I'm speeding up a video by 2x with this command:
ffmpeg -i input.mp4 -r 50 -vcodec mpeg4 -filter:v "setpts=0.5*PTS" -filter:a "atempo=2" output.mp4
However, the output quality is very low. I used
-r 50 because that's what the Ffmpeg guide recommends (here) in order to avoid frame dropping. What can I do to keep a good quality in the output?