mirror of
https://github.com/ekeeke/Genesis-Plus-GX.git
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336 lines
9.9 KiB
C
336 lines
9.9 KiB
C
/*
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SN76489 emulation
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by Maxim in 2001 and 2002
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converted from my original Delphi implementation
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I'm a C newbie so I'm sure there are loads of stupid things
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in here which I'll come back to some day and redo
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Includes:
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- Super-high quality tone channel "oversampling" by calculating fractional positions on transitions
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- Noise output pattern reverse engineered from actual SMS output
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- Volume levels taken from actual SMS output
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07/08/04 Charles MacDonald
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Modified for use with SMS Plus:
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- Added support for multiple PSG chips.
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- Added reset/config/update routines.
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- Added context management routines.
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- Removed SN76489_GetValues().
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- Removed some unused variables.
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25/04/07 Eke-Eke (Genesis Plus GX)
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- Removed stereo GG support (unused)
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- Made SN76489_Update outputs 16bits mono samples
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- Replaced volume table with VGM plugin's one
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05/01/09 Eke-Eke (Genesis Plus GX)
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- Modified Cut-Off frequency (according to Steve Snake: http://www.smspower.org/forums/viewtopic.php?t=1746)
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24/08/10 Eke-Eke (Genesis Plus GX)
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- Removed multichip support (unused)
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- Removed alternate volume table, panning & mute support (unused)
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- Removed configurable Feedback and Shift Register Width (always use Sega ones)
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- Added linear resampling using Blip Buffer (based on Blargg's implementation: http://www.smspower.org/forums/viewtopic.php?t=11376)
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*/
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#include "shared.h"
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#include "blip.h"
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/* Initial state of shift register */
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#define NoiseInitialState 0x8000
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/* Value below which PSG does not output */
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/*#define PSG_CUTOFF 0x6*/
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#define PSG_CUTOFF 0x1
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/* SN76489 clone in Sega's VDP chips (315-5124, 315-5246, 315-5313, Game Gear) */
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#define FB_SEGAVDP 0x0009
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#define SRW_SEGAVDP 16
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typedef struct
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{
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/* Configuration */
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int BoostNoise; /* double noise volume when non-zero */
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/* PSG registers: */
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int Registers[8]; /* Tone, vol x4 */
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int LatchedRegister;
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int NoiseShiftRegister;
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int NoiseFreq; /* Noise channel signal generator frequency */
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/* Output calculation variables */
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int ToneFreqVals[4]; /* Frequency register values (counters) */
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int ToneFreqPos[4]; /* Frequency channel flip-flops */
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int Channels[4]; /* Value of each channel, before stereo is applied */
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/* Blip-Buffer variables */
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int chan_amp[4]; /* current channel amplitudes in delta buffers */
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} SN76489_Context;
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static const int PSGVolumeValues[16] = {
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/* These values are taken from a real SMS2's output */
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/*{892,892,892,760,623,497,404,323,257,198,159,123,96,75,60,0}, */
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/* I can't remember why 892... :P some scaling I did at some point */
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/* these values are true volumes for 2dB drops at each step (multiply previous by 10^-0.1) */
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1516,1205,957,760,603,479,381,303,240,191,152,120,96,76,60,0
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};
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static struct blip_buffer_t* blip; /* delta resampler */
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static SN76489_Context SN76489;
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void SN76489_Init(double PSGClockValue, int SamplingRate)
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{
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SN76489_Shutdown();
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/* SamplingRate*16 instead of PSGClockValue/16 since division would lose some
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precision. blip_alloc doesn't care about the absolute sampling rate, just the
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ratio to clock rate. */
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blip = blip_alloc(PSGClockValue, SamplingRate * 16.0, SamplingRate / 4);
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}
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void SN76489_Reset()
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{
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int i;
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for(i = 0; i <= 3; i++)
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{
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/* Initialise PSG state */
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SN76489.Registers[2*i] = 1; /* tone freq=1 */
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SN76489.Registers[2*i+1] = 0xf; /* vol=off */
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/* Set counters to 0 */
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SN76489.ToneFreqVals[i] = 0;
