For anyone interested, here is how I decided to resolve this for the
lmms-lv2 port:
All three envelopes (vol, cut, res) behave the same: attack controls how
long the envelope takes to reach 100% value, where it says for the length
of hold (or until the note is released). The sustain amount is now a
percentage of the maximum value instead of (1-max) as in LMMS. Thus, decay
is how long it takes to reach the sustain level. Finally, release is how
long it takes for the envelope to reach zero from the current value --
regardless of the current state of the envelope. This provides a continuous
function, and is in-line with all the descriptions of an AHDSR envelope
that I've read. Note, channel (instrument) volume is not a factor here,
that is performed AFTER mixing all the instrument's notes together.
The big mystery in LMMS is how this envelope level (assume the output is
between 0 and 1) maps to instrument parameters. For this, we let amt be the
modulation amount (between -1 and 1). lfo_volume is the value of the volume
LFO (between -1 and 1). Also, I use (x^2) as a shorthand for (|x| + x),
that is: x squared, while maintaining the original sign.
envelope_volume = envelope_level * amt + (1-amt), if amt >= 0
envelope_volume = envelope_level * amt + 1, otherwise
volume = (envelope_volume * lfo_volume)^2, if lfo_modulate_env
volume = (envelope_volume + lfo_volume)^2, otherwise
LMMS, thus maps the envelope value to a velocity quadratically instead of
linear or exponentially. Perhaps due to the perceived volume difference?
Also, the volume envelope "Amount" knob is the ratio of enveloped to
non-enveloped signal -- not the maximum volume. This seems fine to me,
although I might adjust the quadratic mapping to something more musical
such as dB? The cutoff and resonance envelopes do not perform the same
"mixing" operation. Thus, the amt value is actually the magnitude of the
envelope:
envelope_cut = envelope_level * amt
cut = knob_cut + (envelope_cut * lfo_cut)^2 * 6000.0, if lfo_modulate_env
cut = knob_cut + (envelope_cut + lfo_cut)^2 * 6000.0, otherwise
The resonance envelope and LFO behave the same as the cutoff, but is scaled
to 2.0 instead of 6000.0. I personally feel this whole system is adequate
and fairly straight-forward. However, I will try to replace as many of
these magic [0..1] or [-1..1] parameters with proper values and units.
Seconds for attack, hold, decay, release. Percentage for sustain. Perhaps
dB for volume amount and Hz for cutoff -- although that may not be possible
due to the dynamic nature of those two knobs.
-- Paul
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