path: root/pokeytest/pokey.c

/*****************************************************************************/
/*                                                                           */
/* Module:  POKEY Chip Simulator, V1.1                                       */
/* Purpose: To emulate the sound generation hardware of the Atari POKEY chip.*/
/* Author:  Ron Fries                                                        */
/* Date:    September 22, 1996                                               */
/*                                                                           */
/*****************************************************************************/
/*                                                                           */
/*                 License Information and Copyright Notice                  */
/*                 ========================================                  */
/*                                                                           */
/* PokeySound is Copyright(c) 1996 by Ron Fries                              */
/*                                                                           */
/* This library is free software; you can redistribute it and/or modify it   */
/* under the terms of version 2 of the GNU Library General Public License    */
/* as published by the Free Software Foundation.                             */
/*                                                                           */
/* This library is distributed in the hope that it will be useful, but       */
/* WITHOUT ANY WARRANTY; without even the implied warranty of                */
/* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library */
/* General Public License for more details.                                  */
/* To obtain a copy of the GNU Library General Public License, write to the  */
/* Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.   */
/*                                                                           */
/* Any permitted reproduction of these routines, in whole or in part, must   */
/* bear this legend.                                                         */
/*                                                                           */
/*****************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#include "pokey.h"

/* CONSTANT DEFINITIONS */

/* definitions for AUDCx (D201, D203, D205, D207) */
#define NOTPOLY5    0x80     /* selects POLY5 or direct CLOCK */
#define POLY4       0x40     /* selects POLY4 or POLY17 */
#define PURE        0x20     /* selects POLY4/17 or PURE tone */
#define VOL_ONLY    0x10     /* selects VOLUME OUTPUT ONLY */
#define VOLUME_MASK 0x0f     /* volume mask */

/* definitions for AUDCTL (D208) */
#define POLY9       0x80     /* selects POLY9 or POLY17 */
#define CH1_179     0x40     /* selects 1.78979 MHz for Ch 1 */
#define CH3_179     0x20     /* selects 1.78979 MHz for Ch 3 */
#define CH1_CH2     0x10     /* clocks channel 1 w/channel 2 */
#define CH3_CH4     0x08     /* clocks channel 3 w/channel 4 */
#define CH1_FILTER  0x04     /* selects channel 1 high pass filter */
#define CH2_FILTER  0x02     /* selects channel 2 high pass filter */
#define CLOCK_15    0x01     /* selects 15.6999kHz or 63.9210kHz */

#define AUDF1_C     0xd200   
#define AUDC1_C     0xd201
#define AUDF2_C     0xd202
#define AUDC2_C     0xd203
#define AUDF3_C     0xd204
#define AUDC3_C     0xd205
#define AUDF4_C     0xd206
#define AUDC4_C     0xd207
#define AUDCTL_C    0xd208

/* for accuracy, the 64kHz and 15kHz clocks are exact divisions of
   the 1.79MHz clock */
#define DIV_64      28       /* divisor for 1.79MHz clock to 64 kHz */
#define DIV_15      114      /* divisor for 1.79MHz clock to 15 kHz */

/* the size (in entries) of the 4 polynomial tables */
#define POLY4_SIZE  0x000f
#define POLY5_SIZE  0x001f
#define POLY9_SIZE  0x01ff

#ifdef COMP16						   /* if 16-bit compiler */
#define POLY17_SIZE 0x00007fffL    /* reduced to 15 bits for simplicity */
#else
#define POLY17_SIZE 0x0001ffffL    /* else use the full 17 bits */
#endif

/* channel definitions */
#define CHAN1       0
#define CHAN2       1
#define CHAN3       2
#define CHAN4       3
#define SAMPLE      4


#define FALSE       0
#define TRUE        1


/* GLOBAL VARIABLE DEFINITIONS */
 
/* structures to hold the 9 pokey control bytes */ 
static uint8 AUDF[4];    /* AUDFx (D200, D202, D204, D206) */
static uint8 AUDC[4];    /* AUDCx (D201, D203, D205, D207) */
static uint8 AUDCTL;     /* AUDCTL (D208) */      

static uint8 Outbit[4];  /* current state of the output (high or low) */

static uint8 Outvol[4];  /* last output volume for each channel */


/* Initialze the bit patterns for the polynomials. */

/* The 4bit and 5bit patterns are the identical ones used in the pokey chip. */
/* Though the patterns could be packed with 8 bits per byte, using only a */
/* single bit per byte keeps the math simple, which is important for */
/* efficient processing. */

