synthio: support audio outputs that need double buffering

closes #7837

tested on rp2040 pico w on pico dv shield

Author: jepler

shared-module/synthio/MidiTrack.c

@@ -156,7 +156,7 @@ uint8_t common_hal_synthio_miditrack_get_channel_count(synthio_miditrack_obj_t *
 
 void synthio_miditrack_reset_buffer(synthio_miditrack_obj_t *self,
     bool single_channel_output, uint8_t channel) {
-
+    synthio_synth_reset_buffer(&self->synth, single_channel_output, channel);
     self->synth.span.dur = 0;
     self->next_span = 0;
 }
@@ -172,7 +172,7 @@ audioio_get_buffer_result_t synthio_miditrack_get_buffer(synthio_miditrack_obj_t
         self->synth.span = self->track[self->next_span++];
     }
 
-    synthio_synth_synthesize(&self->synth, buffer, buffer_length);
+    synthio_synth_synthesize(&self->synth, buffer, buffer_length, single_channel_output ? 0 : channel);
 
     return (self->synth.span.dur == 0 && self->next_span >= self->total_spans) ?
            GET_BUFFER_DONE : GET_BUFFER_MORE_DATA;

shared-module/synthio/Synthesizer.c

@@ -58,12 +58,13 @@ uint8_t common_hal_synthio_synthesizer_get_channel_count(synthio_synthesizer_obj
 
 void synthio_synthesizer_reset_buffer(synthio_synthesizer_obj_t *self,
     bool single_channel_output, uint8_t channel) {
+    synthio_synth_reset_buffer(&self->synth, single_channel_output, channel);
 }
 
 audioio_get_buffer_result_t synthio_synthesizer_get_buffer(synthio_synthesizer_obj_t *self,
     bool single_channel_output, uint8_t channel, uint8_t **buffer, uint32_t *buffer_length) {
     self->synth.span.dur = SYNTHIO_MAX_DUR;
-    synthio_synth_synthesize(&self->synth, buffer, buffer_length);
+    synthio_synth_synthesize(&self->synth, buffer, buffer_length, single_channel_output ? channel : 0);
     return GET_BUFFER_MORE_DATA;
 }
 

shared-module/synthio/__init__.c

@@ -44,16 +44,27 @@ int synthio_span_count_active_channels(synthio_midi_span_t *span) {
 }
 
 
-void synthio_synth_synthesize(synthio_synth_t *synth, uint8_t **buffer, uint32_t *buffer_length) {
+void synthio_synth_synthesize(synthio_synth_t *synth, uint8_t **bufptr, uint32_t *buffer_length, uint8_t channel) {
+
+    if (channel == synth->other_channel) {
+        *buffer_length = synth->last_buffer_length;
+        *bufptr = (uint8_t *)(synth->buffers[synth->other_buffer_index] + channel);
+        return;
+    }
+
+    synth->buffer_index = !synth->buffer_index;
+    synth->other_channel = 1 - channel;
+    synth->other_buffer_index = synth->buffer_index;
+    int16_t *out_buffer = (int16_t *)(void *)synth->buffers[synth->buffer_index];
+
     uint16_t dur = MIN(SYNTHIO_MAX_DUR, synth->span.dur);
     synth->span.dur -= dur;
-    memset(synth->buffer, 0, synth->buffer_length);
+    memset(out_buffer, 0, synth->buffer_length);
 
     int32_t sample_rate = synth->sample_rate;
     int active_channels = synthio_span_count_active_channels(&synth->span);
     const int16_t *waveform = synth->waveform;
     uint32_t waveform_length = synth->waveform_length;
-    int16_t *out_buffer = synth->buffer;
     if (active_channels) {
         int16_t loudness = 0x3fff / (1 + active_channels);
         for (int chan = 0; chan < CIRCUITPY_SYNTHIO_MAX_CHANNELS; chan++) {
@@ -84,27 +95,38 @@ void synthio_synth_synthesize(synthio_synth_t *synth, uint8_t **buffer, uint32_t
         }
     }
 
