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More accurate fixed-point sqrt() approximation

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Jean-Marc Valin 2024-06-30 01:42:06 -04:00
parent 6f4f3e89c7
commit e9d7842caf
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GPG key ID: 8D2952BBB52C646D
1 changed files with 6 additions and 3 deletions
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9
celt/mathops.c
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@ -126,7 +126,10 @@ opus_val32 celt_sqrt(opus_val32 x)
int k; int k;
opus_val16 n; opus_val16 n;
opus_val32 rt; opus_val32 rt;
static const opus_val16 C[5] = {23175, 11561, -3011, 1699, -664}; /* These coeffs are optimized in fixed-point to minimize both RMS and max error
of sqrt(x) over .25<x<1 without exceeding 32767.
The RMS error is 3.4e-5 and the max is 8.2e-5. */
static const opus_val16 C[6] = {23171, 11574, -2901, 1592, -1002, 336};
if (x==0) if (x==0)
return 0; return 0;
else if (x>=1073741824) else if (x>=1073741824)
@ -134,8 +137,8 @@ opus_val32 celt_sqrt(opus_val32 x)
k = (celt_ilog2(x)>>1)-7; k = (celt_ilog2(x)>>1)-7;
x = VSHR32(x, 2*k); x = VSHR32(x, 2*k);
n = x-32768; n = x-32768;
rt = ADD16(C[0], MULT16_16_Q15(n, ADD16(C[1], MULT16_16_Q15(n, ADD16(C[2], rt = ADD32(C[0], MULT16_16_Q15(n, ADD16(C[1], MULT16_16_Q15(n, ADD16(C[2],
MULT16_16_Q15(n, ADD16(C[3], MULT16_16_Q15(n, (C[4]))))))))); MULT16_16_Q15(n, ADD16(C[3], MULT16_16_Q15(n, ADD16(C[4], MULT16_16_Q15(n, (C[5])))))))))));
rt = VSHR32(rt,7-k); rt = VSHR32(rt,7-k);
return rt; return rt;
} }