static uint8 bit4[POLY4_SIZE] = 
      { 1,1,0,1,1,1,0,0,0,0,1,0,1,0,0 };

static uint8 bit5[POLY5_SIZE] = 
      { 0,0,1,1,0,0,0,1,1,1,1,0,0,1,0,1,0,1,1,0,1,1,1,0,1,0,0,0,0,0,1 };

static uint8 bit17[POLY17_SIZE];  /* Rather than have a table with 131071 */
                            /* entries, I use a random number generator. */
                            /* It shouldn't make much difference since */
                            /* the pattern rarely repeats anyway. */

static uint32 Poly17_size; /* global for the poly 17 size, since it can */
                           /* be changed from 17 bit to 9 bit */

static uint32 Poly_adjust; /* the amount that the polynomial will need */
                           /* to be adjusted to process the next bit */

static uint32 P4=0,   /* Global position pointer for the 4-bit  POLY array */
              P5=0,   /* Global position pointer for the 5-bit  POLY array */
              P17=0;  /* Global position pointer for the 17-bit POLY array */

static uint32 Div_n_cnt[4],   /* Divide by n counter. one for each channel */
              Div_n_max[4];   /* Divide by n maximum, one for each channel */

static uint32 Samp_n_max,     /* Sample max.  For accuracy, it is *256 */
              Samp_n_cnt[2];  /* Sample cnt. */

static uint32 Base_mult;      /* selects either 64Khz or 15Khz clock mult */
       
/*****************************************************************************/
/* In my routines, I treat the sample output as another divide by N counter  */
/* For better accuracy, the Samp_n_cnt has a fixed binary decimal point      */
/* which has 8 binary digits to the right of the decimal point.  I use a two */
/* byte array to give me a minimum of 40 bits, and then use pointer math to  */
/* reference either the 24.8 whole/fraction combination or the 32-bit whole  */
/* only number.  This is mainly used to keep the math simple for             */
/* optimization. See below:                                                  */
/*                                                                           */
/* xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx | xxxxxxxx xxxxxxxx xxxxxxxx.xxxxxxxx */
/*  unused   unused   unused    whole      whole    whole    whole  fraction */
/*                                                                           */
/* Samp_n_cnt[0] gives me a 32-bit int 24 whole bits with 8 fractional bits, */
/* while (uint32 *)((uint8 *)(&Samp_n_cnt[0])+1) gives me the 32-bit whole   */
/* number only.                                                              */
/*****************************************************************************/


/*****************************************************************************/
/* Module:  Pokey_sound_init()                                               */
/* Purpose: to handle the power-up initialization functions                  */
/*          these functions should only be executed on a cold-restart        */
/*                                                                           */
/* Author:  Ron Fries                                                        */
/* Date:    September 22, 1996                                               */
/*                                                                           */
/* Inputs:  freq17 - the value for the '1.79MHz' Pokey audio clock           */
/*          playback_freq - the playback frequency in samples per second     */
/*                                                                           */
/* Outputs: Adjusts local globals - no return value                          */
/*                                                                           */
/*****************************************************************************/

void Pokey_sound_init (uint32 freq17, uint16 playback_freq)
{
   uint8 chan;
   int32 n;

   /* fill the 17bit polynomial with random bits */
   for (n=0; n<POLY17_SIZE; n++)
   {
      bit17[n] = rand() & 0x01;       /* fill poly 17 with random bits */
   }

   /* start all of the polynomial counters at zero */
   Poly_adjust = 0;
   P4 = 0;
   P5 = 0;
   P17 = 0;

   /* calculate the sample 'divide by N' value based on the playback freq. */
   Samp_n_max = ((uint32)freq17 << 8) / playback_freq;

   Samp_n_cnt[0] = 0;  /* initialize all bits of the sample */
   Samp_n_cnt[1] = 0;  /* 'divide by N' counter */

   Poly17_size = POLY17_SIZE;

   for (chan = CHAN1; chan <= CHAN4; chan++)
   {
      Outvol[chan] = 0;
      Outbit[chan] = 0;
      Div_n_cnt[chan] = 0;
      Div_n_max[chan] = 0x7fffffffL;
      AUDC[chan] = 0;
      AUDF[chan] = 0;
   }