-    *buffer_length = dur * SYNTHIO_BYTES_PER_SAMPLE;
-    *buffer = (uint8_t *)synth->buffer;
+    *buffer_length = synth->last_buffer_length = dur * SYNTHIO_BYTES_PER_SAMPLE;
+    *bufptr = (uint8_t *)out_buffer;
+}
+
+void synthio_synth_reset_buffer(synthio_synth_t *synth, bool single_channel_output, uint8_t channel) {
+    if (single_channel_output && channel == 1) {
+        return;
+    }
+    synth->other_channel = -1;
 }
 
 bool synthio_synth_deinited(synthio_synth_t *synth) {
-    return synth->buffer == NULL;
+    return synth->buffers[0] == NULL;
 }
 
 void synthio_synth_deinit(synthio_synth_t *synth) {
-    m_del(uint8_t, synth->buffer, synth->buffer_length);
-    synth->buffer = NULL;
+    m_del(uint8_t, synth->buffers[0], synth->buffer_length);
+    m_del(uint8_t, synth->buffers[1], synth->buffer_length);
+    synth->buffers[0] = NULL;
+    synth->buffers[1] = NULL;
 }
 
 void synthio_synth_init(synthio_synth_t *synth, uint16_t max_dur) {
     synth->buffer_length = MIN(SYNTHIO_MAX_DUR, max_dur) * SYNTHIO_BYTES_PER_SAMPLE;
-    synth->buffer = m_malloc(synth->buffer_length, false);
+    synth->buffers[0] = m_malloc(synth->buffer_length, false);
+    synth->buffers[1] = m_malloc(synth->buffer_length, false);
+    synth->other_channel = -1;
 }
 
 void synthio_synth_get_buffer_structure(synthio_synth_t *synth, bool single_channel_output,
     bool *single_buffer, bool *samples_signed, uint32_t *max_buffer_length, uint8_t *spacing) {
-    *single_buffer = true;
+    *single_buffer = false;
     *samples_signed = true;
     *max_buffer_length = synth->buffer_length;
     *spacing = 1;

shared-module/synthio/__init__.h

@@ -28,7 +28,7 @@
 
 #define SYNTHIO_BITS_PER_SAMPLE (16)
 #define SYNTHIO_BYTES_PER_SAMPLE (SYNTHIO_BITS_PER_SAMPLE / 8)
-#define SYNTHIO_MAX_DUR (512)
+#define SYNTHIO_MAX_DUR (256)
 #define SYNTHIO_SILENCE (0x80)
 
 #include "shared-module/audiocore/__init__.h"
@@ -40,21 +40,25 @@ typedef struct {
 
 typedef struct {
     uint32_t sample_rate;
-    int16_t *buffer;
+    int16_t *buffers[2];
     const int16_t *waveform;
     uint16_t buffer_length;
+    uint16_t last_buffer_length;
+    uint8_t other_channel, buffer_index, other_buffer_index;
     uint16_t waveform_length;
     synthio_midi_span_t span;
     uint32_t accum[CIRCUITPY_SYNTHIO_MAX_CHANNELS];
 } synthio_synth_t;
 
 void synthio_span_init(synthio_midi_span_t *span);
-void synthio_synth_synthesize(synthio_synth_t *synth, uint8_t **buffer, uint32_t *buffer_length);
+void synthio_synth_synthesize(synthio_synth_t *synth, uint8_t **buffer, uint32_t *buffer_length, uint8_t channel);
 void synthio_synth_deinit(synthio_synth_t *synth);
 bool synthio_synth_deinited(synthio_synth_t *synth);
 void synthio_synth_init(synthio_synth_t *synth, uint16_t max_dur);
 void synthio_synth_get_buffer_structure(synthio_synth_t *synth, bool single_channel_output,
     bool *single_buffer, bool *samples_signed, uint32_t *max_buffer_length, uint8_t *spacing);
+void synthio_synth_reset_buffer(synthio_synth_t *synth, bool single_channel_output, uint8_t channel);
 void synthio_synth_parse_waveform(mp_buffer_info_t *bufinfo_waveform, mp_obj_t waveform_obj);
+
 bool synthio_span_change_note(synthio_midi_span_t *span, uint8_t old_note, uint8_t new_note);
 int synthio_span_count_active_channels(synthio_midi_span_t *span);