From 92282c1ae42187c3a5606cddd8cddabdcc7ed8bd Mon Sep 17 00:00:00 2001
From: Jean-Marc Valin <jmvalin@jmvalin.ca>
Date: Wed, 2 Nov 2016 17:59:32 -0400
Subject: [PATCH] Improves CELT bit allocation at very low bitrates
We now try not to fold below band 17 since that produces a lot of harshness.
This mostly helps around 32-40 kb/s.
---
celt/rate.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)
diff --git a/celt/rate.c b/celt/rate.c
index 7dfa5be8..ca4cc870 100644
--- a/celt/rate.c
+++ b/celt/rate.c
@@ -348,12 +348,17 @@ static OPUS_INLINE int interp_bits2pulses(const CELTMode *m, int start, int end,
/*This if() block is the only part of the allocation function that
is not a mandatory part of the bitstream: any bands we choose to
skip here must be explicitly signaled.*/
- /*Choose a threshold with some hysteresis to keep bands from
- fluctuating in and out.*/
+ int depth_threshold;
+ /*We choose a threshold with some hysteresis to keep bands from
+ fluctuating in and out, but we try not to fold below a certain point. */
+ if (codedBands > 17)
+ depth_threshold = j<prev ? 7 : 9;
+ else
+ depth_threshold = 0;
#ifdef FUZZING
if ((rand()&0x1) == 0)
#else
- if (codedBands<=start+2 || (band_bits > ((j<prev?7:9)*band_width<<LM<<BITRES)>>4 && j<=signalBandwidth))
+ if (codedBands<=start+2 || (band_bits > (depth_threshold*band_width<<LM<<BITRES)>>4 && j<=signalBandwidth))
#endif
{
ec_enc_bit_logp(ec, 1, 1);
--
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