   AUDCTL = 0;

   Base_mult = DIV_64;
}


/*****************************************************************************/
/* Module:  Update_pokey_sound()                                             */
/* Purpose: To process the latest control values stored in the AUDF, AUDC,   */
/*          and AUDCTL registers.  It pre-calculates as much information as  */
/*          possible for better performance.  This routine has not been      */
/*          optimized.                                                       */
/*                                                                           */
/* Author:  Ron Fries                                                        */
/* Date:    September 22, 1996                                               */
/*                                                                           */
/* Inputs:  addr - the address of the parameter to be changed                */
/*          val - the new value to be placed in the specified address        */
/*                                                                           */
/* Outputs: Adjusts local globals - no return value                          */
/*                                                                           */
/*****************************************************************************/

void Update_pokey_sound (uint16 addr, uint8 val)
{
    uint32 new_val = 0;
    uint8 chan;
	uint8 chan_mask;

    /* determine which address was changed */
    switch (addr)
    {
       case AUDF1_C:
          AUDF[CHAN1] = val;
          chan_mask = 1 << CHAN1;

          if (AUDCTL & CH1_CH2)    /* if ch 1&2 tied together */
            chan_mask |= 1 << CHAN2;   /* then also change on ch2 */
          break;

       case AUDC1_C:
          AUDC[CHAN1] = val;
          chan_mask = 1 << CHAN1;
          break;

       case AUDF2_C:
          AUDF[CHAN2] = val;
          chan_mask = 1 << CHAN2;
          break;

       case AUDC2_C:
          AUDC[CHAN2] = val;
          chan_mask = 1 << CHAN2;
          break;

       case AUDF3_C:
          AUDF[CHAN3] = val;
          chan_mask = 1 << CHAN3;

          if (AUDCTL & CH3_CH4)   /* if ch 3&4 tied together */
             chan_mask |= 1 << CHAN4;  /* then also change on ch4 */
          break;

       case AUDC3_C:
          AUDC[CHAN3] = val;
          chan_mask = 1 << CHAN3;
          break;

       case AUDF4_C:
          AUDF[CHAN4] = val;
          chan_mask = 1 << CHAN4;
          break;

       case AUDC4_C:
          AUDC[CHAN4] = val;
          chan_mask = 1 << CHAN4;
          break;

       case AUDCTL_C:
          AUDCTL = val;
          chan_mask = 15;		/* all channels */

          /* set poly17 counter to 9- or 17-bit */
          if (AUDCTL & POLY9)
             Poly17_size = POLY9_SIZE;
          else
             Poly17_size = POLY17_SIZE;

          /* determine the base multiplier for the 'div by n' calculations */
          if (AUDCTL & CLOCK_15)
             Base_mult = DIV_15;
          else
             Base_mult = DIV_64;

          break;

       default:
          chan_mask = 0;
          break;
    }

    /************************************************************/
    /* As defined in the manual, the exact Div_n_cnt values are */
    /* different depending on the frequency and resolution:     */
    /*    64 kHz or 15 kHz - AUDF + 1                           */
    /*    1 MHz, 8-bit -     AUDF + 4                           */
    /*    1 MHz, 16-bit -    AUDF[CHAN1]+256*AUDF[CHAN2] + 7    */
    /************************************************************/

    /* only reset the channels that have changed */

    if (chan_mask & (1 << CHAN1))
    {
       /* process channel 1 frequency */
       if (AUDCTL & CH1_179)
          new_val = AUDF[CHAN1] + 4;
       else
          new_val = (AUDF[CHAN1] + 1) * Base_mult;

       if (new_val != Div_n_max[CHAN1])
       {
          Div_n_max[CHAN1] = new_val;
          Div_n_cnt[CHAN1] = 0;
       }
    }

    if (chan_mask & (1 << CHAN2))
    {
       /* process channel 2 frequency */
       if (AUDCTL & CH1_CH2)
          if (AUDCTL & CH1_179)
             new_val = AUDF[CHAN2] * 256 + AUDF[CHAN1] + 7;
          else
             new_val = (AUDF[CHAN2] * 256 + AUDF[CHAN1] + 1) * Base_mult;
       else
          new_val = (AUDF[CHAN2] + 1) * Base_mult;

       if (new_val != Div_n_max[CHAN2])
       {
          Div_n_max[CHAN2] = new_val;
          Div_n_cnt[CHAN2] = 0;
       }
    }

