[FFmpeg-user] Sound level

[Tom Evans]

On Thu, Feb 16, 2012 at 2:44 PM, John Saturday <stozher at gmail.com> wrote:
> Hi jacky,
>
> I have use this command for normalization of sound level at 89 dB.
>
>…
> 2. Commpress to FLAC and view needed replaygain:
>
> flac --replay-gain file.wav
> metaflac --show-tag=REPLAYGAIN_TRACK_GAIN file.flac
>
> Otput from METAFLAC is a like this:
>
> REPLAYGAIN_TRACK_GAIN=+20.91 dB
>
> 3. Open calculator in advanced mode and calculate FFMPEG volume:
>
> volume=256*10^(<gain in dB>/20)=256*10^(20.91/20)=2842.76 ~ 2843
>
> (I'm not sure how to write this in English: 2^2=2x2=4, 2^3=2x2x2=8.
> Other example with negative gain: volume=256*10^(-2.57/20)=190.4 ~
> 190.)
>
> 4. Reencode only audio stream with FFMPEG:
>
> ffmpeg -i file.flv -f mp4 -c:v copy -c:a libfaac -vol:a 2843 -b:a 128k
> output.mp4

Why the hard to understand calculation, and not:

ffmpeg -c:a libfaac -b:a 128k -filter:a volume=-12dB

Cheers

Tom