The author does envelope extraction using Hilbert transform before applying AMDF.
What is the advantage of doing that?
Is speech a narrowband or wideband signal? I read somewhere that hilbert envelope extraction works better for narrowband signals.
When I extracted envelope of a speech signal using Hilbert transform, in some places the envelope was almost the same as the original waveform(Figure 1), whereas in other places it tracked the upper portions of waveforms (Figure 2).
Red curve is the Hilbert envelope, blue curve is the speech waveform.