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Swap the box filter with band-limited synthesis, fixed a regression t…
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…hat prevented filtering of square waves in some scenarios. Fixes #669
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@LIJI32
LIJI32 committed Nov 15, 2024
1 parent fc76063 commit 375fb43
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123 changes: 87 additions & 36 deletions Core/apu.c
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Original file line number Diff line number Diff line change
Expand Up @@ -6,21 +6,83 @@
#include <stdlib.h>
#include "gb.h"

/* Band limited synthesis based on: http://www.slack.net/~ant/bl-synth/ */
static int32_t band_limited_steps[GB_BAND_LIMITED_PHASES][GB_BAND_LIMITED_WIDTH];

static void __attribute__((constructor)) band_limited_init(void)
{
const unsigned master_size = GB_BAND_LIMITED_WIDTH * GB_BAND_LIMITED_PHASES;
double *master = malloc(master_size * sizeof(*master));
nounroll for (unsigned i = 0; i < master_size; i++) {
master[i] = 0.5;
}

const unsigned sine_size = 256 * GB_BAND_LIMITED_PHASES + 2;
const unsigned max_harmonic = sine_size / 2 / GB_BAND_LIMITED_PHASES;
nounroll for (unsigned harmonic = 1; harmonic <= max_harmonic; harmonic += 2) {
double amplitude = 4 / M_PI / harmonic / 2;
double to_angle = M_PI * 2 / sine_size * harmonic;
nounroll for (unsigned i = 0; i < master_size; i++) {
master[i] += sin(((signed)i - (signed)master_size / 2) * to_angle) * amplitude;
}
}

for (unsigned phase = 0; phase < GB_BAND_LIMITED_PHASES; phase++) {
int32_t error = GB_BAND_LIMITED_ONE;
int32_t prev = 0;
for (unsigned i = 0; i < GB_BAND_LIMITED_WIDTH; i++) {
int32_t cur = master[(GB_BAND_LIMITED_PHASES - 1 - phase) + i * GB_BAND_LIMITED_PHASES] * GB_BAND_LIMITED_ONE;
int32_t delta = cur - prev;
error = error - delta;
prev = cur;
band_limited_steps[phase][i] = delta;
}

// Make sure the deltas sum to 1.0
band_limited_steps[phase][GB_BAND_LIMITED_WIDTH / 2 - 1] += error / 2;
band_limited_steps[phase][0] += error - (error / 2);
}
free(master);
}

static void band_limited_update(GB_band_limited_t *band_limited, const GB_sample_t *input, unsigned phase)
{
if (input->packed == band_limited->input.packed) return;
unsigned delay = phase / GB_BAND_LIMITED_PHASES;
phase = phase & (GB_BAND_LIMITED_PHASES - 1);

GB_sample_t delta = {
.left = input->left - band_limited->input.left,
.right = input->right - band_limited->input.right,
};
band_limited->input.packed = input->packed;

for (unsigned i = 0; i < GB_BAND_LIMITED_WIDTH; i++) {
unsigned offset = (i + band_limited->pos + delay) & (sizeof(band_limited->buffer) / sizeof(band_limited->buffer[0]) - 1);
band_limited->buffer[offset].left += delta.left * band_limited_steps[phase][i];
band_limited->buffer[offset].right += delta.right * band_limited_steps[phase][i];
}
}

static void band_limited_read(GB_band_limited_t *band_limited, GB_sample_t *output, uint32_t multiplier)
{
band_limited->output.left += band_limited->buffer[band_limited->pos].left;
band_limited->output.right += band_limited->buffer[band_limited->pos].right;

band_limited->buffer[band_limited->pos].left = band_limited->buffer[band_limited->pos].right = 0;
band_limited->pos = (band_limited->pos + 1) & (sizeof(band_limited->buffer) / sizeof(band_limited->buffer[0]) - 1);

output->left = band_limited->output.left * multiplier / GB_BAND_LIMITED_ONE;
output->right = band_limited->output.right * multiplier / GB_BAND_LIMITED_ONE;
}

