Allow variable mix down sample scale.

Author: relic-se

shared-module/audiodelays/Echo.c

@@ -428,7 +428,7 @@ audioio_get_buffer_result_t audiodelays_echo_get_buffer(audiodelays_echo_obj_t *
                     }
 
                     if (MP_LIKELY(self->bits_per_sample == 16)) {
-                        word = synthio_mix_down_sample(word);
+                        word = synthio_mix_down_sample(word, SYNTHIO_MIX_DOWN_SCALE(2));
                         if (self->freq_shift) {
                             for (uint32_t j = echo_buffer_pos >> 8; j < next_buffer_pos >> 8; j++) {
                                 echo_buffer[j % echo_buf_len] = (int16_t)word;
@@ -453,7 +453,7 @@ audioio_get_buffer_result_t audiodelays_echo_get_buffer(audiodelays_echo_obj_t *
                     }
 
                     word = echo + sample_word;
-                    word = synthio_mix_down_sample(word);
+                    word = synthio_mix_down_sample(word, SYNTHIO_MIX_DOWN_SCALE(2));
 
                     if (MP_LIKELY(self->bits_per_sample == 16)) {
                         word_buffer[i] = (int16_t)((sample_word * (MICROPY_FLOAT_CONST(1.0) - mix)) + (word * mix));

shared-module/audiofilters/Filter.c

@@ -315,7 +315,7 @@ audioio_get_buffer_result_t audiofilters_filter_get_buffer(audiofilters_filter_o
                     // Mix processed signal with original sample and transfer to output buffer
                     for (uint32_t j = 0; j < n_samples; j++) {
                         if (MP_LIKELY(self->bits_per_sample == 16)) {
-                            word_buffer[i + j] = synthio_mix_down_sample((int32_t)((sample_src[i + j] * (MICROPY_FLOAT_CONST(1.0) - mix)) + (self->filter_buffer[j] * mix)));
+                            word_buffer[i + j] = synthio_mix_down_sample((int32_t)((sample_src[i + j] * (MICROPY_FLOAT_CONST(1.0) - mix)) + (self->filter_buffer[j] * mix)), SYNTHIO_MIX_DOWN_SCALE(2));
                             if (!self->samples_signed) {
                                 word_buffer[i + j] ^= 0x8000;
                             }

shared-module/synthio/__init__.c

@@ -129,27 +129,24 @@ static synthio_envelope_definition_t *synthio_synth_get_note_envelope(synthio_sy
 }
 
 
-#define RANGE_LOW (-28000)
-#define RANGE_HIGH (28000)
 #define RANGE_SHIFT (16)
-#define RANGE_SCALE (0xfffffff / (32768 * CIRCUITPY_SYNTHIO_MAX_CHANNELS - RANGE_HIGH))
 
 // dynamic range compression via a downward compressor with hard knee
 //
 // When the output value is within the range +-28000 (about 85% of full scale),
 // it is unchanged. Otherwise, it undergoes a gain reduction so that the
-// largest possible values, (+32768,-32767) * CIRCUITPY_SYNTHIO_MAX_CHANNELS,
+// largest possible values, (+32768,-32767) * count,
 // still fit within the output range
 //
 // This produces a much louder overall volume with multiple voices, without
 // much additional processing.
 //
 // https://en.wikipedia.org/wiki/Dynamic_range_compression
-int16_t synthio_mix_down_sample(int32_t sample) {
-    if (sample < RANGE_LOW) {
-        sample = (((sample - RANGE_LOW) * RANGE_SCALE) >> RANGE_SHIFT) + RANGE_LOW;
-    } else if (sample > RANGE_HIGH) {
-        sample = (((sample - RANGE_HIGH) * RANGE_SCALE) >> RANGE_SHIFT) + RANGE_HIGH;
+int16_t synthio_mix_down_sample(int32_t sample, int32_t scale) {
+    if (sample < SYNTHIO_MIX_DOWN_RANGE_LOW) {
+        sample = (((sample - SYNTHIO_MIX_DOWN_RANGE_LOW) * scale) >> RANGE_SHIFT) + SYNTHIO_MIX_DOWN_RANGE_LOW;
+    } else if (sample > SYNTHIO_MIX_DOWN_RANGE_HIGH) {
+        sample = (((sample - SYNTHIO_MIX_DOWN_RANGE_HIGH) * scale) >> RANGE_SHIFT) + SYNTHIO_MIX_DOWN_RANGE_HIGH;
     }
     return sample;
 }
@@ -344,7 +341,7 @@ void synthio_synth_synthesize(synthio_synth_t *synth, uint8_t **bufptr, uint32_t
     // mix down audio
     for (size_t i = 0; i < dur * synth->channel_count; i++) {
         int32_t sample = out_buffer32[i];
-        out_buffer16[i] = synthio_mix_down_sample(sample);
+        out_buffer16[i] = synthio_mix_down_sample(sample, SYNTHIO_MIX_DOWN_SCALE(CIRCUITPY_SYNTHIO_MAX_CHANNELS));
     }
 
     // advance envelope states

shared-module/synthio/__init__.h

@@ -14,6 +14,10 @@
 #define SYNTHIO_NOTE_IS_PLAYING(synth, i) ((synth)->envelope_state[(i)].state != SYNTHIO_ENVELOPE_STATE_RELEASE)
 #define SYNTHIO_FREQUENCY_SHIFT (16)
 
+#define SYNTHIO_MIX_DOWN_RANGE_LOW (-28000)
+#define SYNTHIO_MIX_DOWN_RANGE_HIGH (28000)
+#define SYNTHIO_MIX_DOWN_SCALE(x) (0xfffffff / (32768 * x - SYNTHIO_MIX_DOWN_RANGE_HIGH))
+
 #include "shared-module/audiocore/__init__.h"
 #include "shared-bindings/synthio/__init__.h"
 
@@ -79,7 +83,7 @@ bool synthio_span_change_note(synthio_synth_t *synth, mp_obj_t old_note, mp_obj_
 void synthio_envelope_step(synthio_envelope_definition_t *definition, synthio_envelope_state_t *state, int n_samples);
 void synthio_envelope_definition_set(synthio_envelope_definition_t *envelope, mp_obj_t obj, uint32_t sample_rate);
 
-int16_t synthio_mix_down_sample(int32_t sample);
+int16_t synthio_mix_down_sample(int32_t sample, int32_t scale);
 
 uint64_t synthio_frequency_convert_float_to_scaled(mp_float_t frequency_hz);
 uint32_t synthio_frequency_convert_float_to_dds(mp_float_t frequency_hz, int32_t sample_rate);