4x Decay Envelope Gen with 4 Trigger ins for 4 EG outs

From BMC website: "This is an all analog synthesizer module with four channels of envelope generation. Each envelope generator has a single input, output and decay knob. This is the same envelope generator circuit as used on BMC18. It's useful for creating percussive sounds."

http://www.bartonmusicalcircuits.com/4xd/