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

[Nicolas George]

Le decadi 20 thermidor, an CCXXI, Francois Visagie a écrit :
> I'm not sure even -request_channels produces the expected result. It merely
> seems to influence the number of input channels guessed:

If it did that, the sound would be completely garbled.

> Input #0, ac3, from 'in.ac3':
>   Duration: 00:00:09.02, start: 0.000000, bitrate: 448 kb/s
>     Stream #0:0: Audio: ac3, 48000 Hz, 5.1(side), fltp, 448 kb/s

Normal.

> Guessed Channel Layout for  Input Stream #0.0 : stereo

You can safely ignore this particular message, it just means that something
somewhere set channels to 2 but neglected to set channel_layout to stereo.

> Input #0, ac3, from 'in.ac3':
>   Duration: 00:00:09.02, start: 0.000000, bitrate: 448 kb/s
>     Stream #0:0: Audio: ac3, 48000 Hz, stereo, fltp, 448 kb/s

Normal.

> Would it therefore be correct to assume that -request_channels leads to only
> that number of channels being extracted, hence no down-mix?

No. -request_channels uses codec-specific ways of extracting sound with the
specified number of channels. It only works for very few codecs that have
that feature.

Regards,

-- 
  Nicolas George
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