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games/snes: implement DSP noise

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I'm not sure if this LFSR is the same one used by the hardware or is
arbitrary, but it matches the noise sequence used by all other snes
emulators I looked at.
This commit is contained in:
Michael Forney 2021-02-17 11:19:50 +01:00
parent 52b54097bf
commit f1d29a9d12
1 changed files with 3 additions and 1 deletions
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4
sys/src/games/snes/dsp.c
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@ -6,7 +6,7 @@
#include "fns.h"
u8int dsp[256], dspstate;
u16int dspcounter, noise;
u16int dspcounter, noise = 0x8000;
static s16int samp[2], echoin[2];
enum {
@ -517,6 +517,8 @@ dspstep(void)
}
if(dspcounter-- == 0)
dspcounter = 0x77ff;
if(envyes(dsp[FLG] & 0x1f))
noise = (noise << 13 ^ noise << 14) & 0x8000 | noise >> 1 & ~1;
break;
case 31: voice(0, 4); voice(2, 1); break;
}