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audio: apply mix and decay in fixed point, not floating point #11415

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3 changes: 2 additions & 1 deletion shared-module/audiodelays/Chorus.c
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Original file line number Diff line number Diff line change
Expand Up @@ -248,6 +248,7 @@ audioio_get_buffer_result_t audiodelays_chorus_get_buffer(audiodelays_chorus_obj
int32_t voices = (int32_t)MAX(synthio_block_slot_get(&self->voices), 1.0);
int32_t mix_down_scale = SYNTHIO_MIX_DOWN_SCALE(voices);
mp_float_t mix = synthio_block_slot_get_limited(&self->mix, MICROPY_FLOAT_CONST(0.0), MICROPY_FLOAT_CONST(1.0));
int32_t mix_scaled = (int32_t)(mix * MICROPY_FLOAT_CONST(32768.0));

mp_float_t f_delay_ms = synthio_block_slot_get(&self->delay_ms);
if (MICROPY_FLOAT_C_FUN(fabs)(self->current_delay_ms - f_delay_ms) >= self->sample_ms) {
Expand Down Expand Up @@ -311,7 +312,7 @@ audioio_get_buffer_result_t audiodelays_chorus_get_buffer(audiodelays_chorus_obj
}

// Add original sample + effect
word = sample_word + (int32_t)(word * mix);
word = sample_word + ((word * mix_scaled) >> 15);
word = synthio_mix_down_sample(word, 2);

if (MP_LIKELY(self->base.bits_per_sample == 16)) {
Expand Down
20 changes: 12 additions & 8 deletions shared-module/audiodelays/Echo.c
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Expand Up @@ -298,6 +298,11 @@ audioio_get_buffer_result_t audiodelays_echo_get_buffer(audiodelays_echo_obj_t *
mp_float_t mix = synthio_block_slot_get_limited(&self->mix, MICROPY_FLOAT_CONST(0.0), MICROPY_FLOAT_CONST(1.0)) * MICROPY_FLOAT_CONST(2.0);
mp_float_t decay = synthio_block_slot_get_limited(&self->decay, MICROPY_FLOAT_CONST(0.0), MICROPY_FLOAT_CONST(1.0));

int32_t decay_scaled = (int32_t)(decay * MICROPY_FLOAT_CONST(32768.0));
int32_t echo_scaled = (int32_t)(MIN(mix, MICROPY_FLOAT_CONST(1.0)) * MICROPY_FLOAT_CONST(32768.0));
int32_t sample_scaled = (int32_t)(MIN(MICROPY_FLOAT_CONST(2.0) - mix,
MICROPY_FLOAT_CONST(1.0)) * MICROPY_FLOAT_CONST(32768.0));

mp_float_t f_delay_ms = synthio_block_slot_get(&self->delay_ms);
if (MICROPY_FLOAT_C_FUN(fabs)(self->current_delay_ms - f_delay_ms) >= self->sample_ms) {
recalculate_delay(self, f_delay_ms);
Expand Down Expand Up @@ -340,16 +345,16 @@ audioio_get_buffer_result_t audiodelays_echo_get_buffer(audiodelays_echo_obj_t *
next_buffer_pos = echo_buffer_pos + self->echo_buffer_rate;

for (uint32_t j = echo_buffer_pos >> 8; j < next_buffer_pos >> 8; j++) {
word = (int16_t)(echo_buffer[(j % echo_buf_len) + echo_buffer_offset] * decay);
word = (int16_t)((echo_buffer[(j % echo_buf_len) + echo_buffer_offset] * decay_scaled) >> 15);

@FoamyGuy FoamyGuy Sep 22, 2026

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Suggested change
word = (int16_t)((echo_buffer[(j % echo_buf_len) + echo_buffer_offset] * decay_scaled) >> 15);
word = synthio_sat16(echo_buffer[(j % echo_buf_len) + echo_buffer_offset] * decay_scaled, 15);

echo_buffer[(j % echo_buf_len) + echo_buffer_offset] = (int16_t)audiofilters_process_filter_chain(&self->filter, self->base.channel_count, !!echo_buffer_offset, word);
}
} else {
echo = echo_buffer[echo_buffer_pos + echo_buffer_offset];
word = (int16_t)(echo * decay);
word = (int16_t)((echo * decay_scaled) >> 15);