    if (chan_mask & (1 << CHAN3))
    {
       /* process channel 3 frequency */
       if (AUDCTL & CH3_179)
          new_val = AUDF[CHAN3] + 4;
       else
          new_val= (AUDF[CHAN3] + 1) * Base_mult;

       if (new_val!= Div_n_max[CHAN3])
       {
          Div_n_max[CHAN3] = new_val;
          Div_n_cnt[CHAN3] = 0;
       }
    }

    if (chan_mask & (1 << CHAN4))
    {
       /* process channel 4 frequency */
       if (AUDCTL & CH3_CH4)
          if (AUDCTL & CH3_179)
             new_val = AUDF[CHAN4] * 256 + AUDF[CHAN3] + 7;
          else
             new_val = (AUDF[CHAN4] * 256 + AUDF[CHAN3] + 1) * Base_mult;
       else
          new_val = (AUDF[CHAN4] + 1) * Base_mult;

       if (new_val != Div_n_max[CHAN4])
       {
          Div_n_max[CHAN4] = new_val;
          Div_n_cnt[CHAN4] = 0;
       }
    }

    /* if channel is volume only, set current output */
    for (chan = CHAN1; chan <= CHAN4; chan++)
    {
       if (chan_mask & (1 << chan))
       {
          /* I've disabled any frequencies that exceed the sampling
             frequency.  There isn't much point in processing frequencies
             that the hardware can't reproduce.  I've also disabled 
             processing if the volume is zero. */

          /* if the channel is volume only */
          /* or the channel is off (volume == 0) */
          /* or the channel freq is greater than the playback freq */
          if ((AUDC[chan] & VOL_ONLY) ||
             ((AUDC[chan] & VOLUME_MASK) == 0) ||
              (Div_n_max[chan] < (Samp_n_max >> 8)))
          {
             /* then set the channel to the selected volume */
             Outvol[chan] = AUDC[chan] & VOLUME_MASK;
             /* and set channel freq to max to reduce processing */
             Div_n_max[chan] = 0x7fffffffL;
          }
       }
    } 
}


/*****************************************************************************/
/* Module:  Pokey_process_2()                                                */
/* Purpose: To fill the output buffer with the sound output based on the     */
/*          pokey chip parameters.  This routine has not been optimized.     */
/*          Though it is not used by the program, I've left it for reference.*/
/*                                                                           */
/* Author:  Ron Fries                                                        */
/* Date:    September 22, 1996                                               */
/*                                                                           */
/* Inputs:  *buffer - pointer to the buffer where the audio output will      */
/*                    be placed                                              */
/*          n - size of the playback buffer                                  */
/*                                                                           */
/* Outputs: the buffer will be filled with n bytes of audio - no return val  */
/*                                                                           */
/*****************************************************************************/

void Pokey_process_2 (register unsigned char *buffer, register uint16 n)
{
    register uint32 *samp_cnt_w_ptr;
    register uint32 event_min;
    register uint8 next_event;
    register uint8 cur_val;
    register uint8 chan;

    /* set a pointer to the whole portion of the samp_n_cnt */
    samp_cnt_w_ptr = (uint32 *)((uint8 *)(&Samp_n_cnt[0])+1);

    /* loop until the buffer is filled */
    while (n)
    {
       /* Normally the routine would simply decrement the 'div by N' */
       /* counters and react when they reach zero.  Since we normally */
       /* won't be processing except once every 80 or so counts, */
       /* I've optimized by finding the smallest count and then */
       /* 'accelerated' time by adjusting all pointers by that amount. */

       /* find next smallest event (either sample or chan 1-4) */
       next_event = SAMPLE;
       event_min = *samp_cnt_w_ptr;

       for (chan = CHAN1; chan <= CHAN4; chan++)
       {
          if (Div_n_cnt[chan] <= event_min)
          {
             event_min = Div_n_cnt[chan];
             next_event = chan;
          }
       }


       /* decrement all counters by the smallest count found */
       for (chan = CHAN1; chan <= CHAN4; chan++)
       {
          Div_n_cnt[chan] -= event_min;
       }

       *samp_cnt_w_ptr -= event_min;

       /* since the polynomials require a mod (%) function which is 
          division, I don't adjust the polynomials on the SAMPLE events,
          only the CHAN events.  I have to keep track of the change,
          though. */
       Poly_adjust += event_min;