static const uint8_t duties[] = {
0, 0, 0, 0, 0, 0, 0, 1,
1, 0, 0, 0, 0, 0, 0, 1,
1, 0, 0, 0, 0, 1, 1, 1,
0, 1, 1, 1, 1, 1, 1, 0,
};

static void refresh_channel(GB_gameboy_t *gb, GB_channel_t index, unsigned cycles_offset)
{
unsigned multiplier = gb->apu_output.cycles_since_render + cycles_offset - gb->apu_output.last_update[index];
gb->apu_output.summed_samples[index].left += gb->apu_output.current_sample[index].left * multiplier;
gb->apu_output.summed_samples[index].right += gb->apu_output.current_sample[index].right * multiplier;
gb->apu_output.last_update[index] = gb->apu_output.cycles_since_render + cycles_offset;
}

bool GB_apu_is_DAC_enabled(GB_gameboy_t *gb, GB_channel_t index)
{
if (gb->model > GB_MODEL_CGB_E) {
Expand Down Expand Up @@ -69,7 +131,6 @@ static uint8_t agb_bias_for_channel(GB_gameboy_t *gb, GB_channel_t index)

static void update_sample(GB_gameboy_t *gb, GB_channel_t index, int8_t value, unsigned cycles_offset)
{

if (gb->model > GB_MODEL_CGB_E) {
/* On the AGB, because no analog mixing is done, the behavior of NR51 is a bit different.
A channel that is not connected to a terminal is idenitcal to a connected channel
Expand Down Expand Up @@ -100,10 +161,9 @@ static void update_sample(GB_gameboy_t *gb, GB_channel_t index, int8_t value, un
output.left = output.right = 0;
}

if (gb->apu_output.current_sample[index].packed != output.packed) {
refresh_channel(gb, index, cycles_offset);
gb->apu_output.current_sample[index] = output;
}
band_limited_update(&gb->apu_output.band_limited[index],
&output,
(gb->apu_output.cycles_since_render + cycles_offset) * GB_BAND_LIMITED_PHASES / gb->apu_output.max_cycles_per_sample);
}

return;
Expand Down Expand Up @@ -131,10 +191,9 @@ static void update_sample(GB_gameboy_t *gb, GB_channel_t index, int8_t value, un
if (likely(!gb->apu_output.channel_muted[index])) {
output = (GB_sample_t){(0xF - value * 2) * left_volume, (0xF - value * 2) * right_volume};
}
if (gb->apu_output.current_sample[index].packed != output.packed) {
refresh_channel(gb, index, cycles_offset);
gb->apu_output.current_sample[index] = output;
}
band_limited_update(&gb->apu_output.band_limited[index],
&output,
(gb->apu_output.cycles_since_render + cycles_offset) * GB_BAND_LIMITED_PHASES / gb->apu_output.max_cycles_per_sample);
}
}

Expand Down Expand Up @@ -210,20 +269,12 @@ static void render(GB_gameboy_t *gb)
}
}
}

GB_sample_t channel_output;
band_limited_read(&gb->apu_output.band_limited[i], &channel_output, multiplier);

if (likely(gb->apu_output.last_update[i] == 0 || gb->apu_output.cycles_since_render == 0)) {
output.left += gb->apu_output.current_sample[i].left * multiplier;
output.right += gb->apu_output.current_sample[i].right * multiplier;
}
else {
refresh_channel(gb, i, 0);
output.left += (signed long) gb->apu_output.summed_samples[i].left * multiplier
/ gb->apu_output.cycles_since_render;
output.right += (signed long) gb->apu_output.summed_samples[i].right * multiplier
/ gb->apu_output.cycles_since_render;
gb->apu_output.summed_samples[i] = (GB_sample_t){0, 0};
}
gb->apu_output.last_update[i] = 0;
output.left += channel_output.left;
output.right += channel_output.right;
}
gb->apu_output.cycles_since_render = 0;

