ref: e9d7842caf2f1cc698efc8ebea1672db5a1fb666
parent: 6f4f3e89c7dfef574f989ac0db1a51b63ed8b987
author: Jean-Marc Valin <jeanmarcv@google.com>
date: Sat Jun 29 21:42:06 EDT 2024
More accurate fixed-point sqrt() approximation
--- a/celt/mathops.c
+++ b/celt/mathops.c
@@ -126,7 +126,10 @@
int k;
opus_val16 n;
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)
return 0;
else if (x>=1073741824)
@@ -134,8 +137,8 @@
k = (celt_ilog2(x)>>1)-7;
x = VSHR32(x, 2*k);
n = x-32768;
- rt = ADD16(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])))))))));
+ 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, ADD16(C[4], MULT16_16_Q15(n, (C[5])))))))))));
rt = VSHR32(rt,7-k);
return rt;
}
--
⑨