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

[Paul B Mahol]

On 7/2/15, Christoph Gerstbauer <christophgerstbauer at gmail.com> wrote:
>
>
> Am 02.07.2015 um 11:04 schrieb Paul B Mahol:
>> 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.
>
> Is anullsrc filter about to be debrecated in some years?
> I think anullsrc will be debrecated because aevalsrc offers mor e
> possibilities to generate sound or silence.
> And anullsrc only generates silence.
>
> Is this correct?

Hmm, I do not think it will get deprecated. It should not get deprecated because
anullsrc is near 10x times faster than aevalsrc in generating silence.

> So would it be better to use aevalsrc (just for generating silence), or
> are there any known bugs at "aevalsrc"?

For generating silence anullsrc is much faster.

>
> Best Regards
> Christoph
>
>>
>>> Best Regards
>>> Christoph
>>>
>>>
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