mirror of
https://github.com/ekeeke/Genesis-Plus-GX.git
synced 2024-12-29 12:41:48 +01:00
312 lines
9.4 KiB
C
312 lines
9.4 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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- Rade 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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25/05/09 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 (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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#include <float.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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static const int PSGVolumeValues[16] =
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{
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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}, *//* 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), normalised at 760 */
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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 SN76489_Context SN76489;
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void SN76489_Init(int PSGClockValue, int SamplingRate)
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{
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SN76489_Context *p = &SN76489;
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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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p->blip_buffer = blip_alloc(PSGClockValue, SamplingRate * 16, SamplingRate / 4);
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SN76489_Reset();
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}
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void SN76489_Reset()
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{
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SN76489_Context *p = &SN76489;
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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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p->Registers[2*i] = 1; /* tone freq=1 */
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p->Registers[2*i+1] = 0xf; /* vol=off */
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p->NoiseFreq = 0x10;
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/* Set counters to 0 */
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p->ToneFreqVals[i] = 0;
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/* Set flip-flops to 1 */
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p->ToneFreqPos[i] = 1;
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/* Clear current amplitudes in Blip delta buffer */
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p->chan_amp[i] = 0;
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}
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p->LatchedRegister=0;
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/* Initialise noise generator */
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p->NoiseShiftRegister=NoiseInitialState;
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/* Clear Blip delta buffer */
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blip_clear(p->blip_buffer);
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}
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void SN76489_Shutdown(void)
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{
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SN76489_Context *p = &SN76489;
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if (p->blip_buffer) blip_free(p->blip_buffer);
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p->blip_buffer = NULL;
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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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SN76489_Context *p = &SN76489;
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if (data&0x80)
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{
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/* Latch/data byte %1 cc t dddd */
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p->LatchedRegister=((data>>4)&0x07);
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p->Registers[p->LatchedRegister] =
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(p->Registers[p->LatchedRegister] & 0x3f0) | /* zero low 4 bits */
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(data&0xf); /* and replace with data */
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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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if (!(p->LatchedRegister%2)&&(p->LatchedRegister<5))
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/* Tone register */
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p->Registers[p->LatchedRegister]=
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(p->Registers[p->LatchedRegister] & 0x00f) | /* zero high 6 bits */
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((data&0x3f)<<4); /* and replace with data */
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else
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/* Other register */
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p->Registers[p->LatchedRegister]=data&0x0f; /* Replace with data */
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}
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switch (p->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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/* Zero frequency changed to 1 to avoid div/0 */
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if (p->Registers[p->LatchedRegister]==0) p->Registers[p->LatchedRegister]=1;
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break;
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case 6: /* Noise */
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p->NoiseShiftRegister=NoiseInitialState; /* reset shift register */
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p->NoiseFreq=0x10<<(p->Registers[6]&0x3); /* set noise signal generator frequency */
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break;
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}
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}
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/* Updates channel amplitude in delta buffer. Call whenever amplitude might have changed. */
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static inline void UpdateChanAmplitude(SN76489_Context* chip, int i, int time)
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{
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/* Build stereo result into buffer */
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int buffer = chip->Channels[i];
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/* Update amplitudes in left and right buffers */
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int delta = buffer - chip->chan_amp[i];
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if (delta != 0)
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{
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chip->chan_amp[i] = buffer;
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blip_add(chip->blip_buffer, time, delta);
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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 inline void UpdateToneAmplitude(SN76489_Context* chip, int i, int time)
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{
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/* Tone channels */
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chip->Channels[i]= PSGVolumeValues[chip->Registers[2 * i + 1]] * chip->ToneFreqPos[i];
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UpdateChanAmplitude(chip, i, time);
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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 inline void UpdateNoiseAmplitude(SN76489_Context* chip, int time)
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{
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/* Noise channel */
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chip->Channels[3] = PSGVolumeValues[chip->Registers[7]] * ( chip->NoiseShiftRegister & 0x1 );
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/* Boost noise volume */
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chip->Channels[3] <<= chip->BoostNoise;
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UpdateChanAmplitude(chip, 3, time);
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}
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/* Runs tone channel for clock_length clocks */
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static inline void RunTone(SN76489_Context* chip, 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(chip, i, 0);
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/* Time of next transition */
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time = chip->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 (chip->Registers[i*2]>PSG_CUTOFF) {
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/* Flip the flip-flop */
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chip->ToneFreqPos[i] = -chip->ToneFreqPos[i];
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} else {
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/* stuck value */
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chip->ToneFreqPos[i] = 1;
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}
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UpdateToneAmplitude(chip, i, time);
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/* Advance to time of next transition */
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time += chip->Registers[i*2] + 1;
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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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chip->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 inline void RunNoise(SN76489_Context* chip, 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 = chip->NoiseFreq;
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if (NoiseFreq == 0x80)
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{
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NoiseFreq = chip->Registers[2*2];
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chip->ToneFreqVals[3] = chip->ToneFreqVals[2];
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}
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/* Update in case a register changed etc. */
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UpdateNoiseAmplitude(chip, 0);
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/* Time of next transition */
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time = chip->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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chip->ToneFreqPos[3] = -chip->ToneFreqPos[3];
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if (chip->ToneFreqPos[3] == 1) {
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/* On the positive edge of the square wave (only once per cycle) */
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int Feedback;
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if ( chip->Registers[6] & 0x4 ) {
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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 = ( ( chip->NoiseShiftRegister & FB_SEGAVDP)
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&& ( (chip->NoiseShiftRegister & FB_SEGAVDP) ^ FB_SEGAVDP) );
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} else /* Periodic noise */
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Feedback=chip->NoiseShiftRegister&1;
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chip->NoiseShiftRegister=(chip->NoiseShiftRegister>>1) | (Feedback << (SRW_SEGAVDP-1));
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UpdateNoiseAmplitude(chip, time);
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}
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/* Advance to time of next transition */
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time += NoiseFreq + 1;
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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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chip->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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SN76489_Context *p = &SN76489;
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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(p->blip_buffer, length);
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/* Run noise first, since it might use current value of third tone frequency counter */
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RunNoise(p, clock_length);
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/* Run tone channels */
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for( i = 0; i <= 2; ++i )
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RunTone(p, i, clock_length);
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/* Read samples into output buffer */
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blip_end_frame(p->blip_buffer,clock_length);
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blip_read_samples(p->blip_buffer,buffer,length,0);
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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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}
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