[FFmpeg-user] Audio channel mapping of stereo channel without re-encode

Claudiu Rad-Lohanel jazzman at misalpina.net
Thu Aug 21 16:45:58 CEST 2014

On 8/21/2014 5:30 PM, Carl Eugen Hoyos wrote:
> Hanno Klamke <Hanno.Klamke <at> streamlab.net> writes:
>> I would like to transcode a video into several
>> outputs. From the source audio I would like to
>> take either only the left or the right channel
>> into my outputs. While only transcoding the
>> video and copy the audio.

I also confronted something similar a while ago.

> This cannot work.
> (And even if you know nothing about audio encoding,
> you should be able to imagine why it cannot work for
> codecs that actually do compress in a useful way.)

Indeed, you are right. But from what I know so far, this is because the 
level of data access that we have now is either at the stream level 
(audio, video, subtitles, etc) after demuxing, either somehow at the 
uncompressed frame level after decoding.
However, this doesn't change the fact that it would be really nice if we 
could have an intermediate access level in audio streams case (and I 
think this is the only one case where one stream actually packs multiple 
'substreams' at least in abstraction) and we could really remap channels 
as we like without reencoding.
But I assume that this would be a nightmare because it must be 
implemented at audio codec level, thus, for every codec alone..

However, it would be something nice for the future, would certainly 
optimize things.

> Carl Eugen
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> ffmpeg-user at ffmpeg.org
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