
Calculates the derivative of the Electroglottogram.
The settings and the algorithm are explained in Sound: To Sound (derivative)...
The derivative of the Electroglottogram is often indicated as dEGG or DEGG. Henrich et al. (2004) used the peaks in the derivative to find the glottal closure instants and sometimes also the glottal opening instants. However, in their paper and also in other papers like, for example, Herbst et al. (2014), the derivative they use is not the exact derivative as calculated in the way explained above. Instead they calculate an approximation of the derivative by taking either the first difference, (dx(t)/dt)[i] = (x[i]  x[i1])/Δt, or by taking the first central difference, (dx(t)/dt)[i] = (x[i+1]  x[i1])/(2Δt).
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