[FFmpeg-user] amix: Hard limit for number of inputs?

Reino Wijnsma rwijnsma at xs4all.nl
Tue Jul 16 19:52:34 EEST 2019

On 16-7-2019 17:45, Paul B Mahol <onemda at gmail.com> wrote:
> On 7/16/19, Reino Wijnsma <rwijnsma at xs4all.nl> wrote:
>> I can't use these filters. Have a look at the Youtube video please.
>> I wish it was a simple matter of just concatenating all segments, but it
>> isn't, because the segments don't align perfectly.
>> Each segments has to start at a very specific time.
>> Then it's a matter of mixing everything at full volume(!), so there's no
>> fading involved at all.
> acrossfade can just *join* it with nofade curve, which basically disables fading.

But that's the point, I need to *mix*all segments, not join/concatthem.

> On 7/16/19, Reino Wijnsma <rwijnsma at xs4all.nl> wrote:
>> Do you happen to know why ffmpeg reports a completely wrong duration?
>> Related question/suggestion:
>> It's really cumbersome to always having to specify the delay for all(!)
>> channels: [1]adelay=158792S|158792S[E2];
>> How about a default behaviour where specifying an amount of
>> milliseconds/samples once(!) applies to all channels: [1]adelay=158792S[E2];
>> And if you want a specific delay for another channel, then you can do so as
>> is already possible.
>> This would make my commandline a lot shorter!
> The thing with amix is that by *reducing volume* by mixing and adding it
> back with volume,
> you basically loose quality. It is more obvious with float sample
> format then double one.

I'm actually trying to prefend volume reduction. I only want to mix all segments.

(I noticed I forgot to attach the Avisynth script I used years ago. Attached now.)
In the Avisynth script I align all segments (with sample precision) and mix everything with full volume: MixAudio(clip1,clip2,1.0,1.0)

ffmpeg.exe -h filter=amix
  weights           <string>     ..F.A.... Set weight for each input. (default "1 1")

I figure "1 1" does the same as MixAudio(clip1,clip2,1.0,1.0), doesn't it? But amix still reduces the volume dramatically.

Let me ask you this: How can I do MixAudio(clip1,clip2,1.0,1.0) with ffmpeg leaving the volume completely untouched?

> Option could be added to amix filter to just sum samples as is.

You mean the adelay filter, right?

> Also what ffmpeg version you are using?
Please see my first post.

On 16-7-2019 18:36, Paul B Mahol <onemda at gmail.com> wrote:
> Actually ffmpeg is behaving correctly, your [29]adelay is missing one S.
Well I'll be darned. A typo. Thanks!
That solves that problem, but the volume issue remains.

-- Reino
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