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Does the AC3 audio format support variable bitrate encoding?
If so, is it possible to use FFmpeg's -vbr option to achieve that?

I'd like to avoid converting from 5.1 to a 2-channel stereo because I've read that FFmpeg doesn't produce good results when mixing the channels.

  • Where did you read that at? Can you elaborate? – llogan Aug 15 '15 at 4:21
  • It's a problem with the option -ac 2 that's needed for downmixing 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? – GetFree Aug 15 '15 at 7:10
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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.

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