[FFmpeg-user] Dim audio in parts where there is a lot of wind

[Paul B Mahol]

On Fri, Jun 2, 2023 at 7:05 PM MacFH - C E Macfarlane - News <
news at macfh.co.uk> wrote:

> On 02/06/2023 07:39, Cecil Westerhof via ffmpeg-user wrote:
> > German Geraskin <german.geraskin at gmail.com> writes:
> >
> >>> I have videos where the audio is mostly reasonable good, but sometimes
> >>> the wind creates havoc. Is there a way to dial the volume a good deal
> >>> down for those parts only?
> >>> It would be even better if that could be down in say half a second
> >>> instead of at once.
> >>
> >> Hi, Cecil
> >>
> >> You can use the volume audio filter.
> >> Example for Linux version of ffmpeg:
> >>
> >>    -af volume=0.5:enable='between(t\,26.12\,28.64)'
> >>
> >> It means - set volume to 0.5 between 26.12 and 28.64 seconds.
> >
> > Thanks, I will try it out. I can do this several times?
> > This is abrupt, there is no possibility to do it smooth?
>
> You can combine it with fades.  For example the following will fade out
> over 2s, mute for 2s, fade in for 2s:
>
> -af
>
> "afade=t=out:st=0:d=2,volume=enable='between(t,2,4)':volume=0,afade=t=in:st=4:d=2"
>
> Notes:
>
> I don't know if it's possible to fade to or from a given level other
> than zero, but perhaps, given the above as a starting point, someone
> else can help further with that.
>

If you use more recent version, there is silence and unity options to set:

https://ffmpeg.org/ffmpeg-filters.html#afade-1



>
> Using the fade and volume filters re-encodes the audio, which may be a
> problem if, as is usual, lossy rather than lossless encodings are being
> used.  You can simply alter the bit rate back to its original level, for
> example ...
>         -b:a 320k
> ... but with lossy encodings repeated re-encoding reduces quality
> somewhat with each iteration, so is something to be avoided.  To avoid
> this, you can separate the soundtrack into chunks such as ...
>
> 0:00-0:02       Temp1.* - Fade in, so re-encoded
> 0:02-5:00       Temp2.* - Good, original sound
> 5:00-5:02       Temp3.* - Fade out, so re-encoded
>
> ... then concatenate these chunks together using the concatenate filter:
>
>         -f concat -i ..\concat.txt
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