From e2a6aaa73158d4e9415a7e053f84f83314e2d9e3 Mon Sep 17 00:00:00 2001
From: "Timothy B. Terriberry" <tterribe@xiph.org>
Date: Wed, 12 Oct 2011 14:24:19 -0400
Subject: [PATCH] Improves accuracy of NLSF2A by reordering the polynomial
roots
---
silk/NLSF2A.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/silk/NLSF2A.c b/silk/NLSF2A.c
index d0a8667bc..d43b61dec 100644
--- a/silk/NLSF2A.c
+++ b/silk/NLSF2A.c
@@ -69,6 +69,15 @@ void silk_NLSF2A(
const opus_int d /* I filter order (should be even) */
)
{
+ /* This ordering was found to maximize quality. It improves numerical accuracy of
+ silk_NLSF2A_find_poly() compared to "standard" ordering. */
+ static const unsigned char ordering16[16] = {
+ 0, 15, 8, 7, 4, 11, 12, 3, 2, 13, 10, 5, 6, 9, 14, 1
+ };
+ static const unsigned char ordering10[10] = {
+ 0, 9, 6, 3, 4, 5, 8, 1, 2, 7
+ };
+ const unsigned char *ordering;
opus_int k, i, dd;
opus_int32 cos_LSF_QA[ SILK_MAX_ORDER_LPC ];
opus_int32 P[ SILK_MAX_ORDER_LPC / 2 + 1 ], Q[ SILK_MAX_ORDER_LPC / 2 + 1 ];
@@ -80,6 +89,7 @@ void silk_NLSF2A(
silk_assert( d==10||d==16 );
/* convert LSFs to 2*cos(LSF), using piecewise linear curve from table */
+ ordering = d == 16 ? ordering16 : ordering10;
for( k = 0; k < d; k++ ) {
silk_assert(NLSF[k] >= 0 );
silk_assert(NLSF[k] <= 32767 );
@@ -98,7 +108,7 @@ void silk_NLSF2A(
delta = silk_LSFCosTab_FIX_Q12[ f_int + 1 ] - cos_val; /* Q12, with a range of 0..200 */
/* Linear interpolation */
- cos_LSF_QA[k] = silk_RSHIFT_ROUND( silk_LSHIFT( cos_val, 8 ) + silk_MUL( delta, f_frac ), 20 - QA ); /* QA */
+ cos_LSF_QA[ordering[k]] = silk_RSHIFT_ROUND( silk_LSHIFT( cos_val, 8 ) + silk_MUL( delta, f_frac ), 20 - QA ); /* QA */
}
dd = silk_RSHIFT( d, 1 );
--
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