       /* if the next event is a channel change */
       if (next_event != SAMPLE)
       {
          /* shift the polynomial counters */
          P4  = (P4  + Poly_adjust) % POLY4_SIZE;
          P5  = (P5  + Poly_adjust) % POLY5_SIZE;
          P17 = (P17 + Poly_adjust) % Poly17_size;
         
          /* reset the polynomial adjust counter to zero */
          Poly_adjust = 0;

          /* adjust channel counter */
          Div_n_cnt[next_event] += Div_n_max[next_event];

          /* From here, a good understanding of the hardware is required */
          /* to understand what is happening.  I won't be able to provide */
          /* much description to explain it here. */

          /* if the output is pure or the output is poly5 and the poly5 bit */
          /* is set */
          if ((AUDC[next_event] & NOTPOLY5) || bit5[P5])
          {
             /* if the PURE bit is set */
             if (AUDC[next_event] & PURE)
             {
                /* then simply toggle the output */
                Outbit[next_event] = !Outbit[next_event];
             }
             /* otherwise if POLY4 is selected */
             else if (AUDC[next_event] & POLY4)
             {
                /* then use the poly4 bit */
                Outbit[next_event] = bit4[P4];
             }
             else
             {
                /* otherwise use the poly17 bit */
                Outbit[next_event] = bit17[P17];
             }
          }

          /* At this point I haven't emulated the filters.  Though I don't
             expect it to be complicated, I don't believe this feature is
             used much anyway.  I'll work on it later. */
          if ((next_event == CHAN1) || (next_event == CHAN3))
          {
             /* INSERT FILTER HERE */
          }

          /* if the current output bit is set */
          if (Outbit[next_event])
          {
             /* then set to the current volume */
             Outvol[next_event] = AUDC[next_event] & VOLUME_MASK;
          }
          else
          {
             /* set the volume to zero */
             Outvol[next_event] = 0;
          }
       }
       else /* otherwise we're processing a sample */
       {
          /* adjust the sample counter - note we're using the 24.8 integer
             which includes an 8 bit fraction for accuracy */
          *Samp_n_cnt += Samp_n_max;

          cur_val = 0;

          /* add the output values of all 4 channels */
          for (chan = CHAN1; chan <= CHAN4; chan++)
          {
             cur_val += Outvol[chan];
          }

          /* multiply the volume by 4 and add 8 to center around 128 */
          /* NOTE: this statement could be eliminated for efficiency, */
          /* though the volume would be lower. */
          cur_val = (cur_val << 2) + 8;
          
          /* add the current value to the output buffer */
          *buffer++ = cur_val;
          
          /* and indicate one less byte in the buffer */
          n--;          
       }
    }
}      

                              
/*****************************************************************************/
/* Module:  Pokey_process()                                                  */
/* Purpose: To fill the output buffer with the sound output based on the     */
/*          pokey chip parameters.  This routine has not been optimized.     */
/*          Though it is not used by the program, I've left it for reference.*/
/*                                                                           */
/* Author:  Ron Fries                                                        */
/* Date:    September 22, 1996                                               */
/*                                                                           */
/* Inputs:  *buffer - pointer to the buffer where the audio output will      */
/*                    be placed                                              */
/*          n - size of the playback buffer                                  */
/*                                                                           */
/* Outputs: the buffer will be filled with n bytes of audio - no return val  */
/*                                                                           */
/*****************************************************************************/

void Pokey_process (register unsigned char *buffer, register uint16 n)
{
    register uint32 *div_n_ptr;
    register uint32 *samp_cnt_w_ptr;
    register uint32 event_min;
    register uint8 next_event;
    register uint8 cur_val;
    register uint8 *out_ptr;
    register uint8 audc;
    register uint8 toggle;


    /* set a pointer to the whole portion of the samp_n_cnt */
    samp_cnt_w_ptr = (uint32 *)((uint8 *)(&Samp_n_cnt[0])+1);
   
    /* set a pointer for optimization */
    out_ptr = Outvol;

    /* The current output is pre-determined and then adjusted based on each */
    /* output change for increased performance (less over-all math). */
    /* add the output values of all 4 channels */
    cur_val  = 2;                 /* start with a small offset */
    cur_val += *out_ptr++;
    cur_val += *out_ptr++;
    cur_val += *out_ptr++;
    cur_val += *out_ptr++;

