[FFmpeg-trac] #2686(avcodec:open): Native AAC encoder collapses at high bitrates on some samples

FFmpeg trac at avcodec.org
Mon Jul 29 20:01:19 CEST 2013


#2686: Native AAC encoder collapses at high bitrates on some samples
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             Reporter:  Kamedo2      |                    Owner:
                 Type:  defect       |                   Status:  open
             Priority:  normal       |                Component:  avcodec
              Version:  git-master   |               Resolution:
             Keywords:  aac          |               Blocked By:
  regression                         |  Reproduced by developer:  1
             Blocking:               |
Analyzed by developer:  0            |
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Comment (by klaussfreire):

 Yes, I was in the middle of tweaking rdlambda scale for VBR (which is what
 gives the tonality boost). It seems way off target for VBR, since a lambda
 that in VBR results in 64kbps, in CBR it will give you about 32 or less.

 With that properly tweaked, we can save lots of bits from noisy bands and
 put them to better use on tonal bands. For VBR, that means lower bitrates
 for the same quality level.

 Increasing cutoff like you did there has the unwanted side effect of
 lowering quality a bit too much on tonal bands, for a set file size. I do
 my tests by searching through -q:a until I get a file roughly the same
 size as a reference CBR-encoded version, and comparing quality among
 those. With higher cutoffs, that procedure resulted in noticeable
 distortion on the HF bands, which is why I left it at 1.6, and it's what I
 believe will be fixed by tweaking rdlambda for VBR.

 It can also be fixed by implementing codebook 13. But that's for another
 (future, way future) patch, since I see no easy way to implement CB 13
 with twoloop, so I'll have to rewrite it.

-- 
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2686#comment:119>
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