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Implement LFSR.

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This commit is contained in:
Thomas Harte
2025-11-10 22:44:00 -05:00
parent ae2936b9c3
commit 8c74e2a323
1 changed files with 31 additions and 7 deletions
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Components/SID/SID.hpp
+31 -7
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@@ -48,11 +48,31 @@ private:
uint32_t phase = PhaseReload;
uint32_t previous_phase = PhaseReload;
static constexpr uint32_t NoiseReload = 0x7'ffff;
uint32_t noise = NoiseReload;
bool did_rise_b23() const {
return previous_phase > phase;
}
uint16_t sawtooth() const {
return phase >> 20;
uint16_t sawtooth_output() const {
return (phase >> 20) ^ 0x800;
}
uint16_t noise_output() const {
// Uses bits: 20, 18, 14, 11, 9, 5, 2 and 0, plus four more zero bits.
return
((noise >> 8) & 0x800) |
((noise >> 7) & 0x400) |
((noise >> 4) & 0x200) |
((noise >> 2) & 0x100) |
((noise >> 1) & 0x080) |
((noise << 2) & 0x040) |
((noise << 4) & 0x020) |
((noise << 5) & 0x010);
}
void update_noise() {
noise =
(noise << 1) |
(((noise >> 17) ^ (noise >> 20)) & 1);
}
} oscillator;
struct ADSR {
@@ -73,7 +93,7 @@ private:
} adsr;
Numeric::SizedInt<8> control;
bool noise() const { return control.bit<7>(); } // TODO
bool noise() const { return control.bit<7>(); }
bool pulse() const { return control.bit<6>(); }
bool sawtooth() const { return control.bit<5>(); }
bool triangle() const { return control.bit<4>(); }
@@ -89,6 +109,10 @@ private:
oscillator.phase = 0;
} else {
oscillator.phase += oscillator.pitch;
if(oscillator.did_rise_b23()) {
oscillator.update_noise();
}
}
// TODO: ADSR.
@@ -107,7 +131,7 @@ private:
static constexpr uint16_t MaxWaveformValue = (1 << 12) - 1;
uint16_t sawtooth_output() const {
return oscillator.sawtooth();
return oscillator.sawtooth_output();
}
uint16_t pulse_output() const {
@@ -115,15 +139,15 @@ private:
}
uint16_t triangle_output(const Voice &prior) const {
const uint16_t sawtooth = oscillator.sawtooth();
const uint16_t sawtooth = oscillator.sawtooth_output();
const uint16_t xor_mask1 = sawtooth;
const uint16_t xor_mask2 = ring_mod() ? prior.sawtooth() : 0;
const uint16_t xor_mask = (xor_mask1 ^ xor_mask2) ? 0xfff : 0x000;
const uint16_t xor_mask = ((xor_mask1 ^ xor_mask2) & 0x800) ? 0xfff : 0x000;
return ((sawtooth << 1) ^ xor_mask) & 0xfff;
}
uint16_t noise_output() const {
return 0; // TODO.
return oscillator.noise_output();
}
uint16_t output(const Voice &prior) const {