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GB Audio: More accurate LFSR implementation for noise channel

This commit is contained in:
Vicki Pfau 2023-06-29 21:56:05 -07:00
parent 9873073400
commit bd15285ea1
1 changed files with 42 additions and 12 deletions
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54
src/gb/audio.c
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@ -385,11 +385,7 @@ void GBAudioWriteNR44(struct GBAudio* audio, uint8_t value) {
if (GBAudioRegisterNoiseControlIsRestart(value)) { if (GBAudioRegisterNoiseControlIsRestart(value)) {
audio->playingCh4 = _resetEnvelope(&audio->ch4.envelope); audio->playingCh4 = _resetEnvelope(&audio->ch4.envelope);
if (audio->ch4.power) { audio->ch4.lfsr = 0;
audio->ch4.lfsr = 0x7F;
} else {
audio->ch4.lfsr = 0x7FFF;
}
if (!audio->ch4.length) { if (!audio->ch4.length) {
audio->ch4.length = 64; audio->ch4.length = 64;
if (audio->ch4.stop && !(audio->frame & 1)) { if (audio->ch4.stop && !(audio->frame & 1)) {
@ -601,24 +597,58 @@ void GBAudioRun(struct GBAudio* audio, int32_t timestamp, int channels) {
} }
} }
if (audio->playingCh4 && (channels & 0x8)) { if (audio->playingCh4 && (channels & 0x8)) {
const uint16_t noiseMaskTable[0x40] = {
0x3f, 0x3e, 0x3c, 0x3d, 0x39, 0x38, 0x3a, 0x3b,
0x33, 0x32, 0x30, 0x31, 0x35, 0x34, 0x36, 0x37,
0x27, 0x26, 0x24, 0x25, 0x21, 0x20, 0x22, 0x23,
0x2b, 0x2a, 0x28, 0x29, 0x2d, 0x2c, 0x2e, 0x2f,
0x0f, 0x0e, 0x0c, 0x0d, 0x09, 0x08, 0x0a, 0x0b,
0x03, 0x02, 0x00, 0x01, 0x05, 0x04, 0x06, 0x07,
0x17, 0x16, 0x14, 0x15, 0x11, 0x10, 0x12, 0x13,
0x1b, 0x1a, 0x18, 0x19, 0x1d, 0x1c, 0x1e, 0x1f
};
const uint16_t noisePopulationTable[0x40] = {
6, 5, 4, 5, 4, 3, 4, 5, 4, 3, 2, 3, 4, 3, 4, 5,
4, 3, 2, 3, 2, 1, 2, 3, 4, 3, 2, 3, 4, 3, 4, 5,
4, 3, 2, 3, 2, 1, 2, 3, 2, 1, 0, 1, 2, 1, 2, 3,
4, 3, 2, 3, 2, 1, 2, 3, 4, 3, 2, 3, 4, 3, 4, 5
};
int32_t cycles = audio->ch4.ratio ? 2 * audio->ch4.ratio : 1; int32_t cycles = audio->ch4.ratio ? 2 * audio->ch4.ratio : 1;
cycles <<= audio->ch4.frequency; cycles <<= audio->ch4.frequency;
cycles *= 8 * audio->timingFactor; cycles *= 8 * audio->timingFactor;
int32_t diff = timestamp - audio->ch4.lastEvent; int32_t diff = timestamp - audio->ch4.lastEvent;
if (diff >= cycles) { if (diff >= cycles) {
int32_t last; int32_t last = 0;
int samples = 0; int samples = 0;
int positiveSamples = 0; int positiveSamples = 0;
int lsb; int lsb;
int coeff = 0x60; int coeff;
if (!audio->ch4.power) { if (audio->ch4.power) {
coeff <<= 8; // TODO: Can this be batched too?
coeff = 0x4040;
} else {
int bits = 0;
// Batch 5 steps at a time when possible
for (; last + cycles * 5 <= diff; last += cycles * 5) {
bits = audio->ch4.lfsr & 0x3F;
audio->ch4.lfsr >>= 5;
audio->ch4.lfsr |= 0x4000 * noiseMaskTable[bits] >> 4;
audio->ch4.lfsr &= 0x7FFF;
samples += 5;
positiveSamples += noisePopulationTable[bits];
} }
for (last = 0; last + cycles <= diff; last += cycles) { lsb = noiseMaskTable[bits] & 1;
lsb = audio->ch4.lfsr & 1; coeff = 0x4000;
}
for (; last + cycles <= diff; last += cycles) {
lsb = (audio->ch4.lfsr ^ (audio->ch4.lfsr >> 1) ^ 1) & 1;
audio->ch4.lfsr >>= 1; audio->ch4.lfsr >>= 1;
audio->ch4.lfsr ^= lsb * coeff; if (lsb) {
audio->ch4.lfsr |= coeff;
} else {
audio->ch4.lfsr &= ~coeff;
}
++samples; ++samples;
positiveSamples += lsb; positiveSamples += lsb;
} }