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# 8e8d9cad1884 x16-emulator / src / vera_psg.c
2.9 KiB · C · last changed chore(): initial commit 15 days ago source blame history raw
03a3459e Andy McCall 1 
// 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 <stdbool.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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struct channel {
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	uint16_t freq;
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	uint16_t volume;
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	bool     left, right;
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	uint8_t  pw;
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	uint8_t  waveform;
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	uint16_t noiseval;
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	uint32_t phase;
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};
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static struct channel channels[16];
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static uint16_t volume_lut[64] = {
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	  0,                                           4,   8,  12,
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	 16,  17,  18,  20,  21,  22,  23,  25,  26,  28,  30,  31,
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	 33,  35,  37,  40,  42,  45,  47,  50,  53,  56,  60,  63,
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	 67,  71,  75,  80,  85,  90,  95, 101, 107, 113, 120, 127,
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	135, 143, 151, 160, 170, 180, 191, 202, 214, 227, 241, 255,
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	270, 286, 303, 321, 341, 361, 382, 405, 429, 455, 482, 511
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};
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static uint16_t noise_state;
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void
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psg_reset(void)
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{
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	memset(channels, 0, sizeof(channels));
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	noise_state = 1;
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}
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void
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psg_writereg(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: channels[ch].freq = (channels[ch].freq & 0xFF00) | val; break;
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		case 1: channels[ch].freq = (channels[ch].freq & 0x00FF) | (val << 8); break;
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		case 2: {
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			channels[ch].right  = (val & 0x80) != 0;
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			channels[ch].left   = (val & 0x40) != 0;
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			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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			channels[ch].pw       = val & 0x3F;
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			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 void
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render(int16_t *left, int16_t *right)
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{
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	int16_t l = 0;
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	int16_t 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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		noise_state = (noise_state << 1) | (((noise_state >> 1) ^ (noise_state >> 2) ^ (noise_state >> 4) ^ (noise_state >> 15)) & 1);
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		struct channel *ch = &channels[i];
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		uint32_t new_phase = (ch->left || ch->right) ? ((ch->phase + ch->freq) & 0x1FFFF) : 0;
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		if ((ch->phase & 0x10000) && !(new_phase & 0x10000)) {
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			ch->noiseval = (noise_state >> 1) & 0x3F;
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		}
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		ch->phase = new_phase;
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		uint32_t v = 0;
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		switch (ch->waveform) {
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			case WF_PULSE: v = ((ch->phase >> 10) > ch->pw) ? 0 : 0x3F; break;
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    		case WF_SAWTOOTH: v = (ch->phase >> 11) ^ ((ch->pw ^ 0x3f) & 0x3f); break;
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			case WF_TRIANGLE: v = ((ch->phase & 0x10000) ? (~(ch->phase >> 10) & 0x3F) : ((ch->phase >> 10) & 0x3F)) ^ ((ch->pw ^ 0x3f) & 0x3f); break;
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			case WF_NOISE: v = ch->noiseval; break;
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		}
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		int16_t sv = (v ^ 0x20);
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		if (sv & 0x20) {
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			sv |= 0xFFC0;
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		}
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		int16_t val = sv * ch->volume;
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		if (ch->left) {
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			l += val >> 3;
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		}
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		if (ch->right) {
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			r += val >> 3;
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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(int16_t *buf, unsigned num_samples)
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{
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	while (num_samples--) {
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		render(&buf[0], &buf[1]);
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		buf += 2;
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	}
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}