    /* loop until the buffer is filled */
    while (n)
    {
       /* Normally the routine would simply decrement the 'div by N' */
       /* counters and react when they reach zero.  Since we normally */
       /* won't be processing except once every 80 or so counts, */
       /* I've optimized by finding the smallest count and then */
       /* 'accelerated' time by adjusting all pointers by that amount. */

       /* find next smallest event (either sample or chan 1-4) */
       next_event = SAMPLE;
       event_min = *samp_cnt_w_ptr;

       /* Though I could have used a loop here, this is faster */
       div_n_ptr = Div_n_cnt;
       if (*div_n_ptr <= event_min)
       {
          event_min = *div_n_ptr;
          next_event = CHAN1;
       }
       div_n_ptr++;
       if (*div_n_ptr <= event_min)
       {
          event_min = *div_n_ptr;
          next_event = CHAN2;
       }
       div_n_ptr++;
       if (*div_n_ptr <= event_min)
       {
          event_min = *div_n_ptr;
          next_event = CHAN3;
       }
       div_n_ptr++;
       if (*div_n_ptr <= event_min)
       {
          event_min = *div_n_ptr;
          next_event = CHAN4;
       }

       /* decrement all counters by the smallest count found */
       /* again, no loop for efficiency */
       *div_n_ptr -= event_min;
       div_n_ptr--;
       *div_n_ptr -= event_min;
       div_n_ptr--;
       *div_n_ptr -= event_min;
       div_n_ptr--;
       *div_n_ptr -= event_min;

       *samp_cnt_w_ptr -= event_min;

       /* since the polynomials require a mod (%) function which is 
          division, I don't adjust the polynomials on the SAMPLE events,
          only the CHAN events.  I have to keep track of the change,
          though. */
       Poly_adjust += event_min;

       /* if the next event is a channel change */
       if (next_event != SAMPLE)
       {
          /* shift the polynomial counters */
          P4  = (P4  + Poly_adjust) % POLY4_SIZE;
          P5  = (P5  + Poly_adjust) % POLY5_SIZE;
          P17 = (P17 + Poly_adjust) % Poly17_size;
         
          /* reset the polynomial adjust counter to zero */
          Poly_adjust = 0;

          /* adjust channel counter */
          Div_n_cnt[next_event] += Div_n_max[next_event];

          /* get the current AUDC into a register (for optimization) */
          audc = AUDC[next_event];

          /* set a pointer to the current output (for opt...) */
          out_ptr = &Outvol[next_event];

          /* assume no changes to the output */
          toggle = FALSE;

          /* From here, a good understanding of the hardware is required */
          /* to understand what is happening.  I won't be able to provide */
          /* much description to explain it here. */

          /* if the output is pure or the output is poly5 and the poly5 bit */
          /* is set */
          if ((audc & NOTPOLY5) || bit5[P5])
          {
             /* if the PURE bit is set */
             if (audc & PURE)
             {               
                /* then simply toggle the output */
                toggle = TRUE;
             }
             /* otherwise if POLY4 is selected */
             else if (audc & POLY4)
             {
                /* then compare to the poly4 bit */
                toggle = (bit4[P4] == !(*out_ptr));
             }
             else
             {
                /* otherwise compare to the poly17 bit */
                toggle = (bit17[P17] == !(*out_ptr));
             }
          }

          /* At this point I haven't emulated the filters.  Though I don't
             expect it to be complicated, I don't believe this feature is
             used much anyway.  I'll work on it later. */
          if ((next_event == CHAN1) || (next_event == CHAN3))
          {
             /* INSERT FILTER HERE */
          }

          /* if the current output bit has changed */
          if (toggle)
          {
             if (*out_ptr)
             {
                /* remove this channel from the signal */
                cur_val -= *out_ptr;
                
                /* and turn the output off */
                *out_ptr = 0;
             }
             else
             {
                /* turn the output on */
                *out_ptr = audc & VOLUME_MASK;

                /* and add it to the output signal */
                cur_val += *out_ptr;
             }
          }
       }
       else /* otherwise we're processing a sample */
       {
          /* adjust the sample counter - note we're using the 24.8 integer
             which includes an 8 bit fraction for accuracy */
          *Samp_n_cnt += Samp_n_max;

          /* add the current value to the output buffer */
          *buffer++ = cur_val << 2;
          
          /* and indicate one less byte in the buffer */
          n--;          
       }
    }
}