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SM8521: "improve" noise

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unconfirmed

issue #2567
This commit is contained in:
tildearrow 2025-06-25 17:13:12 -05:00
parent 83f97a794f
commit c592c44c37
2 changed files with 13 additions and 2 deletions
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12
src/engine/platform/sound/sm8521.c
View file

@ -72,11 +72,19 @@ void sm8521_noise_tick(struct sm8521_noise_t *noise, const int cycle)
noise->base.counter += cycle; noise->base.counter += cycle;
while (noise->base.counter >= (noise->base.t + 1)) while (noise->base.counter >= (noise->base.t + 1))
{ {
// https://github.com/tildearrow/furnace/issues/2567
// unknown algorithm, but don't use rand() // unknown algorithm, but don't use rand()
//
// some research suggests VIC-like noise, although
// that remains to be confirmed
noise->oldLFSR = noise->lfsr & 1;
noise->lfsr = ( noise->lfsr>>1|(((noise->lfsr) ^ (noise->lfsr >> 5) ^ (noise->lfsr >> 8) ^ (noise->lfsr >> 13) ) & 1)<<31); noise->lfsr = ( noise->lfsr>>1|(((noise->lfsr) ^ (noise->lfsr >> 5) ^ (noise->lfsr >> 8) ^ (noise->lfsr >> 13) ) & 1)<<31);
noise->base.counter -= (noise->base.t + 1); noise->base.counter -= (noise->base.t + 1);
if (noise->oldLFSR^(noise->lfsr&1)) {
noise->out ^= 1;
}
} }
noise->base.out = (((noise->lfsr & 0x1) ? 7 : -8) * noise->base.level) >> 1; // scale out to 8bit noise->base.out = ((noise->out ? 7 : -8) * noise->base.level) >> 1; // scale out to 8bit
} }
void sm8521_sound_tick(struct sm8521_t *sm8521, const int cycle) void sm8521_sound_tick(struct sm8521_t *sm8521, const int cycle)
@ -130,6 +138,8 @@ void sm8521_reset(struct sm8521_t *sm8521)
sm8521->noise.base.out = 0; sm8521->noise.base.out = 0;
sm8521->noise.base.counter = 0; sm8521->noise.base.counter = 0;
sm8521->noise.lfsr = 0x89abcdef; sm8521->noise.lfsr = 0x89abcdef;
sm8521->noise.oldLFSR = 1;
sm8521->noise.out = 0;
sm8521->out = 0; sm8521->out = 0;
sm8521->sgda = 0; sm8521->sgda = 0;
sm8521->sgc = 0; sm8521->sgc = 0;

3
src/engine/platform/sound/sm8521.h
View file

@ -65,7 +65,8 @@ struct sm8521_wave_t
struct sm8521_noise_t struct sm8521_noise_t
{ {
struct sm8521_sg_t base; struct sm8521_sg_t base;
unsigned int lfsr; // LFSR unsigned int lfsr, oldLFSR; // LFSR
unsigned char out;
}; };
struct sm8521_t struct sm8521_t