[FFmpeg-user] Audio automatically resampled to 8bit from 24bit when using concat and lavfi nullsrc

[Paul B Mahol]

On Sat, Apr 9, 2022 at 7:29 PM morgan holly via ffmpeg-user <
ffmpeg-user at ffmpeg.org> wrote:

>
> > On Apr 9, 2022, at 11:10 AM, Paul B Mahol <onemda at gmail.com> wrote:
> >
> >
> >
> > On Sat, Apr 9, 2022 at 5:22 PM morgan holly via ffmpeg-user <
> ffmpeg-user at ffmpeg.org <mailto:ffmpeg-user at ffmpeg.org>> wrote:
> > this is part question and part warning. When I run the cmd below FFMPEG
> automatically knocks my 24bit source audio down to 8bits(!). I was able to
> change that by adding 'aformat=sample_fmts=s32' into the filterchain for
> the anullsrc inputs.
> >
> > The warning is: make sure you check your output quality if you use lavfi
> slugs with your audio. 24->8bits, yikes.
> >
> > The question is: Can I specify the format of the nullsource when using
> lavfi. Seems dangerous that it defaults to fmt:u8. Am I doing something
> wrong?
> >
> > You are not setting aformat right after anullsrc filter.
> >
>
> Thanks for the response. I googled it out and ended up with this:
>
> ffmpeg -y -f lavfi -t 1.724 -i
> anullsrc=channel_layout=mono:sample_rate=48000,aformat=s32 -ss 120 -t
> 30.327979 -i /tmp/2ch.mov -f lavfi -t 1.724 -i
> anullsrc=channel_layout=mono:sample_rate=48000,aformat=s32 -filter_complex
> "[0:a:0][1:a:0][2:a:0]concat=n=3:v=0:a=1[sf1_out]" -map [sf1_out]
> /tmp/noisetest.wav
>
> It works, but it does seem odd that the default bit depth of nullsrc would
> be 8.
>

That is because your source is not connected with rest of filters, so
guessing best format never can happen.

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