[FFmpeg-user] 5.1 sound aac

Carl Eugen Hoyos cehoyos at ag.or.at
Mon Nov 21 23:06:17 CET 2011

Adam Kłobukowski <adamklobukowski <at> gmail.com> writes:

> ffmpeg -threads 4 -i video.mkv -vcodec copy -acodec aac -ab 384k -strict 
> experimental -f mp4 video.mp4

> But, it is wrong. I'n fact, with 5.1 sound bitrate is divided per 
> subchannel,

I don't think this is correct.

> so effective bitrate is much lower (and it can be heard).

> There are 2 possibilities:
> 1. ffprobe reports wrong bitrate

Very unlikely

> 2. ffmpeg uses wrong bitrate

Very possible, but please note that ffmpeg tells you the effective bitrate, so
it should be easy for you to tell

> 3. I'm wrong and do not understand what it all means.

What is most important is that you realize the reason you have to use -strict
experimental is that the native AAC encoder (as opposed to the three alternative
AAC encoders) is experimental and does not necessarily produce good results,
this may be more noticeable for multichannel.
I always use high bitrates when I test the native AAC encoder, for
multi-channel, you may test 0.8.7, it may be slightly better than current git
(Please report if you can reproduce!)

What I forgot when this was discussed last week is that work on the native AAC
encoder is as welcome as sponsoring work on the native AAC encoder!

Carl Eugen

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