#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 Kamedo2):
> > Then, search a number of q that have the desired bitrate. Then, make
sure that average tested sample bitrate isn't very far from the "standard"
bitrate.
>
> Just '''how''' do you check bit rate? Because I've noticed {{{ffmpeg -i
file}}} tends to give bogus rates when used on VBR-encoded files (not even
average).
{{{filesize[Byte]*8/Sample_length[Sec]}}}, But be careful of very short
files, it can be bogus too.
> > Also, I think it's beneficial for the end users to set the -q:a value
and typically gets a file with the bitrate around the set value. If one
sets -q:a 256k, one gets a file of roughly 256kbps.
>
> That's not doable without refactoring ffmpeg. -q:a sets the
global_quality parameter, which is specified to have a somewhat
standardized interpretation (1.0 = 100%, what 100% means is what some
other codec means by it, can't remember which OTOMH).
Is LAME breaking the convention?
https://trac.ffmpeg.org/wiki/Encoding%20VBR%20%28Variable%20Bit%20Rate%29%20mp3%20audio
> However, you can get (I think) a similar result by specifying both -q:a
and -b:a, like so:
>
> {{{
> ffmpeg -i somefile.flac -c:a aac -b:a 256k -q:a 1 -strict experimental
somefile.aac
> }}}
>
> Although that seldom gives you 256k. The bitrate there is like a lower
bound (aim for 256k, spend more if needed).
Thank you for the info. Your behavior seems much like the cvbr(most used
mode), Apple iTunes.
--
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2686#comment:170>
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