[FFmpeg-user] Need more detailed description of the behaviour of the "-ac <channelcount>" option

Francois Visagie francois.visagie at gmail.com
Fri Jul 3 09:59:31 CEST 2015

> -----Original Message-----
> From: ffmpeg-user-bounces at ffmpeg.org [mailto:ffmpeg-user-
> bounces at ffmpeg.org] On Behalf Of Christoph Gerstbauer
> Sent: 03 July 2015 08:36
> To: FFmpeg user questions
> Subject: Re: [FFmpeg-user] Need more detailed description of the behaviour
> of the "-ac <channelcount>" option
> Am 02.07.15 um 12:49 schrieb Nicolas George:
> > Le quartidi 14 messidor, an CCXXIII, Christoph Gerstbauer a écrit :
> >> Hello Carl, so what can I do when my target has more audio channel
> >> than the source?
> >> For example if I have a file with 2ch stereo and my target is 4ch
> >> (with ch3 and ch4 silent) how can i do that? I need to generate
> >> silence for the last 2 channels because I dont want to copy ch1ch2 to
> ch3ch4.
> Hi Nicolas!
> I tried the pan filter, it works very well when using ONE audio source
> For example I use this syntax to spread 2ch source to 8ch target:
> -af "pan=8c|c0=c0|c1=c1|c2=c0|c3=c1|c4=c0|c5=c1|c6=c0|c7=c1"
> But what can I do when I want this?
> c1=c1
> c2=c2
> c3=silent
> c4=silent
> c5=silent
> c6=silent
> c7=silent
> c8=silent
> I tried to use the anullsrc (to generate silence), but to use that
anullsrc would
> be a second audio input stream.

Although I haven't tried it in this context, I suspect the '-map' option
should enable you to specify treatment for individual input streams.

> But how can I defind a second audio input stream in pan? I think this is
> possible. In the examples there is no hint for that.
> Am I right?
> mfg chris
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