Framerate conversion

Input duration

Seconds
3600 frames
60 sec(s)
Minutes
60 secs / 60
1 min(s)

Conversion 'conform': 60 → 50fps

Definition
The total # of frames stays the same
3600 frames
Speed
video is slowed down from 60 to 50 fps
-16.67%
Audio
As a result, the audio will sound lower
Correct audio
pitch up by +20% (3.156 semitones)
120%
Convert video
ffmpeg -i [input] -r 50 -filter:v "setpts=1.2*PTS" -y [output]
Convert audio
sox [input] [output] tempo 0.8333
ffmpeg -i [input] -filter:a "atempo=0.8333" -vn [output]
Duration: time
3600 frames / 50 fps
72.00 sec

Conversion 'interpolate': 60 → 50fps

Definition
The total duration stays the same
60 sec
New length
60 seconds * 50 fps
3000 frames
Naive conversion
drop a frame every
6 frames
Convert video
ffmpeg -i [input] -r 50 -y [output]
Convert audio
audio stays exactly the same

Explanation

  • Calculate # frames from time duration and vice versa. e.g. 93 sec @ 24fps = 2232 frames
  • Calculate how audio and video should be manipulated to change from one framerate to another, using sox and/or ffmpeg
  • When is this necessary? If you have a video that was recorded in 25fps(PAL) or 30fps(NTSC), and you want to show it on a cinema screen: you have to convert the framerate to 24 fps.