A review of methods used for the analysis of the electroglottographic signal
Methods relying on the derivative of the electroglottographic signal (DEGG signal)
1. Detection of the peaks by applying thresholds to the DEGG
signal
These methods consist in detecting the maxima located above a given
positive
threshold and the minima located below a given negative threshold,
as
illustrated
in case (d) in the following Figure.
This
threshold
can be
chosen as a percentage of the maximum (respectively minimum) amplitude
of the
windowed
signal.
Several options are open for peak detection. For instance, one may look
for the
local minimum in-between consecutive glottal closing instants, or
opening peaks
may be detected over the whole of the windowed signal and then matched
with closing
peaks.
2. Correlation-based method
The DECOM (DEgg Correlation-based Open quotient Measurement; Henrich
et al., 2004) method is applied
to a four-period windowed DEGG signal which is separated
into
two
parts:
its
positive
part, which shows strong peaks related to glottal
closing instants, and
its negative part, which shows weaker peaks related to glottal opening
instants.
The fundamental period duration is derived from the autocorrelation
function
calculated
on the
positive part of the DEGG signal. The open time is derived from the
intercorrelation
function calculated
between the positive part and the negative part.
Threshold methods applied directly to the EGG signal
In the literature, the main electroglottographic-based methods
for detecting the glottal closing and opening instants use the EGG
signal. As illustrated by cases (a) and (b) in the following Figure, a
threshold method is generally applied (Rothenberg and Mahshie, 1988): a
level line is chosen either as a percentage of the amplitude between
the minimum and maximum of the signal over a glottal period (50 % for a
normal to pressed voice and 35 % for a relaxed voice).
|
Illustration of 4 methods for
measuring the open quotient (or its equivalent, the closed quotient) on
an EGG signal and its derivative: a) on the EGG signal using a
threshold detection (35 % of the difference between the minimum and
maximum value of the signal over a glottal period); b) same method as
a) with a threshold of 50 % ; c) in combining an EGG-based threshold
method (3/7) with a detection of glottal closing instants on the DEGG
signal; d) on the DEGG signal by the detection of opening and closing
peaks. From Henrich et al., 2004. |
|
The crossing points between the level line and the EGG signal
are taken as approximations of the instants of glottal closing and
opening and these are used for the open quotient measurement. These
methods are very convenient for medical purposes, as they are robust
and can be applied even on noisy or weak signals. Nevertheless, the
results of such methods are by their nature imprecise and can be
inaccurate, as compared to what would be measured on a glottal area
signal or on a glottal flow signal (Rothenberg and Mahshie, 1988).
A method combining detection of closing instants on the DEGG
signal and opening instants on the EGG signal
An improvement over EGG threshold methods has been proposed by
Howard (1995); Howard et al. (1990): using the DEGG closing peak for
detecting the glottal closing instant, and an EGG-based threshold
method for detecting the glottal opening instant. This is illustrated
by case (c) in the Figure above.
[Note: the references are on the main page.]