# Real-Time Visual Feedback in Singing Pedagogy: Current Trends and Future Directions
## 3 The Oscillatory Subsystem
### Vocal Folds Contact Variations  
The electroglottograph is a non-invasive device that has two electrodes placed on each side of the thyroid notch. When the vocal folds contact, an imperceptible electric current is sent between the electrodes. The resulting signal displays vocal fold contact area in terms of a waveform corresponding to the voltage variation across the glottis; it reaches a maximum at full contact and a minimum when the vocal folds are separated (Figure 6) [37]. Real-time visual feedback of EGG shapes can be provided by several software systems, such as SpeechStudio (Laryngograph, UK) and VOCVista Video Pro (Sygyt Software, Bochum, Germany). By visualizing EGG shapes, different degrees of vocal fold adduction can be monitored. For example, the top panel of Figure 7 shows EGG signals and their corresponding derivative (dEGG), as displayed by the VOCVista Video Pro software. This was recorded from a male singer sustaining the vowel /a/ with different degrees of glottal adduction on the same pitch and sound pressure level; the longer the vocal fold contact, the broader and more knee-like the EGG shape. The rightmost bottom EGG shapes illustrate the variation from breathy (hypofunctional) to pressed (hyperfunctional) phonation. Minimal glottal adduction, as used in breathy phonation, results in a waveform with a narrow pulse. In flow phonation, the glottal closure is complete and the pulse is wider. The pulse is still wider for firmer glottal adduction, as it is in neutral and pressed phonation [34; 39]. Firm phonation refers to an elevated but not maximal degree of adduction, reflected in a wide pulse that is still narrower than it is for pressed phonation. Real-time displays of EGG signals for phonation types may be advantageous for a developing singer, for both aesthetic and vocal health reasons. As mentioned, glottal adduction determines phonation types which, in turn, determine voice timbre [20; 40]. Moreover, habitual use of pressed phonation (hyperfunctional) may lead to phonotrauma [33]. Finally, in classically trained singing, flow phonation is often considered a baseline phonation type and seems to be associated with a more resonant voice quality [41]. Figure 6: Electroglottograph signal for three vocal fold vibratory cycles recorded by an electrolaryngograph, here displayed by the software Sopran. The sudden voltage changes (red arrows) represent an initiation of vocal fold contact. VoeeVista Video Pro software also offers a waveform analysis of EGG (by [PERSON])[42], showing continuous sequences rather than single EGG periods. The amplitude of the waveform patterns reflect the pulse width and, thus, glottal adduction. The waveform can be complemented with a display of the EGG derivative sequence (_d_EGG waveform), as well as a narrow band spectrogram [38]. This combination may help a student learn how to avoid or intentionally produce phonatory events, such as register breaks. Figure 8 shows an example of one voice break during an ascending glissando sum by a male singer. The red arrows highlight_f_o variations, while the red box highlights a voice break. The chaotic event in the right part of both EGG and _d_EGG waveforms illustrates a loss of vocal fold contact. More recently, real-time visual feedback of EGG shapes and their related metrics are also freely available using the software FonaDyn ([[https://github.com/ElsevierSoftwareX/SOFTX_2019_251](https://github.com/ElsevierSoftwareX/SOFTX_2019_251)]([https://github.com/ElsevierSoftwareX/SOFTX_2019_251](https://github.com/ElsevierSoftwareX/SOFTX_2019_251)), accessed on 6 September 2021) [43]. Apart  
Figure 8: VoeeVista Video Pro display of an ascending glissando sum by a male. Top panel: EGG waveform. Middle panel: _d_EGG waveform. Bottom panel: narrow band spectrogram. Red box highlights a voice break and the red arrow highlights_f_o variations. Figure 7: Electrolaryngograph (EGG) shapes displayed by the VoeeVista Video Pro software for a single vibratory cycle of a male singer sustaining the vowel /a/ with the indicated phonation types.