The electroglottograph is a device for measuring how much electricity flows across the larynx. Because the larynx conducts electricity better when the vocal folds are touching, the amplitude (voltage) of the EGG signal is positively correlated with how closed the vocal folds are. Refer to Alexis Michaud’s web site for general information about electroglottography.
Data collection with the EG2-PCX electroglottograph
The EGG has two batteries inside. It needs to be connected to its AC adapter for several hours to charge, but then the batteries last a long time. It is not meant to be powered directly from the AC adapter, so you need to make sure ahead of time that the batteries are charged. If turning the BATTERY switch to “A” or “B” makes the battery light turn green, it has power. A picture of our EGG machine is below in Figure 1.
The waveform produced by the electroglottograph is not technically an audio signal, but it has a lot in common with an audio signal. You can treat it as audio for the purposes for recording it, and then analyze it as an EGG signal. For recording purposes, the EGG signal and the audio signal are a stereo audio signal.
The EG2-PCX contains a microphone preamp and an A/D converter. It can interface with the device you are using as a recorder with an analog or digital connection. The simplest option is to connect it to a computer with a USB cable and record in Praat. This involves the following connections:
- Connect the electrodes to the ethernet-type jack labeled “ELECTODES”
- Connect a dynamic microphone to the small microphone jack on the front of the unit or the XLR jack on the back. You can use a condenser mic if it has its own power source. Set the “Mic Input” toggle switch on the back of the EG2-PCX to the input you want to use.
- Use a USB cable to connect the EG2-PCX to a computer. You will need to use the USB port if using the EGG and the ultrasound machine simultaneously.
Open up the “Recording devices” menu on the computer and make sure it recognizes the EG2-PCX as “Microphone/USB Audio Device” and that it is selected as default. You will probably need to click on the device’s “Properties” button, set the format to “2 channel, 16 bit, 44100 Hz) and adjust the levels. You can also control the signal levels using the switches on the EG2-PCX.
The EGG has four electrodes that are held against the larynx by a velcro collar. Make sure the electrodes are attached to the velcro so that the gaps between the electrodes run parallel to the velcro strap. Coat each of the four electrodes with a thin layer of gel, being careful not to get gel in between the paired electrodes (which would connect them to each other). Then place the velcro collar so that there is one set of electrodes on each side of the larynx (with wires pointing down) and secure it behind the neck with the velcro straps, as in Figure 1. You may need to move them closer together or farther apart on the velcro.
With the EGG box powered on, watch the green, yellow, and red LEDs labeled ELECTRODE PLACEMENT, which will tell you whether the electrodes are too high or too low against the speaker’s larynx, and adjust the electrodes until one of the green LEDs in the middle is lit when the speaker is at rest.
When the speaker produces a vowel, the LEDs labeled SIGNAL STRENGTH should light up, showing the signal strength, and the ELECTRODE PLACEMENT LEDs, which double as LARYNGEAL MOVEMENT LEDs, should reflect laryngeal movement.
Open praat and click “New… Record stereo Sound…”. Click “Record” and have your speaker speak naturally. You may need to adjust the record level some more (in the audio control panel or on the EG2-PCX) in order to get both volume indicators to peak high in the green range but not hit the red range.
If you are satisfied with the record level for both channels, record some speech. Save the recording to the object list and then inspect it. Channel 1 should look like a normal audio recording and Channel 2 should look like a different kind of waveform.
Working with EGG data
Useful measurements can be made from the first derivative of the EGG signal (the DEGG signal). A function is a mapping between two sets of values. The derivative of a function represents the slope of the function, i.e., how the function is changing over time. The EGG signal is a function expressing the closedness of the glottis over time: its largest values occur at times when the vocal folds are closed the tightest and its smallest values occur at times when the vocal folds are open the widest. DEGG is calulated by measuring the slope of the EGG waveform at every point in time. Its largest values occur when the EGG signal is increasing the fastest (the moment when the vocal folds begin to touch) and its smallest values occur when the EGG signal is decreasing the fastest (the moment when the vocal folds begin to separate). A nice illustration of the relationship betwen EGG, DEGG, and glottal opening and closing found here.
The Open Quotient (OQ) is how much time the glottis is open during voicing. The Closed Quotient (CQ) is how much time the glottis is closed during voicing. At any point in time, OQ + CQ = 1. One pitch period is the time interval between two peaks or two troughs in the DEGG signal (and also in the EGG signal). To find the Closed Quotient, we measure the time between a DEGG peak and the following DEGG minimum, and divide it by the time between two peaks. To find the Open Quotient, we measure the time between a DEGG minimum and the following DEGG peak, and divide it by the time between two peaks. It’s best to measure several pitch periods in the DEGG and average them for a more accurate open/closed quotient.
Three scripts are available for working with EGG recordings:
- EGG_and_DEGG_script.praat (made by Marc Brunelle) calculates DEGG and combines EGG and DEGG into a stereo recording. To use the script, first extract the EGG channel from your stereo recording (Convert… Extract one channel… 2) and rename it “EGG”, and the run the script.
- EGG_DEGG7.praat is a modified version of EGG_and_DEGG_script.praat. It does basically the same thing, but instead of extracting a channel and renaming it, you select the original stereo file. It smooths the EGG and DEGG signals by averaging across samples within a short window, and averages across periods of the DEGG signal, to improve signal-to-noise ratio. Also calculates the AEGG (acceleration) which may be useful for selecting the opening peak that you want.
- OQ_Praat_JM.zip for semi-automatically measuring open quotients from a DEGG signal. A zip file that contains the praat script as well as sample input files.