sn76496u.c

   1 {
   2         int i;
   3         DATATYPE *buf = (DATATYPE *)buffer;
   4         struct SN76496 *R = &sn[chip];
   5
   6
   7         /* If the volume is 0, increase the counter */
   8         for (i = 0;i < 4;i++)
   9         {
  10                 if (R->Volume[i] == 0)
  11                 {
  12                         /* note that I do count += length, NOT count = length + 1. You might think */
  13                         /* it's the same since the volume is 0, but doing the latter could cause */
  14                         /* interferencies when the program is rapidly modulating the volume. */
  15                         if (R->Count[i] <= length*STEP) R->Count[i] += length*STEP;
  16                 }
  17         }
  18
  19         while (length > 0)
  20         {
  21                 int vol[4];
  22                 unsigned int out;
  23                 int left;
  24
  25
  26                 /* vol[] keeps track of how long each square wave stays */
  27                 /* in the 1 position during the sample period. */
  28                 vol[0] = vol[1] = vol[2] = vol[3] = 0;
  29
  30                 for (i = 0;i < 3;i++)
  31                 {
  32                         if (R->Output[i]) vol[i] += R->Count[i];
  33                         R->Count[i] -= STEP;
  34                         /* Period[i] is the half period of the square wave. Here, in each */
  35                         /* loop I add Period[i] twice, so that at the end of the loop the */
  36                         /* square wave is in the same status (0 or 1) it was at the start. */
  37                         /* vol[i] is also incremented by Period[i], since the wave has been 1 */
  38                         /* exactly half of the time, regardless of the initial position. */
  39                         /* If we exit the loop in the middle, Output[i] has to be inverted */
  40                         /* and vol[i] incremented only if the exit status of the square */
  41                         /* wave is 1. */
  42                         while (R->Count[i] <= 0)
  43                         {
  44                                 R->Count[i] += R->Period[i];
  45                                 if (R->Count[i] > 0)
  46                                 {
  47                                         R->Output[i] ^= 1;
  48                                         if (R->Output[i]) vol[i] += R->Period[i];
  49                                         break;
  50                                 }
  51                                 R->Count[i] += R->Period[i];
  52                                 vol[i] += R->Period[i];
  53                         }
  54                         if (R->Output[i]) vol[i] -= R->Count[i];
  55                 }
  56
  57                 left = STEP;
  58                 do
  59                 {
  60                         int nextevent;
  61
  62
  63                         if (R->Count[3] < left) nextevent = R->Count[3];
  64                         else nextevent = left;
  65
  66                         if (R->Output[3]) vol[3] += R->Count[3];
  67                         R->Count[3] -= nextevent;
  68                         if (R->Count[3] <= 0)
  69                         {
  70                                 if (R->RNG & 1) R->RNG ^= R->NoiseFB;
  71                                 R->RNG >>= 1;
  72                                 R->Output[3] = R->RNG & 1;
  73                                 R->Count[3] += R->Period[3];
  74                                 if (R->Output[3]) vol[3] += R->Period[3];
  75                         }
  76                         if (R->Output[3]) vol[3] -= R->Count[3];
  77
  78                         left -= nextevent;
  79                 } while (left > 0);
  80
  81                 out = vol[0] * R->Volume[0] + vol[1] * R->Volume[1] +
  82                                 vol[2] * R->Volume[2] + vol[3] * R->Volume[3];
  83
  84                 if (out > MAX_OUTPUT * STEP) out = MAX_OUTPUT * STEP;
  85
  86     *(buf++) = ((DATACONV(out)*3)>>3); // Dave: a bit quieter
  87
  88                 length--;
  89         }
  90 }