114 lines
2.8 KiB
C
114 lines
2.8 KiB
C
// Commander X16 Emulator
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// Copyright (c) 2020 Frank van den Hoef
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// All rights reserved. License: 2-clause BSD
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#include "vera_psg.h"
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#include <stdlib.h>
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#include <string.h>
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enum waveform {
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WF_PULSE = 0,
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WF_SAWTOOTH,
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WF_TRIANGLE,
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WF_NOISE,
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};
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static uint8_t volume_lut[64] = {0, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 6, 6, 7, 7, 7, 8, 8, 9, 9, 10, 11, 11, 12, 13, 14, 14, 15, 16, 17, 18, 19, 21, 22, 23, 25, 26, 28, 29, 31, 33, 35, 37, 39, 42, 44, 47, 50, 52, 56, 59, 63};
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void
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psg_reset(struct VERA_PSG* psg)
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{
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memset(psg->channels, 0, sizeof(psg->channels));
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psg->noiseOut = 0;
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psg->noiseState = 1;
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}
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void
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psg_writereg(struct VERA_PSG* psg, uint8_t reg, uint8_t val)
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{
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reg &= 0x3f;
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int ch = reg / 4;
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int idx = reg & 3;
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switch (idx) {
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case 0: psg->channels[ch].freq = (psg->channels[ch].freq & 0xFF00) | val; break;
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case 1: psg->channels[ch].freq = (psg->channels[ch].freq & 0x00FF) | (val << 8); break;
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case 2: {
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psg->channels[ch].right = (val & 0x80) != 0;
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psg->channels[ch].left = (val & 0x40) != 0;
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psg->channels[ch].volume = volume_lut[val & 0x3F];
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break;
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}
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case 3: {
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psg->channels[ch].pw = val & 0x3F;
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psg->channels[ch].waveform = val >> 6;
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break;
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}
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}
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}
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static inline void
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render(struct VERA_PSG* psg, int16_t *left, int16_t *right)
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{
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int l = 0;
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int r = 0;
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for (int i = 0; i < 16; i++) {
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// In FPGA implementation, noise values are generated every system clock and
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// the channel update is run sequentially. So, even if both two channels are
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// fetching a noise value in the same sample, they should have different values
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psg->noiseOut = ((psg->noiseOut << 1) | (psg->noiseState & 1)) & 63;
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psg->noiseState = (psg->noiseState << 1) | (((psg->noiseState >> 1) ^ (psg->noiseState >> 2) ^ (psg->noiseState >> 4) ^ (psg->noiseState >> 15)) & 1);
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struct VERAChannel *ch = &psg->channels[i];
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unsigned new_phase = (ch->phase + ch->freq) & 0x1FFFF;
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if ((ch->phase & 0x10000) != (new_phase & 0x10000)) {
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ch->noiseval = psg->noiseOut;
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}
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ch->phase = new_phase;
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uint8_t v = 0;
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switch (ch->waveform) {
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case WF_PULSE: v = (ch->phase >> 10) > ch->pw ? 0 : 63; break;
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case WF_SAWTOOTH: v = ch->phase >> 11; break;
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case WF_TRIANGLE: v = (ch->phase & 0x10000) ? (~(ch->phase >> 10) & 0x3F) : ((ch->phase >> 10) & 0x3F); break;
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case WF_NOISE: v = ch->noiseval; break;
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}
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int8_t sv = (v ^ 0x20);
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if (sv & 0x20) {
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sv |= 0xC0;
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}
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int val = (int)sv * (int)ch->volume;
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if (ch->left) {
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l += val;
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}
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if (ch->right) {
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r += val;
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}
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if (ch->left || ch->right) {
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ch->lastOut = val;
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} else {
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ch->lastOut = 0;
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}
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}
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*left = l;
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*right = r;
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}
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void
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psg_render(struct VERA_PSG* psg, int16_t *bufL, int16_t *bufR, unsigned num_samples)
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{
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while (num_samples--) {
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render(psg, bufL, bufR);
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bufL++;
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bufR++;
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}
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}
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