My previous post ended up in moderation queue because I quoted StainlessS's post with lots of links
Anyway, abreviation:
raffriff42: I tried this:
Code:
ffmpeg -i out.m4a -filter:a "atempo=0.5" -vn outputaudio.aac
same as:
ffmpeg -i source.mp4 -filter_complex "[0:a]atempo=0.5[a]" -map "[a]" outputaud.m4a
and it sounded horrible. While the speed sounded correct, it had that metallic echo all over it. Couldn't understand anything. It sounded nothing like VLC's 0.5x slowdown. Note: I used 0.5 and not 2.0 like in your example because I wanted it to slow down, not speed up by 2x.
StainlessS: I always try changing AR like this... without re-encoding. Most of the time it works great:
Code:
ffmpeg -i in.mp4 -vcodec copy -acodec copy -aspect 16:9 output.mp4