This is a slew rate limiter which allows the rise and fall times to be changed independently. The top pot controls rise time, the bottom one controls fall.
The actual slew rate limiter acts as an inverter, so the first opamp is mainly just an inverter to fix that. The next opamp is a simple comparator, and the final opamp is an integrator. The comparator and integrator work together to limit the slew rate, and the connection between them has current running through a different pot for each direction via the diodes.
I've tuned the capacitor and pots so that they could be used for a wide range of applications, from simple portamento to extreme envelope/peak detection.
