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

[Christoph Gerstbauer]

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 
stream.
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.
But how can I defind a second audio input stream in pan? I think this is 
not possible. In the examples there is no hint for that.

Am I right?

mfg chris