synthio: slow ramp overall envelope back up

.. and account releasing notes at their sustain level until they're
done.

this ameliorates the effect where multiple releasing notes
don't seem to actually be releasing, but stay at a constant volume.

Author: jepler

shared-module/synthio/__init__.c

@@ -151,7 +151,12 @@ STATIC uint32_t synthio_synth_sum_envelope(synthio_synth_t *synth) {
     uint32_t result = 0;
     for (int chan = 0; chan < CIRCUITPY_SYNTHIO_MAX_CHANNELS; chan++) {
         if (synth->span.note_obj[chan] != SYNTHIO_SILENCE) {
-            result += synth->envelope_state[chan].level;
+            synthio_envelope_state_t *state = &synth->envelope_state[chan];
+            if (state->state == SYNTHIO_ENVELOPE_STATE_RELEASE) {
+                result += synth->envelope_definition.sustain_level;
+            } else {
+                result += state->level;
+            }
         }
     }
     return result;
@@ -177,25 +182,38 @@ void synthio_synth_synthesize(synthio_synth_t *synth, uint8_t **bufptr, uint32_t
 
     int32_t sample_rate = synth->sample_rate;
     uint32_t total_envelope = synthio_synth_sum_envelope(synth);
+    if (total_envelope < synth->total_envelope) {
+        // total envelope is decreasing. Slowly let remaining notes get louder
+        // the time constant is arbitrary, on the order of 1s at 48kHz
+        total_envelope = synth->total_envelope = (
+            total_envelope + synth->total_envelope * 255) / 256;
+    } else {
+        // total envelope is steady or increasing, so just store this as
+        // the high water mark
+        synth->total_envelope = total_envelope;
+    }
     if (total_envelope > 0) {
         uint16_t ovl_loudness = 0x7fffffff / MAX(0x8000, total_envelope);
+
         for (int chan = 0; chan < CIRCUITPY_SYNTHIO_MAX_CHANNELS; chan++) {
-            if (synth->span.note_obj[chan] == SYNTHIO_SILENCE) {
+            mp_obj_t note_obj = synth->span.note_obj[chan];
+            if (note_obj == SYNTHIO_SILENCE) {
                 synth->accum[chan] = 0;
                 continue;
             }
-            // adjust loudness by envelope
-            uint16_t loudness = (ovl_loudness * synth->envelope_state[chan].level) >> 16;
 
             if (synth->envelope_state[chan].level == 0) {
-                // note is truly finished
+                // note is truly finished, but we only just noticed
                 synth->span.note_obj[chan] = SYNTHIO_SILENCE;
+                continue;
             }
 
+            // adjust loudness by envelope
+            uint16_t loudness = (ovl_loudness * synth->envelope_state[chan].level) >> 16;
+
             uint32_t dds_rate;
             const int16_t *waveform = synth->waveform;
             uint32_t waveform_length = synth->waveform_length;
-            mp_obj_t note_obj = synth->span.note_obj[chan];
             if (mp_obj_is_small_int(note_obj)) {
                 uint8_t note = mp_obj_get_int(note_obj);
                 uint8_t octave = note / 12;

shared-module/synthio/__init__.h

@@ -62,6 +62,7 @@ typedef struct {
 
 typedef struct synthio_synth {
     uint32_t sample_rate;
+    uint32_t total_envelope;
     int16_t *buffers[2];
     const int16_t *waveform;
     uint16_t buffer_length;