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sn76496.c
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C/C++ Source or Header
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2000-04-04
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9KB
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334 lines
/***************************************************************************
sn76496.c
Routines to emulate the Texas Instruments SN76489 / SN76496 programmable
tone /noise generator. Also known as (or at least compatible with) TMS9919.
Noise emulation is not accurate due to lack of documentation. The noise
generator uses a shift register with a XOR-feedback network, but the exact
layout is unknown. It can be set for either period or white noise; again,
the details are unknown.
***************************************************************************/
#include "driver.h"
#define MAX_OUTPUT 0x7fff
#define STEP 0x10000
/* Formulas for noise generator */
/* bit0 = output */
/* noise feedback for white noise mode */
#define FB_WNOISE 0x12000 /* bit15.d(16bits) = bit0(out) ^ bit2 */
//#define FB_WNOISE 0x14000 /* bit15.d(16bits) = bit0(out) ^ bit1 */
//#define FB_WNOISE 0x28000 /* bit16.d(17bits) = bit0(out) ^ bit2 (same to AY-3-8910) */
//#define FB_WNOISE 0x50000 /* bit17.d(18bits) = bit0(out) ^ bit2 */
/* noise feedback for periodic noise mode */
/* it is correct maybe (it was in the Megadrive sound manual) */
//#define FB_PNOISE 0x10000 /* 16bit rorate */
#define FB_PNOISE 0x08000 /* JH 981127 - fixes Do Run Run */
/* noise generator start preset (for periodic noise) */
#define NG_PRESET 0x0f35
struct SN76496
{
int Channel;
int SampleRate;
unsigned int UpdateStep;
int VolTable[16]; /* volume table */
int Register[8]; /* registers */
int LastRegister; /* last register written */
int Volume[4]; /* volume of voice 0-2 and noise */
unsigned int RNG; /* noise generator */
int NoiseFB; /* noise feedback mask */
int Period[4];
int Count[4];
int Output[4];
};
static struct SN76496 sn[MAX_76496];
static void SN76496Write(int chip,int data)
{
struct SN76496 *R = &sn[chip];
/* update the output buffer before changing the registers */
stream_update(R->Channel,0);
if (data & 0x80)
{
int r = (data & 0x70) >> 4;
int c = r/2;
R->LastRegister = r;
R->Register[r] = (R->Register[r] & 0x3f0) | (data & 0x0f);
switch (r)
{
case 0: /* tone 0 : frequency */
case 2: /* tone 1 : frequency */
case 4: /* tone 2 : frequency */
R->Period[c] = R->UpdateStep * R->Register[r];
if (R->Period[c] == 0) R->Period[c] = R->UpdateStep;
if (r == 4)
{
/* update noise shift frequency */
if ((R->Register[6] & 0x03) == 0x03)
R->Period[3] = 2 * R->Period[2];
}
break;
case 1: /* tone 0 : volume */
case 3: /* tone 1 : volume */
case 5: /* tone 2 : volume */
case 7: /* noise : volume */
R->Volume[c] = R->VolTable[data & 0x0f];
break;
case 6: /* noise : frequency, mode */
{
int n = R->Register[6];
R->NoiseFB = (n & 4) ? FB_WNOISE : FB_PNOISE;
n &= 3;
/* N/512,N/1024,N/2048,Tone #3 output */
R->Period[3] = (n == 3) ? 2 * R->Period[2] : (R->UpdateStep << (5+n));
/* reset noise shifter */
R->RNG = NG_PRESET;
R->Output[3] = R->RNG & 1;
}
break;
}
}
else
{
int r = R->LastRegister;
int c = r/2;
switch (r)
{
case 0: /* tone 0 : frequency */
case 2: /* tone 1 : frequency */
case 4: /* tone 2 : frequency */
R->Register[r] = (R->Register[r] & 0x0f) | ((data & 0x3f) << 4);
R->Period[c] = R->UpdateStep * R->Register[r];
if (R->Period[c] == 0) R->Period[c] = R->UpdateStep;
if (r == 4)
{
/* update noise shift frequency */
if ((R->Register[6] & 0x03) == 0x03)
R->Period[3] = 2 * R->Period[2];
}
break;
}
}
}
WRITE_HANDLER( SN76496_0_w ) { SN76496Write(0,data); }
WRITE_HANDLER( SN76496_1_w ) { SN76496Write(1,data); }
WRITE_HANDLER( SN76496_2_w ) { SN76496Write(2,data); }
WRITE_HANDLER( SN76496_3_w ) { SN76496Write(3,data); }
static void SN76496Update(int chip,INT16 *buffer,int length)
{
int i;
struct SN76496 *R = &sn[chip];
/* If the volume is 0, increase the counter */
for (i = 0;i < 4;i++)
{