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/* Set flip-flops to 1 */
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SN76489.ToneFreqPos[i] = 1;
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/* Clear channels output */
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SN76489.Channels[i] = 0;
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/* Clear current amplitudes in delta buffer */
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SN76489.chan_amp[i] = 0;
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}
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SN76489.LatchedRegister=0;
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/* Initialise noise generator */
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SN76489.NoiseShiftRegister=NoiseInitialState;
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SN76489.NoiseFreq = 0x10;
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SN76489.BoostNoise = config.psgBoostNoise;
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/* Clear Blip delta buffer */
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if (blip) blip_clear(blip);
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}
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void SN76489_Shutdown(void)
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{
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if (blip) blip_free(blip);
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blip = NULL;
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}
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void SN76489_BoostNoise(int boost)
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{
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SN76489.BoostNoise = boost;
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SN76489.Channels[3]= PSGVolumeValues[SN76489.Registers[7]] << boost;
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}
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void SN76489_SetContext(uint8 *data)
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{
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memcpy(&SN76489, data, sizeof(SN76489_Context));
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}
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void SN76489_GetContext(uint8 *data)
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{
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memcpy(data, &SN76489, sizeof(SN76489_Context));
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}
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uint8 *SN76489_GetContextPtr(void)
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{
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return (uint8 *)&SN76489;
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}
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int SN76489_GetContextSize(void)
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{
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return sizeof(SN76489_Context);
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}
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void SN76489_Write(int data)
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{
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if (data & 0x80)
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{
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/* Latch byte %1 cc t dddd */
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SN76489.LatchedRegister = (data >> 4) & 0x07;
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}
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int LatchedRegister = SN76489.LatchedRegister;
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switch (LatchedRegister)
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{
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case 0:
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case 2:
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case 4: /* Tone channels */
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if (data & 0x80)
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{
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/* Data byte %1 cc t dddd */
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SN76489.Registers[LatchedRegister] = (SN76489.Registers[LatchedRegister] & 0x3f0) | (data & 0xf);
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}
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else
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{
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/* Data byte %0 - dddddd */
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SN76489.Registers[LatchedRegister] = (SN76489.Registers[LatchedRegister] & 0x00f) | ((data & 0x3f) << 4);
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}
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/* Zero frequency changed to 1 to avoid div/0 */
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if (SN76489.Registers[LatchedRegister] == 0) SN76489.Registers[LatchedRegister] = 1;
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break;
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case 1:
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case 3:
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case 5: /* Channel attenuation */
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SN76489.Registers[LatchedRegister] = data & 0x0f;
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SN76489.Channels[LatchedRegister>>1] = PSGVolumeValues[data&0x0f];
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break;
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case 6: /* Noise */
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SN76489.Registers[6] = data & 0x0f;
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SN76489.NoiseShiftRegister = NoiseInitialState; /* reset shift register */
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SN76489.NoiseFreq = 0x10 << (data&0x3); /* set noise signal generator frequency */
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break;
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case 7: /* Noise attenuation */
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SN76489.Registers[7] = data & 0x0f;
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SN76489.Channels[3] = PSGVolumeValues[data&0x0f] << SN76489.BoostNoise;
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break;
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}
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}
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/* Updates tone amplitude in delta buffer. Call whenever amplitude might have changed. */
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static void UpdateToneAmplitude(int i, int time)
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{
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int delta = (SN76489.Channels[i] * SN76489.ToneFreqPos[i]) - SN76489.chan_amp[i];
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if (delta != 0)
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{
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SN76489.chan_amp[i] += delta;
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blip_add(blip, time, delta);
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}
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}
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/* Updates noise amplitude in delta buffer. Call whenever amplitude might have changed. */
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static void UpdateNoiseAmplitude(int time)
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{
