mirror of
https://github.com/ekeeke/Genesis-Plus-GX.git
synced 2024-11-13 22:35:10 +01:00
339 lines
10 KiB
C
339 lines
10 KiB
C
/*
|
|
SN76489 emulation
|
|
by Maxim in 2001 and 2002
|
|
converted from my original Delphi implementation
|
|
|
|
I'm a C newbie so I'm sure there are loads of stupid things
|
|
in here which I'll come back to some day and redo
|
|
|
|
Includes:
|
|
- Super-high quality tone channel "oversampling" by calculating fractional positions on transitions
|
|
- Noise output pattern reverse engineered from actual SMS output
|
|
- Volume levels taken from actual SMS output
|
|
|
|
07/08/04 Charles MacDonald
|
|
Modified for use with SMS Plus:
|
|
- Added support for multiple PSG chips.
|
|
- Added reset/config/update routines.
|
|
- Added context management routines.
|
|
- Removed SN76489_GetValues().
|
|
- Removed some unused variables.
|
|
|
|
25/04/07 Eke-Eke (Genesis Plus GX)
|
|
- Removed stereo GG support (unused)
|
|
- Made SN76489_Update outputs 16bits mono samples
|
|
- Replaced volume table with VGM plugin's one
|
|
|
|
05/01/09 Eke-Eke (Genesis Plus GX)
|
|
- Modified Cut-Off frequency (according to Steve Snake: http://www.smspower.org/forums/viewtopic.php?t=1746)
|
|
|
|
24/08/10 Eke-Eke (Genesis Plus GX)
|
|
- Removed multichip support (unused)
|
|
- Removed alternate volume table, panning & mute support (unused)
|
|
- Removed configurable Feedback and Shift Register Width (always use Sega ones)
|
|
- Added linear resampling using Blip Buffer (based on Blargg's implementation: http://www.smspower.org/forums/viewtopic.php?t=11376)
|
|
*/
|
|
|
|
#include "shared.h"
|
|
#include "blip.h"
|
|
|
|
/* Initial state of shift register */
|
|
#define NoiseInitialState 0x8000
|
|
|
|
/* Value below which PSG does not output */
|
|
/*#define PSG_CUTOFF 0x6*/
|
|
#define PSG_CUTOFF 0x1
|
|
|
|
/* SN76489 clone in Sega's VDP chips (315-5124, 315-5246, 315-5313, Game Gear) */
|
|
#define FB_SEGAVDP 0x0009
|
|
#define SRW_SEGAVDP 16
|
|
|
|
typedef struct
|
|
{
|
|
/* Configuration */
|
|
int BoostNoise; /* double noise volume when non-zero */
|
|
|
|
/* PSG registers: */
|
|
int Registers[8]; /* Tone, vol x4 */
|
|
int LatchedRegister;
|
|
int NoiseShiftRegister;
|
|
int NoiseFreq; /* Noise channel signal generator frequency */
|
|
|
|
/* Output calculation variables */
|
|
int ToneFreqVals[4]; /* Frequency register values (counters) */
|
|
int ToneFreqPos[4]; /* Frequency channel flip-flops */
|
|
int Channels[4]; /* Value of each channel, before stereo is applied */
|
|
|
|
/* Blip-Buffer variables */
|
|
int chan_amp[4]; /* current channel amplitudes in delta buffers */
|
|
} SN76489_Context;
|
|
|
|
static const int PSGVolumeValues[16] = {
|
|
/* These values are taken from a real SMS2's output */
|
|
/*{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 */
|
|
|
|
/* these values are true volumes for 2dB drops at each step (multiply previous by 10^-0.1) */
|
|
|
|
1516,1205,957,760,603,479,381,303,240,191,152,120,96,76,60,0
|
|
};
|
|
|
|
static struct blip_buffer_t* blip; /* delta resampler */
|
|
|
|
static SN76489_Context SN76489;
|
|
|
|
void SN76489_Init(double PSGClockValue, int SamplingRate)
|
|
{
|
|
SN76489_Shutdown();
|
|
|
|
/* SamplingRate*16 instead of PSGClockValue/16 since division would lose some
|
|
precision. blip_alloc doesn't care about the absolute sampling rate, just the
|
|
ratio to clock rate. */
|
|
blip = blip_alloc(PSGClockValue, SamplingRate * 16.0, SamplingRate / 4);