if (R->Volume[i] == 0)
{
/* note that I do count += length, NOT count = length + 1. You might think */
/* it's the same since the volume is 0, but doing the latter could cause */
/* interferencies when the program is rapidly modulating the volume. */
if (R->Count[i] <= length*STEP) R->Count[i] += length*STEP;
}
}
while (length > 0)
{
int vol[4];
unsigned int out;
int left;
/* vol[] keeps track of how long each square wave stays */
/* in the 1 position during the sample period. */
vol[0] = vol[1] = vol[2] = vol[3] = 0;
for (i = 0;i < 3;i++)
{
if (R->Output[i]) vol[i] += R->Count[i];
R->Count[i] -= STEP;
/* Period[i] is the half period of the square wave. Here, in each */
/* loop I add Period[i] twice, so that at the end of the loop the */
/* square wave is in the same status (0 or 1) it was at the start. */
/* vol[i] is also incremented by Period[i], since the wave has been 1 */
/* exactly half of the time, regardless of the initial position. */
/* If we exit the loop in the middle, Output[i] has to be inverted */
/* and vol[i] incremented only if the exit status of the square */
/* wave is 1. */
while (R->Count[i] <= 0)
{
R->Count[i] += R->Period[i];
if (R->Count[i] > 0)
{
R->Output[i] ^= 1;
if (R->Output[i]) vol[i] += R->Period[i];
break;
}
R->Count[i] += R->Period[i];
vol[i] += R->Period[i];
}
if (R->Output[i]) vol[i] -= R->Count[i];
}
left = STEP;
do
{
int nextevent;
if (R->Count[3] < left) nextevent = R->Count[3];
else nextevent = left;
if (R->Output[3]) vol[3] += R->Count[3];
R->Count[3] -= nextevent;
if (R->Count[3] <= 0)
{
if (R->RNG & 1) R->RNG ^= R->NoiseFB;
R->RNG >>= 1;
R->Output[3] = R->RNG & 1;
R->Count[3] += R->Period[3];
if (R->Output[3]) vol[3] += R->Period[3];
}
if (R->Output[3]) vol[3] -= R->Count[3];
left -= nextevent;
} while (left > 0);
out = vol[0] * R->Volume[0] + vol[1] * R->Volume[1] +
vol[2] * R->Volume[2] + vol[3] * R->Volume[3];
if (out > MAX_OUTPUT * STEP) out = MAX_OUTPUT * STEP;
*(buffer++) = out / STEP;
length--;
}
}
static void SN76496_set_clock(int chip,int clock)
{
struct SN76496 *R = &sn[chip];
/* the base clock for the tone generators is the chip clock divided by 16; */
/* for the noise generator, it is clock / 256. */
/* Here we calculate the number of steps which happen during one sample */
/* at the given sample rate. No. of events = sample rate / (clock/16). */
/* STEP is a multiplier used to turn the fraction into a fixed point */
/* number. */
R->UpdateStep = ((double)STEP * R->SampleRate * 16) / clock;
}
static void SN76496_set_gain(int chip,int gain)
{
struct SN76496 *R = &sn[chip];
int i;
double out;
gain &= 0xff;
/* increase max output basing on gain (0.2 dB per step) */
out = MAX_OUTPUT / 3;
while (gain-- > 0)
out *= 1.023292992; /* = (10 ^ (0.2/20)) */
/* build volume table (2dB per step) */
for (i = 0;i < 15;i++)
{
/* limit volume to avoid clipping */
if (out > MAX_OUTPUT / 3) R->VolTable[i] = MAX_OUTPUT / 3;
else R->VolTable[i] = out;
out /= 1.258925412; /* = 10 ^ (2/20) = 2dB */
}
R->VolTable[15] = 0;
}
static int SN76496_init(const struct MachineSound *msound,int chip,int clock,int volume,int sample_rate)
{
int i;
struct SN76496 *R = &sn[chip];
char name[40];
sprintf(name,"SN76496 #%d",chip);
R->Channel = stream_init(name,volume,sample_rate,chip,SN76496Update);
if (R->Channel == -1)
return 1;
R->SampleRate = sample_rate;
SN76496_set_clock(chip,clock);
for (i = 0;i < 4;i++) R->Volume[i] = 0;
R->LastRegister = 0;
for (i = 0;i < 8;i+=2)
{
R->Register[i] = 0;
R->Register[i + 1] = 0x0f; /* volume = 0 */
}
for (i = 0;i < 4;i++)
{
R->Output[i] = 0;
R->Period[i] = R->Count[i] = R->UpdateStep;
}
R->RNG = NG_PRESET;
R->Output[3] = R->RNG & 1;
return 0;
}
int SN76496_sh_start(const struct MachineSound *msound)
{
int chip;
const struct SN76496interface *intf = msound->sound_interface;
for (chip = 0;chip < intf->num;chip++)
{
if (SN76496_init(msound,chip,intf->baseclock[chip],intf->volume[chip] & 0xff,Machine->sample_rate) != 0)
return 1;
SN76496_set_gain(chip,(intf->volume[chip] >> 8) & 0xff);
}
return 0;
}