Hi all,
I was investigating the behaviour of the -p switch of flac. This
switch should enable exhaustive search for the optimal qlp
coefficient precision, but the resulting files are usually 0.5%
larger then when not using the switch. I stumbled upon this code
in stream_encoder
if(encoder->protected_->do_qlp_coeff_prec_search) {
min_qlp_coeff_precision = FLAC__MIN_QLP_COEFF_PRECISION;
/* try to ensure a 32-bit datapath throughout for
16bps(+1bps for side channel) or less */
if(subframe_bps <= 17) {
max_qlp_coeff_precision = flac_min(32 - subframe_bps -
lpc_order, FLAC__MAX_QLP_COEFF_PRECISION);
max_qlp_coeff_precision =
flac_max(max_qlp_coeff_precision, min_qlp_coeff_precision);
}
else
max_qlp_coeff_precision = FLAC__MAX_QLP_COEFF_PRECISION;
}
else {
min_qlp_coeff_precision = max_qlp_coeff_precision =
encoder->protected_->qlp_coeff_precision;
}
However, a little further, I found similar (but slightly
different)code
/* try to keep qlp coeff precision such that only 32-bit math
is required for decode of <=16bps streams */
if(subframe_bps <= 16) {
FLAC__ASSERT(order > 0);
FLAC__ASSERT(order <= FLAC__MAX_LPC_ORDER);
qlp_coeff_precision = flac_min(qlp_coeff_precision, 32 -
subframe_bps - FLAC__bitmath_ilog2(order));
}
The difference here is the ilog2 function. I presume this is a
mistake, because subtracting the order (32 - 17 - 8) leaves few
possible precision steps to try. Because the first 'quote' of
the code is always followed by the second, I attach a patch that
makes the first match the second.
>From e7f3dccb36b132f6d94608f069e3e59f7224cc6c Mon Sep 17 00:00:00 2001
From: Martijn van Beurden <[email protected]>
Date: Mon, 28 Jul 2014 13:04:56 +0200
Subject: [PATCH] Fix bug when using -p switch during compression
When using the -p switch during encoding, the encoder should try
different qlp predictor precision steps. However, some faulty code
was too severely restricting the possible steps. This patch lifts
the restriction to match a restriction coded a little further in
the process. This doesn't make using -p worth your while, but at
least it doesn't create larger files now
---
src/libFLAC/stream_encoder.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/src/libFLAC/stream_encoder.c b/src/libFLAC/stream_encoder.c
index 9dfb0c5..8d4cfcc 100644
--- a/src/libFLAC/stream_encoder.c
+++ b/src/libFLAC/stream_encoder.c
@@ -3428,9 +3428,9 @@ FLAC__bool process_subframe_(
}
if(encoder->protected_->do_qlp_coeff_prec_search) {
min_qlp_coeff_precision = FLAC__MIN_QLP_COEFF_PRECISION;
- /* try to ensure a 32-bit datapath throughout for 16bps(+1bps for side channel) or less */
- if(subframe_bps <= 17) {
- max_qlp_coeff_precision = flac_min(32 - subframe_bps - lpc_order, FLAC__MAX_QLP_COEFF_PRECISION);
+ /* try to keep qlp coeff precision such that only 32-bit math is required for decode of <=16bps streams */
+ if(subframe_bps <= 16) {
+ max_qlp_coeff_precision = flac_min(32 - subframe_bps - FLAC__bitmath_ilog2(lpc_order), FLAC__MAX_QLP_COEFF_PRECISION);
max_qlp_coeff_precision = flac_max(max_qlp_coeff_precision, min_qlp_coeff_precision);
}
else
--
1.9.1
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