@FoamyGuy FoamyGuy Sep 22, 2026

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Suggested change
word = (int16_t)((echo * decay_scaled) >> 15);
word = synthio_sat16(echo * decay_scaled, 15);

echo_buffer[echo_buffer_pos++ + echo_buffer_offset] = (int16_t)audiofilters_process_filter_chain(&self->filter, self->base.channel_count, !!echo_buffer_offset, word);
}

word = (int16_t)(echo * MIN(mix, MICROPY_FLOAT_CONST(1.0)));
word = (int16_t)((echo * echo_scaled) >> 15);

@FoamyGuy FoamyGuy Sep 22, 2026

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Suggested change
word = (int16_t)((echo * echo_scaled) >> 15);
word = synthio_sat16(echo * echo_scaled, 15);


if (MP_LIKELY(self->base.bits_per_sample == 16)) {
word_buffer[i] = word;
Expand Down Expand Up @@ -418,13 +423,13 @@ audioio_get_buffer_result_t audiodelays_echo_get_buffer(audiodelays_echo_obj_t *
next_buffer_pos = echo_buffer_pos + self->echo_buffer_rate;
} else {
echo = echo_buffer[echo_buffer_pos + echo_buffer_offset];
word = (int32_t)(echo * decay + sample_word);
word = ((echo * decay_scaled) >> 15) + sample_word;

@FoamyGuy FoamyGuy Sep 22, 2026

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Suggested change
word = ((echo * decay_scaled) >> 15) + sample_word;
synthio_sat16(echo * decay_scaled, 15) + sample_word;

}

if (MP_LIKELY(self->base.bits_per_sample == 16)) {
if (self->freq_shift) {
for (uint32_t j = echo_buffer_pos >> 8; j < next_buffer_pos >> 8; j++) {
word = (int32_t)(echo_buffer[(j % echo_buf_len) + echo_buffer_offset] * decay + sample_word);
word = ((echo_buffer[(j % echo_buf_len) + echo_buffer_offset] * decay_scaled) >> 15) + sample_word;

@FoamyGuy FoamyGuy Sep 22, 2026

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Suggested change
word = ((echo_buffer[(j % echo_buf_len) + echo_buffer_offset] * decay_scaled) >> 15) + sample_word;
word = synthio_sat16(echo_buffer[(j % echo_buf_len) + echo_buffer_offset] * decay_scaled, 15) + sample_word;

word = synthio_mix_down_sample(word, SYNTHIO_MIX_DOWN_SCALE(2));
echo_buffer[(j % echo_buf_len) + echo_buffer_offset] = (int16_t)audiofilters_process_filter_chain(&self->filter, self->base.channel_count, !!echo_buffer_offset, word);
}
Expand All @@ -435,7 +440,7 @@ audioio_get_buffer_result_t audiodelays_echo_get_buffer(audiodelays_echo_obj_t *
} else {
if (self->freq_shift) {
for (uint32_t j = echo_buffer_pos >> 8; j < next_buffer_pos >> 8; j++) {
word = (int32_t)(echo_buffer[(j % echo_buf_len) + echo_buffer_offset] * decay + sample_word);
word = ((echo_buffer[(j % echo_buf_len) + echo_buffer_offset] * decay_scaled) >> 15) + sample_word;

@FoamyGuy FoamyGuy Sep 22, 2026

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Suggested change
word = ((echo_buffer[(j % echo_buf_len) + echo_buffer_offset] * decay_scaled) >> 15) + sample_word;
word = synthio_sat16(echo_buffer[(j % echo_buf_len) + echo_buffer_offset] * decay_scaled, 15) + sample_word;

