#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):
Attached the current WIP.
An explanation of what caused the bug for high q values: there was a bug
in psy's threshold reduction for hole avoidance. When a second pass was
needed, it would accumulate errors due to a simple typo (reduction +=
instead of reduction =).
I don't have the 3GPP spec to check, but I just noticed the code made no
sense with the +=, but did with =.
Then there's the ESC_BT thing.
I think most serious anomalies have been fixed in this bug, I haven't had
time to properly test CBR, but it seems to mostly work now. That was very
subtle bit reservoir a bug on my "lookahead" patch that didn't surface
until I fixed psy.
Anyway, I still would like to make VBR achieve lower bitrates without
having to resort to LP filtering. I somehow sense it should be possible.
In any case, I made CBR also use the same scalefactor-band-based LP
filtering to remove the need for the butterworth that didn't save many
bits anyway, and now it responds to the -cutoff argument, so if you don't
like the default cutoff you can override yourself. It seemed worth
parameterizing since I've found some sources that sound better at low bit
rates with higher cutoffs, and some that don't. So it's source-dependent.
Anyway, enjoy the patch, I'm not sure I'll have time to work on a more
permanent (one that I'd push to trunk) one till next weekend.
--
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2686#comment:115>
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