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games/snes: use 4-point hermite interpolation to resample

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This is noticeably better than nearest-neighbor.
This commit is contained in:
Michael Forney 2021-02-22 16:12:24 +01:00
parent 619fbe051b
commit 472958e3e7
1 changed files with 25 additions and 6 deletions
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31
sys/src/games/snes/dsp.c
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@ -49,7 +49,6 @@ enum { RELEASE, ATTACK, DECAY, SUSTAIN };
enum { Freq = 44100 };
static s16int sbuf[2*2000], *sbufp;
static int stime;
static int fd;
void
@ -61,16 +60,36 @@ audioinit(void)
sbufp = sbuf;
}
static int
hermite(int *x, int t)
{
int y;
y = (x[0] - x[6]) / 2 + (x[4] - x[2]) * 3 / 2;
y = y * t >> 15;
y += x[6] - x[4] * 5 / 2 + x[2] * 2 - x[0] / 2;
y = y * t >> 15;
y += (x[2] - x[6]) / 2;
y = y * t >> 15;
y += x[4];
return y;
}
static void
audiosample(s16int *s)
{
stime -= 1<<16;
static int x[8], t;
x[0] = s[0];
x[1] = s[1];
do {
sbufp[0] = s[0];
sbufp[1] = s[1];
sbufp[0] = hermite(x, t);
sbufp[1] = hermite(x + 1, t);
sbufp += 2;
stime += (32000<<16)/Freq;
} while(stime < 0);
t += (32000<<15)/Freq;
} while(t < 1<<15);
t -= 1<<15;
memmove(x + 2, x, sizeof(x) - 2 * sizeof(x[0]));
}
int