README.md
ADSR Envelope

This circuit operates by using the LM13700 as a variable current source charging
a capacitor. It is configured as a non-inverting op-amp charging the capacitor
to a reference voltage V_ref at the + pin with a slew rate limited by the current
control. The capacitor's voltage will change linearly based on I_abc until it settles
at V_ref. Then a bit of digital logic to create a 3-state state machine to select the
correct  V_ref and I_abc.



States:


Attack: V_gate=5V, V_ref=V_in, I_abc controlled by R_attack
Decay: V_gate=5V, V_ref=V_sustain, I_abc controlled by R_decay
Release: V_gate=0, V_ref=0, I_abc controlled by R_release


The Attack, Decay, and Release knobs control I_abc in each state,
while the Sustain knob divides down V_in to set V_ref in the
Decay state.

A comparator, capacitor, and a few transistors create cheap analog
a flip-flop that turns on when the output hits V_in (flipping from
the Attack state to the Decay state), and turns off when V_gate
turns off.

Voltage control of the parameters could be possible by exposing
access to I_abc, but it doesn't make much sense to me to expose
those parameters with any control other than a knob - it's not the
kind of thing you need to modulate automatically.

Choice of the capacitor and the knob resistance control the time
range. Large values for both mean a longer charge time, but a large
capacitance effects the fastest charge time too, since I_abc is
capped at 2mA.

TODO: power usage