is just a sawtooth into a filter. The filter is the "MoogDirty" which is a Moog-style filter (no connection with the firms of that name) with an overdrive parameter. The four knobs control, in order, 1) cutoff, 2) resonance, 3) drive, 4) 1/2/3/4-pole selection.
This filter is happy to self-resonate to extremely high levels, BUT as you increase drive it chokes back down. There are awesome sounds to be had at medium-high drive and medium-high resonance. At high drive with no resonance, it's not so interesting: more like a normal square wave.
The PURPLE trace on the oscilloscope shows which of the four outputs we're currently listening to. In order to keep the patch as simple as possible no anti-click was built into the patch.
Note the sound is recorded directly from Moselle: no hardware or external processing was used.