Ignoring standards
Oh, that awful, misleading thread. The ATSC A/52A (AC-3) standard (refer to sections 6.1.12 & 7.8, and also this post by pandy) provides detailed info about how proper downmixing should occur for AC-3.
Note that the Low Frequency Effects (LFE) channel may be omitted when downmixing, and I believe that's what ffmpeg
does.
Changing default downmix method
If you want to change the downmix method then try the suggestion by pandy in the same thread:
-af aformat=sample_fmts=fltp:channel_layouts=downmix
I did not test this, so I'm not sure it will do what you want.
Also, investigate the -dmix_mode
private option. See ffmpeg -h encoder=ac3
.
VBR
The -vbr
option is used as a "private" encoding option for some encoders, not as a global option:
Private options are specific to the given container, device or codec.
These options are listed under their corresponding containers/devices/codecs.
...meaning the use of this option and what it does may vary depending on the encoder; if the encoder even supports this option (refer to ffmpeg -h encoder=<encoder name>
). Other audio encoders may use -qscale:a
(or -q:a
) for VBR.
The encoder ac3
ignores both -vbr
and -qscale:a
/-q:a
.
Where did you read that at? Can you elaborate? – llogan – 2015-08-15T04:21:19.283
It's a problem with the option
– GetFree – 2015-08-15T07:10:00.463-ac 2
that's needed for downmixing https://ffmpeg.zeranoe.com/forum/viewtopic.php?f=17&t=1851. Either way, the question still holds, does FFmpeg support variable bitrate for the AC3 format?