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int delta = (SN76489.Channels[3] * ( SN76489.NoiseShiftRegister & 0x1 )) - SN76489.chan_amp[3];
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if (delta != 0)
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{
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SN76489.chan_amp[3] += delta;
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blip_add(blip, time, delta);
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}
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}
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/* Runs tone channel for clock_length clocks */
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static void RunTone(int i, int clock_length)
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{
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int time;
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/* Update in case a register changed etc. */
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UpdateToneAmplitude(i, 0);
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/* Time of next transition */
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time = SN76489.ToneFreqVals[i];
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/* Process any transitions that occur within clocks we're running */
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while (time < clock_length)
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{
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if (SN76489.Registers[i*2]>PSG_CUTOFF) {
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/* Flip the flip-flop */
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SN76489.ToneFreqPos[i] = -SN76489.ToneFreqPos[i];
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} else {
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/* stuck value */
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SN76489.ToneFreqPos[i] = 1;
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}
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UpdateToneAmplitude(i, time);
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/* Advance to time of next transition */
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time += SN76489.Registers[i*2];
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}
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/* Calculate new value for register, now that next transition is past number of clocks we're running */
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SN76489.ToneFreqVals[i] = time - clock_length;
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}
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/* Runs noise channel for clock_length clocks */
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static void RunNoise(int clock_length)
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{
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int time;
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/* Noise channel: match to tone2 if in slave mode */
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int NoiseFreq = SN76489.NoiseFreq;
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if (NoiseFreq == 0x80)
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{
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NoiseFreq = SN76489.Registers[2*2];
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SN76489.ToneFreqVals[3] = SN76489.ToneFreqVals[2];
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}
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/* Update in case a register changed etc. */
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UpdateNoiseAmplitude(0);
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/* Time of next transition */
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time = SN76489.ToneFreqVals[3];
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/* Process any transitions that occur within clocks we're running */
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while ( time < clock_length )
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{
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/* Flip the flip-flop */
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SN76489.ToneFreqPos[3] = -SN76489.ToneFreqPos[3];
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if (SN76489.ToneFreqPos[3] == 1)
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{
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/* On the positive edge of the square wave (only once per cycle) */
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int Feedback = SN76489.NoiseShiftRegister;
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if ( SN76489.Registers[6] & 0x4 )
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{
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/* White noise */
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/* Calculate parity of fed-back bits for feedback */
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/* Do some optimised calculations for common (known) feedback values */
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/* If two bits fed back, I can do Feedback=(nsr & fb) && (nsr & fb ^ fb) */
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/* since that's (one or more bits set) && (not all bits set) */
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Feedback = ((Feedback & FB_SEGAVDP) && ((Feedback & FB_SEGAVDP) ^ FB_SEGAVDP));
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}
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else /* Periodic noise */
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Feedback = Feedback & 1;
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SN76489.NoiseShiftRegister = (SN76489.NoiseShiftRegister >> 1) | (Feedback << (SRW_SEGAVDP - 1));
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UpdateNoiseAmplitude(time);
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}
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/* Advance to time of next transition */
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time += NoiseFreq;
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}
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/* Calculate new value for register, now that next transition is past number of clocks we're running */
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SN76489.ToneFreqVals[3] = time - clock_length;
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}
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void SN76489_Update(INT16 *buffer, int length)
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{
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int i;
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/* Determine how many clocks we need to run until 'length' samples are available */
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int clock_length = blip_clocks_needed(blip, length);
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/* Run noise first, since it might use current value of third tone frequency counter */
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RunNoise(clock_length);
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/* Run tone channels */
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for( i = 0; i <= 2; ++i )
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RunTone(i, clock_length);
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/* Read samples into output buffer */
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blip_end_frame(blip, clock_length);
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blip_read_samples(blip, buffer, length, 0);
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}
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