From fdd867534a8f53ddb3f2845fc1dd48d13e734125 Mon Sep 17 00:00:00 2001
From: Gregory Maxwell <greg@xiph.org>
Date: Wed, 13 Apr 2011 17:08:22 -0400
Subject: [PATCH] Stop adjusting the VBR drift during silent frames, this
 prevents overshoot after silent spans. This also results in undershoot on
 files with lots of silence, but this is desirable in terms of increasing
 quality consistency. It will be even better if we later include some
 open-loop correction of the targets.

---
 libcelt/celt.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/libcelt/celt.c b/libcelt/celt.c
index 6331553f..d6c2b4dc 100644
--- a/libcelt/celt.c
+++ b/libcelt/celt.c
@@ -1425,17 +1425,23 @@ int celt_encode_with_ec_float(CELTEncoder * restrict st, const celt_sig * pcm, i
      nbAvailableBytes = IMAX(min_allowed,nbAvailableBytes);
      nbAvailableBytes = IMIN(nbCompressedBytes,nbAvailableBytes+nbFilledBytes) - nbFilledBytes;
 
-     if(silence)
-     {
-       nbAvailableBytes = 2;
-       target = 2*8<<BITRES;
-     }
-
      /* By how much did we "miss" the target on that frame */
      delta = target - vbr_rate;
 
      target=nbAvailableBytes<<(BITRES+3);
 
+     /*If the frame is silent we don't adjust our drift, otherwise
+       the encoder will shoot to very high rates after hitting a
+       span of silence, but we do allow the bitres to refill.
+       This means that we'll undershoot our target in CVBR/VBR modes
+       on files with lots of silence. */
+     if(silence)
+     {
+       nbAvailableBytes = 2;
+       target = 2*8<<BITRES;
+       delta = 0;
+     }
+
      if (st->vbr_count < 970)
      {
         st->vbr_count++;