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

Paul B Mahol onemda at gmail.com
Thu Jul 2 11:04:56 CEST 2015

On 7/2/15, Christoph Gerstbauer <christophgerstbauer at gmail.com> wrote:
>> 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?

See anullsrc and aevalsrc.

> Best Regards
> Christoph
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