synthio: make sustain level relative to attack level

and re-vamp overall envelope calculation again.

Now, if you set a low overall attack level like 0.2 this avoids the
"diminishing volume" effect when many notes sound at once. You need
simply choose a maximum attack level that is appropriate for the max
number of voices that will actually be played.

shared-bindings/synthio/__init__.c

@@ -79,8 +79,8 @@ static const mp_arg_t envelope_properties[] = {
 //|         :param float attack_time: The time in seconds it takes to ramp from 0 volume to attack_volume
 //|         :param float decay_time: The time in seconds it takes to ramp from attack_volume to sustain_volume
 //|         :param float release_time: The time in seconds it takes to ramp from sustain_volume to release_volume. When a note is released before it has reached the sustain phase, the release is done with the same slope indicated by ``release_time`` and ``sustain_level``
-//|         :param float attack_level: The relative level, in the range ``0.0`` to ``1.0`` of the peak volume of the attack phase
+//|         :param float attack_level: The level, in the range ``0.0`` to ``1.0`` of the peak volume of the attack phase
-//|         :param float sustain_level: The relative level, in the range ``0.0`` to ``1.0`` of the volume of the sustain phase
+//|         :param float sustain_level: The level, in the range ``0.0`` to ``1.0`` of the volume of the sustain phase relative to the attack level
 //|         """
 //|     attack_time: float
 //|     """The time in seconds it takes to ramp from 0 volume to attack_volume"""
@@ -92,10 +92,10 @@ static const mp_arg_t envelope_properties[] = {
 //|     """The time in seconds it takes to ramp from sustain_volume to release_volume. When a note is released before it has reached the sustain phase, the release is done with the same slope indicated by ``release_time`` and ``sustain_level``"""
 //|
 //|     attack_level: float
-//|     """The relative level, in the range ``0.0`` to ``1.0`` of the peak volume of the attack phase"""
+//|     """The level, in the range ``0.0`` to ``1.0`` of the peak volume of the attack phase"""
 //|
 //|     sustain_level: float
-//|     """The relative level, in the range ``0.0`` to ``1.0`` of the volume of the sustain phase"""
+//|     """The level, in the range ``0.0`` to ``1.0`` of the volume of the sustain phase relative to the attack level"""
 //|
 
 STATIC mp_obj_t synthio_envelope_make_new(const mp_obj_type_t *type_in, size_t n_args, size_t n_kw, const mp_obj_t *all_args) {

shared-module/synthio/__init__.c

@@ -77,7 +77,7 @@ void synthio_envelope_definition_set(synthio_envelope_definition_t *envelope, mp
     mp_obj_tuple_get(obj, &len, &fields);
 
     envelope->attack_level = (int)(32767 * mp_obj_get_float(fields[3]));
-    envelope->sustain_level = (int)(32767 * mp_obj_get_float(fields[4]));
+    envelope->sustain_level = (int)(32767 * mp_obj_get_float(fields[4]) * mp_obj_get_float(fields[3]));
 
     envelope->attack_step = convert_time_to_rate(
         sample_rate, fields[0], envelope->attack_level);
@@ -152,10 +152,10 @@ STATIC uint32_t synthio_synth_sum_envelope(synthio_synth_t *synth) {
     for (int chan = 0; chan < CIRCUITPY_SYNTHIO_MAX_CHANNELS; chan++) {
         if (synth->span.note_obj[chan] != SYNTHIO_SILENCE) {
             synthio_envelope_state_t *state = &synth->envelope_state[chan];
-            if (state->state == SYNTHIO_ENVELOPE_STATE_RELEASE) {
+            if (state->state == SYNTHIO_ENVELOPE_STATE_ATTACK) {
-                result += synth->envelope_definition.sustain_level;
-            } else {
                 result += state->level;
+            } else {
+                result += synth->envelope_definition.attack_level;
             }
         }
     }