// Do not have mix_down for 8 bit so just hard cap samples into 1 byte
word = MIN(MAX(word, -128), 127);
echo_buffer[(j % echo_buf_len) + echo_buffer_offset] = (int8_t)audiofilters_process_filter_chain(&self->filter, self->base.channel_count, !!echo_buffer_offset, word);
Expand All @@ -447,8 +452,7 @@ audioio_get_buffer_result_t audiodelays_echo_get_buffer(audiodelays_echo_obj_t *
}
}

word = (int32_t)((sample_word * MIN(MICROPY_FLOAT_CONST(2.0) - mix, MICROPY_FLOAT_CONST(1.0)))
+ (echo * MIN(mix, MICROPY_FLOAT_CONST(1.0))));
word = ((sample_word * sample_scaled) >> 15) + ((echo * echo_scaled) >> 15);

@FoamyGuy FoamyGuy Sep 22, 2026

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This version shifts and truncates each separately before adding. It could instead sum first and then shift to stay more precise.

Suggested change
word = ((sample_word * sample_scaled) >> 15) + ((echo * echo_scaled) >> 15);
word = synthio_shift_round_to_zero(sample_word * sample_scaled + echo * echo_scaled, 15);

This is the helper function that could be added to synthio/init.h so it can be used here and a few other places.

static inline int32_t synthio_shift_round_to_zero(int32_t n, int rshift) {
    if (n < 0) {
        n += (1 << rshift) - 1;
    }
    return n >> rshift;
}

word = synthio_mix_down_sample(word, SYNTHIO_MIX_DOWN_SCALE(2));

if (MP_LIKELY(self->base.bits_per_sample == 16)) {
Expand Down
10 changes: 7 additions & 3 deletions shared-module/audiodelays/MultiTapDelay.c
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Expand Up @@ -374,6 +374,11 @@ audioio_get_buffer_result_t audiodelays_multi_tap_delay_get_buffer(audiodelays_m
mp_float_t mix = synthio_block_slot_get_limited(&self->mix, MICROPY_FLOAT_CONST(0.0), MICROPY_FLOAT_CONST(1.0)) * MICROPY_FLOAT_CONST(2.0);
mp_float_t decay = synthio_block_slot_get_limited(&self->decay, MICROPY_FLOAT_CONST(0.0), MICROPY_FLOAT_CONST(1.0));

int32_t decay_scaled = (int32_t)(decay * MICROPY_FLOAT_CONST(32768.0));
int32_t tap_scaled = (int32_t)(MIN(mix, MICROPY_FLOAT_CONST(1.0)) * MICROPY_FLOAT_CONST(32768.0));
int32_t sample_scaled = (int32_t)(MIN(MICROPY_FLOAT_CONST(2.0) - mix,
MICROPY_FLOAT_CONST(1.0)) * MICROPY_FLOAT_CONST(32768.0));

int16_t *sample_src = NULL;
int8_t *sample_hsrc = NULL;
if (self->sample != NULL) {
Expand Down Expand Up @@ -424,7 +429,7 @@ audioio_get_buffer_result_t audiodelays_multi_tap_delay_get_buffer(audiodelays_m
}

// Apply decay and add sample
delay_word = (int32_t)(delay_word * decay) + sample_word;
delay_word = ((delay_word * decay_scaled) >> 15) + sample_word;

@FoamyGuy FoamyGuy Sep 21, 2026 •
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This should use synthio_sat16(). The old code rounds toward zero, but the new code always goes toward -infinity. The helper should resolve it.

delay_word = synthio_sat16(delay_word * decay_scaled, 15) + sample_word;

@FoamyGuy FoamyGuy Sep 22, 2026

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Suggested change
delay_word = ((delay_word * decay_scaled) >> 15) + sample_word;
delay_word = synthio_sat16(delay_word * decay_scaled, 15) + sample_word;


if (MP_LIKELY(self->base.bits_per_sample == 16)) {
delay_word = synthio_mix_down_sample(delay_word, SYNTHIO_MIX_DOWN_SCALE(2));
Expand All @@ -436,8 +441,7 @@ audioio_get_buffer_result_t audiodelays_multi_tap_delay_get_buffer(audiodelays_m
}

