[sdiy] what is the amplitude envelope of a signal

[Czech Martin]

Following the (long, excuse me) e-mail about my FS experiments
you can see that I have some little signal processing tools.
But one: envelope follower.

I thought that programming this was an easy thing, everbody
knows what the amplitude envelope of a signal is, and the
methods for analog circuits are well known (rectifier, filter,
bleeder).

Unfortunately it turns out that extracting the amplitude envelope
of an audio signal is not so easy.
I used speech as sample signal, and all my experiments showed
that considerable feedthrough of the original speech into
the envelope signal was present.

Which led me to think more about these envelpes.
If I think about them, I feel I know them.
If I try to explain I do not know them (stolen from Augustinus).

What the heck is the amplitude envelope of an audio signal?
(please give a formal description, i.e. something so precise
that programming would be easy).

m.c.