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swan: Fix noise emulation

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Adrian Siekierka 2025-03-09 21:03:13 +01:00 committed by tildearrow
parent 14bdbd852e
commit c4561b15a5
1 changed files with 6 additions and 7 deletions
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13
src/engine/platform/sound/swan.c
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@ -127,7 +127,6 @@ void swan_sound_out(swan_sound_t *snd, uint16_t port, uint8_t value) {
}
static void swan_sound_subtick(swan_sound_t *snd, uint32_t cycles) {
// TODO: Do period counters update when a channel is inactive?
for (int ch = 0; ch < 4; ch++) {
if (snd->ch_ctrl & (1 << ch)) {
snd->period_counter[ch] += cycles;
@ -135,6 +134,12 @@ static void swan_sound_subtick(swan_sound_t *snd, uint32_t cycles) {
while (snd->period_counter[ch] >= step) {
snd->sample_index[ch] = (snd->sample_index[ch] + 1) & 0x1F;
snd->period_counter[ch] -= step;
// Update noise counter
if (ch == 3 && (snd->ch_ctrl & SND_CH4_NOISE) && (snd->noise_ctrl & SND_NOISE_ENABLE)) {
uint8_t lfsr_new = (1 ^ (snd->noise_lfsr >> 7) ^ (snd->noise_lfsr >> lfsr_tap_bits[snd->noise_ctrl & 7])) & 0x1;
snd->noise_lfsr = (snd->noise_lfsr << 1) | lfsr_new;
}
}
}
}
@ -178,12 +183,6 @@ void swan_sound_tick(swan_sound_t *snd) {
// Update counters
swan_sound_subtick(snd, 128);
// Update noise counter
if (snd->noise_ctrl & SND_NOISE_ENABLE) {
uint8_t lfsr_new = (1 ^ (snd->noise_lfsr >> 7) ^ (snd->noise_lfsr >> lfsr_tap_bits[snd->noise_ctrl & 7])) & 0x1;
snd->noise_lfsr = (snd->noise_lfsr << 1) | lfsr_new;
}
// Calculate synthesizer channels
if (!(snd->test_flags & SND_TEST_HOLD_CH)) {
for (int i = 0; i < 4; i++) {