Electroglottograph or EGG is a system which gives an information on the closure of vocal folds by measuring the electrical resistance between two electrodes placed around the neck. Even if the signal provides only an approximate value of the glottis surface, it gives a very good information about period of vocal folds vibration, especially because we are at the source of the phonation (no influence of vocal tract) and because there is no aerodynamic noise.
EGG signal is composed of :
|Raw waveform of E.G.G|
For a good analysis, the system filters as following:
1) filtering for voice
- high-pass filter (to cut low frequencies)
- anti-aliasing low-pass filter
2) filtering for swallowing
- low-pass filter (to keep low frequencies)
The waveform of the glottal cycle can be displayed in different ways. Upward movement of the curve can indicate more (1) or less (2) vocal fold contact: :
(1) upward <=> closure
(2) upward <=> opening
The waveform depends on :
Electrode placement is important. The amplitude of EGG signal depends of this placement and gives only a relative information.
Closed quotient consist in measuring the ratio between the closed phase and the complete cycle. A method is proposed by Scherer, Vail et Rockwell (1993), « Examination of the laryngeal Adduction Measure EGGW », NCVS status and Progress Report, 73-82. A threshold H must be set to distinguish closed and open parts of the cycle (authors proposed a threshold from 25% to 35 % by comparing with stroboscopic data).
Method to compute Closed quotient
Closed quotient=Tc / (Tc +To)