blurb (from http://www.sdiy.org/pinky/data/arseq.html) - This circuit is quite simple but an excellent envelope generator.
It has a counter, four attack release generators (ARgen) and a mixer.
It was originally intended to be a sequencer with controllable slew on each stage,
but it only took an extra pot and two diodes on each stage to turn it into what it is now.
The ARgen circuit is pretty common, I have seen at least 6 variations of the circuit around the net.
Most likely it evolved from the old diode function generators of analogue computer days.
Each stage takes two clock signals, the 1st starts the attack slope.
If the slope is steep enough the output will reach the sustain value set by the level pot for that stage.
The next clock signal puts that stage into release mode and the output starts to drop.
The 3rd clock signal starts the attack on the next stage, if the previous is still 'releasing'
the signals from each stage will sum in the output mixer.
LEDs indicate the envelope of each stage, so more than one LED may be lit at once,
indicating the output signal is a mix of two or maybe more stages.