Yamaha DX21 - Manual (Page 29)

Each of the 4 operators available in DX21 can be programmed with its own envelope.
The envelope applied to a carrier will, generally, contribute to the overall volume
envelope of the note, while an envelope applied to a modulator will contribute to
the timbre envelope of the note.
Here is a copy of the envelope diagram printed above the LCD on the DX21 panel.
This envelope diagram can be used as a guide in visualizing DX21 envelope settings
while you program or edit a voice.
Each envelope generator can be programmed with five different parameters: AT-
TACK RATE (AR), DECAY 1 RATE (D1R), DECAY 1 LEVEL (D1L), DECAY 2 RATE
(D2R), and RELEASE RATE (RR). The RATE parameters determine how fast the
envelope moves from one level to the next. The term LEVEL is used rather than
“volume” because the envelope of the operator you're working on could affect
volume or timbre, depending on whether it is a carrier or a modulator.
Any note begins at zero level when you press a key, then begins to approach
maximum EG level at a rate determined by the setting of AR (Attack Rate). The
envelope may reach maximum level instantly, or it may take over 30 seconds de-
pending on the setting of AR.
When the envelope reaches maximum level it immediately begins moving towards
the next level in the envelope—D1L (Decay 1 Level)—at a speed determined by the
setting of D1R (Decay 1 Rate).
The change from maximum EG level to D1R can be either a decrease in level or a
sustain at maximum level, depending on the values you choose for D1L.
After reaching D1L, the envelope then begins to decay toward zero level at a speed
determined by the setting of the D2R (Decay 2 Rate) parameter. If D2R is set to
0 (no decay), however, the note will be sustained at D1L for as long as you hold
the key. Now, when you release the key you have been holding the envelope will
immediately begin to decay towards 0 level at a speed determined by RR (Release
Rate). In fact, at whatever point in the envelope you release the key the envelope
will immediately begin moving toward 0 level at the set Release Rate. Thus ends
the note envelope “history”.
THE PITCH EG
In addition to the EG functions just described, the DX21 also has a "PITCH EG".
This pitch envelope generator is programmed in a similar way to the individual
operator EGs, but it affects the overall pitch of the voice rather than level and timbre.
This is useful for creating certain types of voices-whistling, for example, in which
the pitch of the note usually starts out a bit low and then sweeps up to the proper
pitch.
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