// Mix sample with tap output
word = (int32_t)((sample_word * MIN(MICROPY_FLOAT_CONST(2.0) - mix, MICROPY_FLOAT_CONST(1.0)))
+ (word * MIN(mix, MICROPY_FLOAT_CONST(1.0))));
word = ((sample_word * sample_scaled) >> 15) + ((word * tap_scaled) >> 15);

@FoamyGuy FoamyGuy Sep 22, 2026

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Suggested change
word = ((sample_word * sample_scaled) >> 15) + ((word * tap_scaled) >> 15);
word = synthio_shift_round_to_zero(sample_word * sample_scaled + word * tap_scaled, 15);

word = synthio_mix_down_sample(word, SYNTHIO_MIX_DOWN_SCALE(2));

if (MP_LIKELY(self->base.bits_per_sample == 16)) {
Expand Down
9 changes: 6 additions & 3 deletions shared-module/audiofilters/Filter.c
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Expand Up @@ -195,6 +195,9 @@ audioio_get_buffer_result_t audiofilters_filter_get_buffer(audiofilters_filter_o
shared_bindings_synthio_lfo_tick(self->base.sample_rate, n / self->base.channel_count);
mp_float_t mix = synthio_block_slot_get_limited(&self->mix, MICROPY_FLOAT_CONST(0.0), MICROPY_FLOAT_CONST(1.0));

int32_t wet_scaled = (int32_t)(mix * MICROPY_FLOAT_CONST(32768.0));
int32_t dry_scaled = (int32_t)((MICROPY_FLOAT_CONST(1.0) - mix) * MICROPY_FLOAT_CONST(32768.0));

@FoamyGuy FoamyGuy Sep 22, 2026

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Suggested change
int32_t dry_scaled = (int32_t)((MICROPY_FLOAT_CONST(1.0) - mix) * MICROPY_FLOAT_CONST(32768.0));
int32_t dry_scaled = 32768 - wet_scaled;

Both wet and dry truncate toward zero. Could cause off by 1 error when they're summed.


if (mix <= MICROPY_FLOAT_CONST(0.01) || !self->filter.states) { // if mix is zero pure sample only or no biquad filter objects are provided
for (uint32_t i = 0; i < n; i++) {
if (MP_LIKELY(self->base.bits_per_sample == 16)) {
Expand Down Expand Up @@ -238,15 +241,15 @@ audioio_get_buffer_result_t audiofilters_filter_get_buffer(audiofilters_filter_o
bool buf_offset = (j % self->base.channel_count) == 1;
uint32_t k = j / self->base.channel_count;

@FoamyGuy FoamyGuy Sep 22, 2026

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This section has different behavior rounding always down instead of toward zero. It should use the new helper function.

Add here:

int32_t filtered_word = self->filter_buffer[k + SYNTHIO_MAX_DUR * buf_offset];

if (MP_LIKELY(self->base.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[k + SYNTHIO_MAX_DUR * buf_offset] * mix)), SYNTHIO_MIX_DOWN_SCALE(2));
word_buffer[i + j] = synthio_mix_down_sample(((sample_src[i + j] * dry_scaled) >> 15) + ((self->filter_buffer[k + SYNTHIO_MAX_DUR * buf_offset] * wet_scaled) >> 15), SYNTHIO_MIX_DOWN_SCALE(2));

@FoamyGuy FoamyGuy Sep 22, 2026

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Suggested change
word_buffer[i + j] = synthio_mix_down_sample(((sample_src[i + j] * dry_scaled) >> 15) + ((self->filter_buffer[k + SYNTHIO_MAX_DUR * buf_offset] * wet_scaled) >> 15), SYNTHIO_MIX_DOWN_SCALE(2));
word_buffer[i + j] = synthio_mix_down_sample(synthio_shift_round_to_zero(sample_src[i + j] * dry_scaled + filtered_word * wet_scaled, 15), SYNTHIO_MIX_DOWN_SCALE(2));

if (!self->base.samples_signed) {
word_buffer[i + j] ^= 0x8000;
}
} else {
if (self->base.samples_signed) {
hword_buffer[i + j] = (int8_t)((sample_hsrc[i + j] * (MICROPY_FLOAT_CONST(1.0) - mix)) + (self->filter_buffer[k + SYNTHIO_MAX_DUR * buf_offset] * mix));
hword_buffer[i + j] = (int8_t)(((sample_hsrc[i + j] * dry_scaled) >> 15) + ((self->filter_buffer[k + SYNTHIO_MAX_DUR * buf_offset] * wet_scaled) >> 15));