Expand Down Expand Up @@ -295,7 +346,7 @@ static void render(GB_gameboy_t *gb)
}
}

static void update_square_sample(GB_gameboy_t *gb, GB_channel_t index)
static void update_square_sample(GB_gameboy_t *gb, GB_channel_t index, unsigned cycles)
{
if (gb->apu.square_channels[index].sample_surpressed) {
if (gb->model > GB_MODEL_CGB_E) {
Expand All @@ -308,7 +359,7 @@ static void update_square_sample(GB_gameboy_t *gb, GB_channel_t index)
update_sample(gb, index,
duties[gb->apu.square_channels[index].current_sample_index + duty * 8]?
gb->apu.square_channels[index].current_volume : 0,
0);
cycles);
}

static inline void update_wave_sample(GB_gameboy_t *gb, unsigned cycles)
Expand Down Expand Up @@ -424,7 +475,7 @@ static void tick_square_envelope(GB_gameboy_t *gb, GB_channel_t index)
}

if (gb->apu.is_active[index]) {
update_square_sample(gb, index);
update_square_sample(gb, index, 0);
}
}

Expand Down Expand Up @@ -785,7 +836,7 @@ restart:;
gb->apu.pcm_mask[0] &= i == GB_SQUARE_1? 0xF0 : 0x0F;
}
gb->apu.square_channels[i].did_tick = true;
update_square_sample(gb, i);
update_square_sample(gb, i, cycles - cycles_left);

uint8_t duty = gb->io_registers[i == GB_SQUARE_1? GB_IO_NR11 :GB_IO_NR21] >> 6;
uint8_t edge_sample_index = inline_const(uint8_t[], {7, 7, 5, 1})[duty];
Expand Down Expand Up @@ -1250,7 +1301,7 @@ void GB_apu_write(GB_gameboy_t *gb, uint8_t reg, uint8_t value)
nrx2_glitch(gb, &gb->apu.square_channels[index].current_volume,
value, gb->io_registers[reg], &gb->apu.square_channels[index].volume_countdown,
&gb->apu.square_channels[index].envelope_clock);
update_square_sample(gb, index);
update_square_sample(gb, index, 0);
}

break;
Expand Down Expand Up @@ -1335,7 +1386,7 @@ void GB_apu_write(GB_gameboy_t *gb, uint8_t reg, uint8_t value)
started sound). The playback itself is not instant which is why we don't update the sample for other
cases. */
if (gb->apu.is_active[index]) {
update_square_sample(gb, index);
update_square_sample(gb, index, 0);
}

gb->apu.square_channels[index].volume_countdown = gb->io_registers[index == GB_SQUARE_1 ? GB_IO_NR12 : GB_IO_NR22] & 7;
Expand Down
20 changes: 15 additions & 5 deletions Core/apu.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -5,7 +5,12 @@
#include <stdio.h>
#include "defs.h"

#define GB_BAND_LIMITED_WIDTH 16
#define GB_BAND_LIMITED_PHASES 32

#ifdef GB_INTERNAL
#define GB_BAND_LIMITED_ONE 0x10000 // fixed point value equal to 1

/* Speed = 1 / Length (in seconds) */
#define DAC_DECAY_SPEED 20000
#define DAC_ATTACK_SPEED 20000
Expand Down Expand Up @@ -165,17 +170,22 @@ typedef enum {
GB_AUDIO_FORMAT_WAV,
} GB_audio_format_t;

typedef struct {
struct {
int32_t left, right;
} buffer[GB_BAND_LIMITED_WIDTH * 2], output;
uint8_t pos;
GB_sample_t input;
} GB_band_limited_t;

typedef struct {
unsigned sample_rate;

unsigned sample_cycles; // Counts by sample_rate until it reaches the clock frequency
unsigned max_cycles_per_sample;

// Samples are NOT normalized to MAX_CH_AMP * 4 at this stage!
unsigned cycles_since_render;
unsigned last_update[GB_N_CHANNELS];
GB_sample_t current_sample[GB_N_CHANNELS];
GB_sample_t summed_samples[GB_N_CHANNELS];
uint32_t cycles_since_render;
GB_band_limited_t band_limited[GB_N_CHANNELS];
double dac_discharge[GB_N_CHANNELS];
bool channel_muted[GB_N_CHANNELS];
bool edge_triggered[GB_N_CHANNELS];
Expand Down

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