[FFmpeg-user] split audio/video and merge later gets async

[Frank Tetzel]

Hi,

i've recorded a video with ffmpeg:
$ ffmpeg -f x11grab -r 25 -s 798x438 -i :0.0+1,461 -f alsa -ac 1 -ar
44100 -i default:CARD=PCH -c:v libx264 -crf 0 -preset ultrafast -c:a
flac grab.mkv

Now, i want to process the audio outside of ffmpeg with sox for noise
reduction and encode it with neroaac. After that i want to merge audio
and video with ffmpeg again. But the result is always async
(progressively getting more).

Even if i don't process the audio file (except convert to wav). Just
splitting and merging gets out of sync.
$ ffmpeg -i grab.mkv -vn -c:a pcm_s16le grab-aud.wav
$ ffmpeg -i grab.mkv -i grab-aud.wav -map 0:0 -map 1:0 -c copy merge.mkv

I guess the problem is the conversion to wav, because if i copy the
flac stream it works, but can't be processed by neroaac.

So, does anybody know a way to split audio as wav and merge it back,
and get it synchronized?

Regards,
Frank.