Envelope Generators are used to create individual events that begin, continue, and end in a predictable way. The Odyssey's two envelopes were used to shape changes in pitch, pulse width, filter cutoff, and volume, and ODC 2800 builds on this with envelopes that offer more control and routing flexibility than the original.
The Odyssey differentiated its two envelopes by their number of stages. It had one AR (Attack/Release) envelope and one ADSR (Attack/Decay/Sustain/Release) envelope. ODC 2800's two Envelope Generators are both ADSRs, with sliders for:
Attack: The time it takes from pressing the key to reaching peak level. Because so much of a sound's timbre relies on the transient that begins the note, setting Attack to be very fast (like a drum hit) or slow (like bowing a string) does a lot to define "what kind of a sound" is being played.
Decay: The time it takes from reaching peak level to drop to the Sustain level. If the Sustain level is set lower than the peak level, the Decay time is the "other half" of the transient; short Decays can simulate sharp sounds like claves, and longer Decays suggest the ringing of a drum head or an undamped plucked string.
Sustain: The level at which the sound sustains after the Decay is over, set as a percentage of the peak level. 100% eliminates the level change in the Decay, and 0% prevents the sound from sustaining.
Release: The time it takes from releasing the key until the envelope returns to zero.
Attack, Decay, and Release all have a time range of 5 ms to 4000 ms (4 seconds).
Vel- The original Odyssey's keyboard wasn't dynamically sensitive, but ODC 2800 adds the ability for envelopes to respond to MIDI Velocity messages for each key played. This can be set from 0% (no sensitivity) to 100% (very dynamic sensitivity).
Below the sliders are switches to determine how each envelope is triggered.
Each ADSR can respond to KYBD GATE (playing a note on the keyboard, the usual way you play a synth) or LFO 1 REPEAT, where envelopes are triggered by LFO 1.
The Repeat function can be set to KYBD REPEAT, where the retriggering envelopes are only activated while at least one key is held down, or AUTO REPEAT, where the envelopes retrigger constantly and playing notes simply changes their pitch.
Note that the LFO 1 settings apply to how LFO 1 Repeat affects playback, especially in Poly mode. For example, setting the LFO to Mono will cause the envelopes of all notes held down to retrigger at the same time, whereas setting it to Poly will have each envelope retrigger on its own, out of sync with the others. Experiment with the Voices set to 4, 8, or 16, and hear how repeating notes respond to play (using voice stealing to stop some notes while starting others). You'll find that while 16 might get a little crazy, you can quickly get great results with 4.
Drift
In between the two sets of ADSR sliders, you'll find a set of three small Drift trimpots.
These controls allow you to add a bit of drift – an instability in voltage output over time, which was part of why vintage analog synths had a "living, breathing" character to them. The controls are:
VCO DRIFT: Adds pitch variation to the VCOs, ranging from 0 to 20.00 cents. This can "thicken" polyphonic playing, as well as providing slightly different tuning to subsequent notes. While it's tempting to grab two-fisted chords with VCO Drift turned all the way up, more subtle settings will lend themselves to melodies and simpler chords/intervals.
VCF DRIFT: Adds variability to the keyboard tracking of the VCF, from 0 to 20.00 cents. This is especially noticeable when using keyboard tracking on the VCF when its resonance is self-oscillating, but can also add small changes in brightness for each note, a much more subtle effect than VCO Drift.
EG DRIFT: Controls the drift in Envelope Generator times, making each note slightly different from others in terms of its envelope response.