@FoamyGuy FoamyGuy Sep 22, 2026

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Suggested change
hword_buffer[i + j] = (int8_t)(((sample_hsrc[i + j] * dry_scaled) >> 15) + ((self->filter_buffer[k + SYNTHIO_MAX_DUR * buf_offset] * wet_scaled) >> 15));
hword_buffer[i + j] = (int8_t)synthio_shift_round_to_zero(sample_hsrc[i + j] * dry_scaled + filtered_word * wet_scaled, 15);

} else {
hword_buffer[i + j] = (uint8_t)(((int8_t)(((uint8_t)sample_hsrc[i + j]) ^ 0x80) * (MICROPY_FLOAT_CONST(1.0) - mix)) + (self->filter_buffer[k + SYNTHIO_MAX_DUR * buf_offset] * mix)) ^ 0x80;
hword_buffer[i + j] = (uint8_t)((((int8_t)(((uint8_t)sample_hsrc[i + j]) ^ 0x80) * dry_scaled) >> 15) + ((self->filter_buffer[k + SYNTHIO_MAX_DUR * buf_offset] * wet_scaled) >> 15)) ^ 0x80;

@FoamyGuy FoamyGuy Sep 22, 2026

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Suggested change
hword_buffer[i + j] = (uint8_t)((((int8_t)(((uint8_t)sample_hsrc[i + j]) ^ 0x80) * dry_scaled) >> 15) + ((self->filter_buffer[k + SYNTHIO_MAX_DUR * buf_offset] * wet_scaled) >> 15)) ^ 0x80;
hword_buffer[i + j] = (uint8_t)synthio_shift_round_to_zero((int8_t)(((uint8_t)sample_hsrc[i + j]) ^ 0x80) * dry_scaled + filtered_word * wet_scaled, 15) ^ 0x80;

}
}
}
Expand Down
25 changes: 12 additions & 13 deletions shared-module/audiomixer/Mixer.c
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Expand Up @@ -106,20 +106,19 @@ static inline uint32_t mult16signed(uint32_t val, int32_t lomul, int32_t himul)
__asm__ volatile ("pkhbt %0, %1, %2, lsl #16" : "=r" (val) : "r" (lo), "r" (hi)); // pack
return val;
#else
uint32_t result = 0;
for (int8_t i = 0; i < 2; i++) {
float mod_mul = (float)(i ? himul : lomul) / (float)((1 << 15) - 1);
int16_t ai = (val >> (sizeof(uint16_t) * 8 * i));
int32_t intermediate = (int32_t)(ai * mod_mul);
if (intermediate > SHRT_MAX) {
intermediate = SHRT_MAX;
} else if (intermediate < SHRT_MIN) {
intermediate = SHRT_MIN;
}
intermediate &= 0x0000FFFF;
result |= (((uint32_t)intermediate)) << (sizeof(int16_t) * 8 * i);
int32_t lo = ((int32_t)(int16_t)val * lomul) >> 15;
int32_t hi = ((int32_t)(int16_t)(val >> 16) * himul) >> 15;
if (lo > SHRT_MAX) {
lo = SHRT_MAX;
} else if (lo < SHRT_MIN) {
lo = SHRT_MIN;
}
return result;
if (hi > SHRT_MAX) {
hi = SHRT_MAX;
} else if (hi < SHRT_MIN) {
hi = SHRT_MIN;
}
return ((uint32_t)hi << 16) | ((uint32_t)lo & 0xFFFF);
#endif
}

Expand Down
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