|
|
}
|
|
|
|
void SN76489_Reset()
|
|
{
|
|
SN76489_Context *chip = &SN76489;
|
|
int i;
|
|
|
|
for(i = 0; i <= 3; i++)
|
|
{
|
|
/* Initialise PSG state */
|
|
chip->Registers[2*i] = 1; /* tone freq=1 */
|
|
chip->Registers[2*i+1] = 0xf; /* vol=off */
|
|
|
|
/* Set counters to 0 */
|
|
chip->ToneFreqVals[i] = 0;
|
|
|
|
/* Set flip-flops to 1 */
|
|
chip->ToneFreqPos[i] = 1;
|
|
|
|
/* Clear channels output */
|
|
chip->Channels[i] = 0;
|
|
|
|
/* Clear current amplitudes in delta buffer */
|
|
chip->chan_amp[i] = 0;
|
|
}
|
|
|
|
chip->LatchedRegister=0;
|
|
|
|
/* Initialise noise generator */
|
|
chip->NoiseShiftRegister=NoiseInitialState;
|
|
chip->NoiseFreq = 0x10;
|
|
|
|
/* Clear Blip delta buffer */
|
|
if (blip) blip_clear(blip);
|
|
}
|
|
|
|
void SN76489_Shutdown(void)
|
|
{
|
|
if (blip) blip_free(blip);
|
|
blip = NULL;
|
|
}
|
|
|
|
void SN76489_BoostNoise(int boost)
|
|
{
|
|
SN76489.BoostNoise = boost;
|
|
SN76489.Channels[3]= PSGVolumeValues[SN76489.Registers[7]] << boost;
|
|
}
|
|
|
|
void SN76489_SetContext(uint8 *data)
|
|
{
|
|
memcpy(&SN76489, data, sizeof(SN76489_Context));
|
|
}
|
|
|
|
void SN76489_GetContext(uint8 *data)
|
|
{
|
|
memcpy(data, &SN76489, sizeof(SN76489_Context));
|
|
}
|
|
|
|
uint8 *SN76489_GetContextPtr(void)
|
|
{
|
|
return (uint8 *)&SN76489;
|
|
}
|
|
|
|
int SN76489_GetContextSize(void)
|
|
{
|
|
return sizeof(SN76489_Context);
|
|
}
|
|
|
|
void SN76489_Write(int data)
|
|
{
|
|
SN76489_Context *chip = &SN76489;
|
|
|
|
if (data & 0x80)
|
|
{
|
|
/* Latch byte %1 cc t dddd */
|
|
chip->LatchedRegister = (data >> 4) & 0x07;
|
|
}
|
|
|
|
int LatchedRegister = chip->LatchedRegister;
|
|
|
|
switch (LatchedRegister)
|
|
{
|
|
case 0:
|
|
case 2:
|
|
case 4: /* Tone channels */
|
|
if (data & 0x80)
|
|
{
|
|
/* Data byte %1 cc t dddd */
|
|
chip->Registers[LatchedRegister] = (chip->Registers[LatchedRegister] & 0x3f0) | (data & 0xf);
|
|
}
|
|
else
|
|
{
|
|
/* Data byte %0 - dddddd */
|
|
chip->Registers[LatchedRegister] = (chip->Registers[LatchedRegister] & 0x00f) | ((data & 0x3f) << 4);
|
|
}
|
|
/* Zero frequency changed to 1 to avoid div/0 */
|
|
if (chip->Registers[LatchedRegister] == 0) chip->Registers[LatchedRegister] = 1;
|
|
break;
|
|
|
|
case 1:
|
|
case 3:
|
|
case 5: /* Channel attenuation */
|
|
chip->Registers[LatchedRegister] = data & 0x0f;
|
|
chip->Channels[LatchedRegister>>1] = PSGVolumeValues[data&0x0f];
|
|
break;
|
|
|
|
case 6: /* Noise */
|
|
chip->Registers[6] = data & 0x0f;
|
|
chip->NoiseShiftRegister = NoiseInitialState; /* reset shift register */
|
|
chip->NoiseFreq = 0x10 << (data&0x3); /* set noise signal generator frequency */
|
|
break;
|
|
|
|
case 7: /* Noise attenuation */
|
|
chip->Registers[7] = data&0x0f;
|
|
chip->Channels[3] = PSGVolumeValues[data&0x0f] << chip->BoostNoise;
|
|
break;
|
|
}
|
|
}
|
|
|
|
/* Updates tone amplitude in delta buffer. Call whenever amplitude might have changed. */
|
|
static void UpdateToneAmplitude(SN76489_Context* chip, int i, int time)
|
|
{
|
|
int delta = (chip->Channels[i] * chip->ToneFreqPos[i]) - chip->chan_amp[i];
|
|
if (delta != 0)
|
|
{
|
|
chip->chan_amp[i] += delta;
|
|
blip_add(blip, time, delta);
|
|
}
|
|
}
|
|
|
|
/* Updates noise amplitude in delta buffer. Call whenever amplitude might have changed. */
|
|
static void UpdateNoiseAmplitude(SN76489_Context* chip, int time)
|
|
{
|
|
int delta = (chip->Channels[3] * ( chip->NoiseShiftRegister & 0x1 )) - chip->chan_amp[3];
