From 78c892284150e12f6b08b287bdf7e62307c6985f Mon Sep 17 00:00:00 2001 From: Niels Möller Date: Thu, 12 Mar 2015 13:26:40 +0100 Subject: dca: Document the permuted order in the lfe_fir_64 and lfe_fir_128 tables --- libavcodec/dcadata.c | 28 +++++++++++++++++++++++++++- 1 file changed, 27 insertions(+), 1 deletion(-) (limited to 'libavcodec/dcadata.c') diff --git a/libavcodec/dcadata.c b/libavcodec/dcadata.c index 4c69891ca3..47fb9c53c1 100644 --- a/libavcodec/dcadata.c +++ b/libavcodec/dcadata.c @@ -7295,7 +7295,21 @@ DECLARE_ALIGNED(16, const float, ff_dca_fir_32bands_nonperfect)[512] = { +1.390191784E-007 }; -/* pre-scale lfe fir coefficients */ +/* pre-scale lfe fir coefficients + * + * Permuted compared to table D.8 in the spec. First, the table in the + * spec has twice the number of coefficients, but they are reflected, + * T[i] = T[511 - i]. Furthermore, the indices are permuted, so that + * + * lfe_fir_64[8 * k + j] = T[64 * j + k] + * + * for 0 <= k < 32 and 0 <= j < 8. (This corresponds to adding a + * leading zero to the 8-bit index, and rotating it right by 3 bits + * as a 9-bit field). Values with j >= 4 can be reflected, as + * + * lfe_fir_64[8 * k + j] = T[64 * (7 - j) + (63 - k)] + * + * so all values in T get included. */ #define SCALE(c) ((c) / (256.0f * 32768.0f)) DECLARE_ALIGNED(16, const float, ff_dca_lfe_fir_64)[256] = { SCALE(2.658434386830777e-4), SCALE(9.029330685734748e-3), @@ -7428,6 +7442,18 @@ DECLARE_ALIGNED(16, const float, ff_dca_lfe_fir_64)[256] = { SCALE(3.165979683399200e-2), SCALE(1.527829794213176e-3), }; +/* Permuted compared to table D.8 in the spec. First, the table in the + * spec has twice the number of coefficients, but they are reflected, + * T[i] = T[511 - i]. Furthermore, the indices are permuted, so that + * + * lfe_fir_128[4 * k + j] = T[128 * j + k] + * + * for 0 <= k < 64 and 0 <= j < 4. Values with j >= 2 can be + * reflected, as + * + * lfe_fir_128[4 * k + j] = T[128 * (3 - j) + (127 - k)] + * + * so all values in T get included. */ DECLARE_ALIGNED(16, const float, ff_dca_lfe_fir_128)[256] = { SCALE(0.00053168571), SCALE(0.15878495574), SCALE(0.68603444099), SCALE(0.15492856503), SCALE(0.00016358691), SCALE(0.16269733012), SCALE(0.68591803312), SCALE(0.15112841129), -- cgit v1.2.3