Genesis-Plus-GX/source/sound/sn76496.c

#include "shared.h"

#define MAX_OUTPUT  0x7fff 
#define STEP        0x10000
#define FB_WNOISE   0x14002
#define FB_PNOISE   0x08000
#define NG_PRESET   0x0F35



struct SN76496 sn[MAX_76496];

void SN76496Write (int chip, int data)
{
  struct SN76496 *R = &sn[chip];
  int n;

  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 */
	  {
	    n = R->Register[6];
	    R->NoiseFB = (n & 4) ? FB_WNOISE : FB_PNOISE;
	    n &= 3;
	    /* N/512,N/1024,N/2048,Tone #3 output */

	    /*
	     * Amended from Mame 1.04s
	     * 
	     * R->Period[3] = (n == 3) ? 2 * R->Period[2] : (R->UpdateStep << (5+n));
	     */
	    R->Period[3] =
	      ((n & 3) ==
	       3) ? 2 * R->Period[2] : (R->UpdateStep << (5 + (n & 3)));

	    /* 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;

	  /*
	   * Additions from Mame 1.04
	   * Start Here
	   */

	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];
	  R->Register[r] = (R->Register[r] & 0x3f0) | (data & 0x0f);
	  break;
	case 6:		/* noise  : frequency, mode */
	  {
	    R->Register[r] = (R->Register[r] & 0x3f0) | (data & 0x0f);
	    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) ==
	       3) ? 2 * R->Period[2] : (R->UpdateStep << (5 + (n & 3)));
	    /* reset noise shifter */
	    R->RNG = NG_PRESET;
	    R->Output[3] = R->RNG & 1;
	  }
	  break;
	}
    }
}


void SN76496Update (int chip, signed short int *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--;
    }
}



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;
}



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;
}



int SN76496_init (int chip, int clock, int volume, int sample_rate)
{
  int i;
  struct SN76496 *R = &sn[chip];

  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 (int clock, int volume, int rate)
{
  SN76496_init (0, clock, volume & 0xff, rate);
  SN76496_set_gain (0, (volume >> 8) & 0xff);
  return 0;
}