synthio: allow negative amplitudes

Previously, negative amplitudes were clamped to zero.
Now, they are allowed to range from -ALMOST_ONE to +ALMOST_ONE.
This is useful in certain circumstances, such as using synthio
to create CV-like outputs that can be positive or negative, by
using the amplitude property of the note.

shared-module/synthio/Note.c

@@ -214,7 +214,7 @@ STATIC uint32_t pitch_bend(uint32_t frequency_scaled, int32_t bend_value) {
 #define ONE MICROPY_FLOAT_CONST(1.)
 #define ALMOST_ONE (MICROPY_FLOAT_CONST(32767.) / 32768)
 
-uint32_t synthio_note_step(synthio_note_obj_t *self, int32_t sample_rate, int16_t dur, uint16_t loudness[2]) {
+uint32_t synthio_note_step(synthio_note_obj_t *self, int32_t sample_rate, int16_t dur, int16_t loudness[2]) {
     int panning = synthio_block_slot_get_scaled(&self->panning, -ALMOST_ONE, ALMOST_ONE);
     int left_panning_scaled, right_panning_scaled;
     if (panning >= 0) {
@@ -225,7 +225,7 @@ uint32_t synthio_note_step(synthio_note_obj_t *self, int32_t sample_rate, int16_
         left_panning_scaled = 32767 + panning;
     }
 
-    int amplitude = synthio_block_slot_get_scaled(&self->amplitude, ZERO, ALMOST_ONE);
+    int amplitude = synthio_block_slot_get_scaled(&self->amplitude, -ALMOST_ONE, ALMOST_ONE);
     left_panning_scaled = (left_panning_scaled * amplitude) >> 15;
     right_panning_scaled = (right_panning_scaled * amplitude) >> 15;
     loudness[0] = (loudness[0] * left_panning_scaled) >> 15;

shared-module/synthio/Note.h

@@ -55,6 +55,6 @@ typedef struct synthio_note_obj {
 } synthio_note_obj_t;
 
 void synthio_note_recalculate(synthio_note_obj_t *self, int32_t sample_rate);
-uint32_t synthio_note_step(synthio_note_obj_t *self, int32_t sample_rate, int16_t dur, uint16_t loudness[2]);
+uint32_t synthio_note_step(synthio_note_obj_t *self, int32_t sample_rate, int16_t dur, int16_t loudness[2]);
 void synthio_note_start(synthio_note_obj_t *self, int32_t sample_rate);
 bool synthio_note_playing(synthio_note_obj_t *self);

shared-module/synthio/__init__.c

@@ -172,7 +172,7 @@ int16_t mix_down_sample(int32_t sample) {
     return sample;
 }
 
-static bool synth_note_into_buffer(synthio_synth_t *synth, int chan, int32_t *out_buffer32, int16_t dur, uint16_t loudness[2]) {
+static bool synth_note_into_buffer(synthio_synth_t *synth, int chan, int32_t *out_buffer32, int16_t dur, int16_t loudness[2]) {
     mp_obj_t note_obj = synth->span.note_obj[chan];
 
     int32_t sample_rate = synth->sample_rate;
@@ -298,7 +298,7 @@ STATIC mp_obj_t synthio_synth_get_note_filter(mp_obj_t note_obj) {
     return mp_const_none;
 }
 
-STATIC void sum_with_loudness(int32_t *out_buffer32, int32_t *tmp_buffer32, uint16_t loudness[2], size_t dur, int synth_chan) {
+STATIC void sum_with_loudness(int32_t *out_buffer32, int32_t *tmp_buffer32, int16_t loudness[2], size_t dur, int synth_chan) {
     if (synth_chan == 1) {
         for (size_t i = 0; i < dur; i++) {
             *out_buffer32++ += (*tmp_buffer32++ *loudness[0]) >> 16;
@@ -344,7 +344,7 @@ void synthio_synth_synthesize(synthio_synth_t *synth, uint8_t **bufptr, uint32_t
             continue;
         }
 
-        uint16_t loudness[2] = {synth->envelope_state[chan].level, synth->envelope_state[chan].level};
+        int16_t loudness[2] = {synth->envelope_state[chan].level, synth->envelope_state[chan].level};
 
         if (!synth_note_into_buffer(synth, chan, tmp_buffer32, dur, loudness)) {
             // for some other reason, such as being above nyquist, note

tests/circuitpython/synth_note_amplitude.py

@@ -3,8 +3,8 @@ from synthnotehelper import *
 
 @synth_test
 def gen(synth):
-    l = LFO(bend_out, offset=0.2, scale=0.8, rate=4, once=True)
+    l = LFO(sine, offset=0.2, scale=0.8, rate=2)
     yield [l]
-    n = Note(128, amplitude=l)
+    n = Note(8, amplitude=l)
     synth.press(n)
-    yield 1 / 4
+    yield 2