[FFmpeg-user] Preserving perceived loudness when downmixing audio from 5.1 AC3 to stereo AAC

Francois Visagie francois.visagie at gmail.com
Sat Aug 3 11:04:11 CEST 2013

> -----Original Message-----
> From: ffmpeg-user-bounces at ffmpeg.org [mailto:ffmpeg-user-
> bounces at ffmpeg.org] On Behalf Of Carl Eugen Hoyos
> Sent: 02 August 2013 20:01
> To: ffmpeg-user at ffmpeg.org
> Subject: Re: [FFmpeg-user] Preserving preserved loudness when
> downmixing audio from 5.1 AC3 to stereo AAC
> Francois Visagie <francois.visagie <at> gmail.com> writes:
> > . output from '-ac 2' is noticeably softer than the original
> >
> > . '-filter:a aformat=channel_layouts=stereo' seems to
> > have the same volume as the original
> >
> > . '-filter:a_aresample=ocl=3' is noticeably softer
> > than the original
> I tested the following three command lines with random
> 5.1 ac3 input:
> $ ffmpeg -i input -ac 2 out1.wav
> $ ffmpeg -i input -filter:a format=channel_layouts=stereo out2.wav

You mean aformat?

> $ ffmpeg -i input -filter:a aresample=ocl=3 out3.wav
> As expected, the three output files are bit-identical.

Thanks for doing this testing, which I repeated here.

n32-static.7z which I was using up to now, out2.wav differs from the other
two. However, it's the only one equally loud as the input file.

n32-static.7z the files are bit-identical as you found. However, all three
are now significantly softer than the original.

I don't think this is the desired outcome, to achieve unified audio
processing but with significant reduction in perceived volume in the
process. Is there any further testing I can do to help investigate and
hopefully improve this situation?

> (Both format and ac 2 auto-insert the aresample filter
> as loglevel debug will tell you.)
> The following makes a difference, it does only work
> for certain input codecs though (not aac):
> $ ffmpeg -request_channels 2 -i input out4.wav
> Carl Eugen
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