds. During this interval the EGG waveform is convex. Then, as the upper margin separates the waveform of the EGG changes to concave. It is however questionable whether this should be used as a reliable marker of the instant of opening (because of the impact of additional mechanisms that can mask or delay it in the EGG signal). There is evidence for the so-called mucus bridging effect (Childers & Krishnamurthy, 1985; Titze & Talkin, 1979), in which a strand of mucus can bridge the glottis i as the vocal folds initially open. When the mucus bridge breaks, the EGG waveform records a sharp fall even though the glottis is already open.
It is usually supposed that the EGG reflects the contact area between the vocal folds. This area can be viewed laterally and transversely. Assuming that the depth of contact does not change during vocal fold vibration, the lateral contact area depends on the length of contact area along the upper margins of the vocal folds. Childers, Smith and Moore (1984) found a strong correlation between the EGG and the length of the vocal folds contact. Scherer et al. (1988) show a linear dependence of the EGG signal on the contact area between the folds.
The description of the EGG waveform fails for voices with a continuously open glottis. In that case, the variation of the larynx impedance does not correspond to the glottal area. Particularly the amplitude of the Lx waveform fluctuation may, but need not, change in accordance with the reduced contact between the folds.
In conclusion it can be assumed that the EGG waveform provides an adequate representation of the approximation and the closure of the vocal folds. The open phase of the folds movement is better represented by other means, especially by inverse filtering and invasive visual inspection (stroboscopy).