|
|
if (delta != 0)
|
|
{
|
|
chip->chan_amp[3] += delta;
|
|
blip_add(blip, time, delta);
|
|
}
|
|
}
|
|
|
|
/* Runs tone channel for clock_length clocks */
|
|
static void RunTone(SN76489_Context* chip, int i, int clock_length)
|
|
|
|
{
|
|
int time;
|
|
|
|
/* Update in case a register changed etc. */
|
|
UpdateToneAmplitude(chip, i, 0);
|
|
|
|
/* Time of next transition */
|
|
time = chip->ToneFreqVals[i];
|
|
|
|
/* Process any transitions that occur within clocks we're running */
|
|
while (time < clock_length)
|
|
{
|
|
if (chip->Registers[i*2]>PSG_CUTOFF) {
|
|
/* Flip the flip-flop */
|
|
chip->ToneFreqPos[i] = -chip->ToneFreqPos[i];
|
|
} else {
|
|
/* stuck value */
|
|
chip->ToneFreqPos[i] = 1;
|
|
}
|
|
UpdateToneAmplitude(chip, i, time);
|
|
|
|
/* Advance to time of next transition */
|
|
time += chip->Registers[i*2];
|
|
}
|
|
|
|
/* Calculate new value for register, now that next transition is past number of clocks we're running */
|
|
chip->ToneFreqVals[i] = time - clock_length;
|
|
}
|
|
|
|
/* Runs noise channel for clock_length clocks */
|
|
static void RunNoise(SN76489_Context* chip, int clock_length)
|
|
|
|
{
|
|
int time;
|
|
|
|
/* Noise channel: match to tone2 if in slave mode */
|
|
int NoiseFreq = chip->NoiseFreq;
|
|
if (NoiseFreq == 0x80)
|
|
{
|
|
NoiseFreq = chip->Registers[2*2];
|
|
chip->ToneFreqVals[3] = chip->ToneFreqVals[2];
|
|
}
|
|
|
|
/* Update in case a register changed etc. */
|
|
UpdateNoiseAmplitude(chip, 0);
|
|
|
|
/* Time of next transition */
|
|
time = chip->ToneFreqVals[3];
|
|
|
|
/* Process any transitions that occur within clocks we're running */
|
|
while ( time < clock_length )
|
|
{
|
|
/* Flip the flip-flop */
|
|
chip->ToneFreqPos[3] = -chip->ToneFreqPos[3];
|
|
if (chip->ToneFreqPos[3] == 1)
|
|
{
|
|
/* On the positive edge of the square wave (only once per cycle) */
|
|
int Feedback = chip->NoiseShiftRegister;
|
|
if ( chip->Registers[6] & 0x4 )
|
|
{
|
|
/* White noise */
|
|
/* Calculate parity of fed-back bits for feedback */
|
|
/* Do some optimised calculations for common (known) feedback values */
|
|
/* If two bits fed back, I can do Feedback=(nsr & fb) && (nsr & fb ^ fb) */
|
|
/* since that's (one or more bits set) && (not all bits set) */
|
|
Feedback = ((Feedback & FB_SEGAVDP) && ((Feedback & FB_SEGAVDP) ^ FB_SEGAVDP));
|
|
}
|
|
else /* Periodic noise */
|
|
Feedback = Feedback & 1;
|
|
|
|
chip->NoiseShiftRegister = (chip->NoiseShiftRegister >> 1) | (Feedback << (SRW_SEGAVDP - 1));
|
|
UpdateNoiseAmplitude(chip, time);
|
|
}
|
|
|
|
/* Advance to time of next transition */
|
|
time += NoiseFreq;
|
|
}
|
|
|
|
/* Calculate new value for register, now that next transition is past number of clocks we're running */
|
|
chip->ToneFreqVals[3] = time - clock_length;
|
|
}
|
|
|
|
void SN76489_Update(INT16 *buffer, int length)
|
|
{
|
|
int i;
|
|
|
|
SN76489_Context *chip = &SN76489;
|
|
|
|
/* Determine how many clocks we need to run until 'length' samples are available */
|
|
int clock_length = blip_clocks_needed(blip, length);
|
|
|
|
/* Run noise first, since it might use current value of third tone frequency counter */
|
|
RunNoise(chip, clock_length);
|
|
|
|
/* Run tone channels */
|
|
for( i = 0; i <= 2; ++i )
|
|
RunTone(chip, i, clock_length);
|
|
|
|
/* Read samples into output buffer */
|
|
blip_end_frame(blip, clock_length);
|
|
blip_read_samples(blip, buffer, length, 0);
|
|
}
|