Electroglottography & Inverse Filtering Procedures

Electroglottography & Inverse Filtering Procedures are the important procedures for the analysis of the voice. Here is o

1. Sri Aurobindo Institute Of Speech & Hearing Topic – Electroglottography & Inverse Filtering Procedures Submitted To – Mrs. Garima Dixit Garge Presented By – Mr. Piyush Malviya

2. ELECTROGLOTTOGRAPHY - Electroglottography (EGG) is a noninvasive technique for obtaining an estimate of vocal fold contact patterns during phonation. A gold electrode is placed on each side of the thyroid cartilage at a level corresponding to the position of the vocal folds. A weak high‐frequency electrical current is passed between the electrodes. As vocal fold contact area changes, there are changes in the electrical resistance between the electrodes. When the glottis is opening or open, resistance increases; when the glottis is closing or closed, resistance decreases. The resulting Lx waveform, called an electroglottogram or laryngogram , reveals summary information about vocal fold contact over time. The EGG can be used to visualize various types of voice quality. For example, Figure 6.14 shows electroglottograms for a prolonged / i / vowel produced with a normal voice quality, a breathy voice quality, and a hoarse voice quality.

3. Electroglottography (EGG) is a technique for the measurement of vocal fold contact area based on the principle that tissue conducts current. A high frequency low current signal is passed between the vocal folds via electrodes located on the externa neck over the thyroid lamina. When the vocal folds touch, greater current flows than when they are open . There 13 a proportional variation of current when the vocal folds are less than maximally open or closed.

4. Electroglottographic recordings can be used to determine when the vocal folds are closed and how fast they are closing. If carefully interpreted, it is possible to determine characteristics of the opening of the vocal folds from an electroglottographic recording. Devices to record the electroglottographic signal are readily available. Currently these include the Voiscope and Laryngograph ( Laryngograph , Ltd.), the single channel SCI or the dual channel MCZ-l (Glottal Enterprises), and the unit from F] Electronics. Electrical output of the electroglottograph can easily be converted to hard copy using oscillograph or similar graphic recording device, or from a computer generated display. The literature on EGG is primarily qualitative in nature, based on interpretation of the waveform. Several studies have related the shape of the EGG waveform to the underlying physiology of vocal fold vibration. Information has also been reported on characteristic waveforms in patients with vocal pathology.

5. There have been many attempts to quantify the electroglottograph signal. EGG reflects the state of the vocal folds in a way that can be easily demonstrated and interpreted to patients. However, a limitation of the technique is that it cannot be used with all patients. Because the technique depends on vocal fold contact, the signal is considerably diminished or even absent in patients with lack of good contact, such as those with unilateral paralysis, or aphonia . It may also be difficult to obtain a clear waveform in the presence of severe hoarseness. The thick or large necks of some patients hinder transduction of the current and result in a poor EGG tracing. Manufacturers of EGGS have greatly enhanced its clinical value. For example, the MCZ-l EGG made by Glottal Enterprises uses two identical circuits and two sets of electrodes. The output of each electrode pair is compared and displayed 'on a meter. When the meter reading is O, the output of the two channels is identical and the vocal folds are centered between the two electrode pairs. This simple monitoring device helps to insure the proper placement of the electrodes and an optimal EGG signal.

6. INVERSE FILTERING PROCEDURES – In the inverse filtering procedure, the voice signal emitted at the lips is analyzed to remove the resonant effects of the vocal tract, producing an estimate of the waveform produced at the waveform. According to the acoustic theory of speech production, speech is the product of a sound source and a filter. That is, the sound output of the vocal folds is modified by the resonant characteristics of the vocal tract. If the resonant characteristics of the vocal tract are known, it should be possible to retrieve the characteristics of the output of the vocal folds from the orally emitted speech signal. Inverse filtering has been performed on the acoustic sound pressure waveform. We have used the airflow waveform for inverse filtering of normal and voice-disordered subjects. We routinely record the EGG signal simultaneously with the inverse filtered airflow signal to extract information about the vibratory characteristics of the vocal folds during the complete cycle.

7. The techniques complement each other in that airflow will often not be present during the closed phase of the vocal folds, but the EGG provides information about vibratory characteristics of the vocal folds during that phase. Thus, with both techniques, we are able to obtain a more complete picture of vocal fold vibratory characteristics during speech. The result of inverse filtering the oral airflow waveform is called a flow glottogram and an example is shown in Figure 7.13 . Four channels are collected directly into the computer using CSpeech . These are the (a) acoustic waveform, (b) raw oral airflow waveform, (c) EGG waveform, and (d) intraoral air pressure pulses associated with the stop plosives in the utterance (not shown in Figure 7.13 but see Figure 7.14 ).

8. The oral airflow waveform is inverse filtered using CGlott , a computer program separate from but complementary to CSpeech . The result is the inverse filtered or flow glottogram shown in Figure 7.13 . Collection of electroglottographic and inverse filtered airflow waveforms is routine in our clinic. The following measures are obtained from computer-assisted analysis of the waveform: (a) each cycle’s minimum (or leakage) and ac airflows, (b) the ratio of the time of the airflow pulse relative to the total period (airflow duty cycle), (c) the ratio of the closing and opening slopes of the airflow pulse (airflow speed quotient), (d) the ratio of the open time to the total period of the electroglottographic waveform (abduction quotient),(e) the closing time of the electroglottogram waveform, and (f) lung pressure.

9. REFERENCES Boone, D. R., McFarlane, S. C, Von Berg, S. L. & Zraick , R, I. (2013): The Voice and Voice Therapy. (9th Ed.). Englewood Cliffs, Prentice-Hall, Inc. New Jersy . Colton, R. H, Casper, J. K. & Leonard, R. (2006). Understanding voice problems. Baltimore : Williams & Wilkins.