DR2 ENVELOPE GENERATOR
Envelopes are 'control signals' used on synthesizers to control the course of pitch, volume, filter frequencies, ... The DR2 uses a quite peculiar type of envelopes, all generated by a central envelope generator.
Envelopes used in drum synthesis aren't very complex. In synthesizers you most often see ADSR envelopes, or even more complex envelopes such as the Modor NF1's 4stage envelopes. In drum synthesis you need decayonly envelopes, a control value starting at high level (1) and dropping to low level (0). A drum sound starts loud, and drops to silence. It's pitch starts high, and drops to a low pitch. So, you don't need attack, sustain or release settings. Only a DECAY setting.
However, there's another very important property of envelopes used in drum sound synthesis: its curvature! Does the envelope drop to zero in a uniform, linear way? Or does it drop fast in the beginning, slowing down towards the end? The envelopes in the DR2 have, next to the DECAY rate, also a CURVE parameter. Behind the CURVE parameter of the DR2's envelope generators there are 5 different curvature types: linear, squared linear, exponential, squared reciprocal and reciprocal.
Linear full left, 8 o′clock setting of the CURVE knob: The linear envelope drops uniformly from 1 to 0. For the math freaks, it follows a y = (1 − ax) curve, a being a decay speed parameter 


Squared linear at 10 o′clock setting of the CURVE knob. In between the linear and exponential curve is the squared linear curve: y = (1 − ax)^{2} 

Exponential central, 12 o′clock setting: This is the most extensively used curve in drum synthesis, as it matches to many natural decay processes. It drops fast in the beginning, and slows down while dropping, theoretically never coming to an end. It follows a y = e^{−ax} curve 

Squared reciprocal 2 o′clock setting: In between the exponential and reciprocal curve is the squared reciprocal, 

Reciprocal Full right, 4 o′clock setting: The reciprocal curve also drops fast at the beginning, even faster then an exponential curve, but it slows down a lot earlier, resulting in a different curvature with a longer tail that never really seems to stop decaying. It follows a y = a/(x+a) curve 
Of course, there is a soft transition between these 5 types. With the CURVEknob at 11 o′clock, you'll get a mix of exponential and squared linear curvature, for example...
What does that sound like? In the first example below a simple bass drum sound gets a pitch envelope with exponential, linear and reciprocal curves. In the second example the same is done with the amp decay.
dr2pitchenv.mp3 // Exponential, Linear and Reciprocal pitch envelopes on a bassdrum...
dr2ampenv.mp3 // Exponential, Linear and Reciprocal amp envelopes on a bassdrum...