[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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