MXR envelope follower..?how works?

[Paul Perry]

Looking at a poor copy of the original 1979 MXR envelope filter ckt,
has a CD4016 quad switch and a CD4069 hex inverter.... and that's it!
Presume the switches are used to make switched resistors for the 
filter, with the pulse width modulated by the signal envelope.... but
I can't for the life of me see how any of it works.
Any advice, comments , gratefully received.
Also wonder how it sounds!
If any good, it is a miracle of minimalism, and cheap to boot.

paul perry melbourne australia