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

[Christoph Gerstbauer]

> Then please don't use it, use an audiofilter instead.
> (The one that fits your needs.)
>
> I am not convinced that the current mapping of -ac
> to aresample is immutable.
> (Even if it is, you can still use the filter and
> you are on the safe side.)
>
> Carl Eugen

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.

If I have a target with less channels than the source I use the 
-map_channel option:
Like: ffmpeg -i "24bitaudio8CH.wav" -acodec pcm_s16le -map_channel 0.1.0 
-map_channel 0.1.1 -ac 2 "16bitaudio2CH_ch1ch2.wav"

So is this possible?:
ffmpeg -i "2ch_audio_source.wav" -i "mono-silence" -acodec pcm_s16le 
-map_channel 0.1.0 -map_channel 0.1.1 -map_channel 1.0.0 -map_channel 
1.0.0  "4ch_audio_target.wav"
Is there a filter which can generate silence as input source?

Best Regards
Christoph