# Framerate conversion

### Input duration

• `1500` frames
• = `60` second(s)
• = `1` minute(s)

### Conversion 'conform': 25 → 24fps

• The total # of frames `1500` stays the same.
• Frames are played at 24 instead of 25 (slower).
• The video speed will slow down by `4%`
• As a result, the audio will sound lower
• To correct the audio: pitch up by 4.167% (0.707 semitones)
• Video conversion:
`ffmpeg -i [input] -r 24 -filter:v "setpts=1.0417*PTS" -y [output]`
• Audio conversion:
`sox [input] [output] tempo 0.96ffmpeg -i [input] -filter:a "atempo=0.96" -vn [output]`
• Duration of output = 62.500 sec (1500 frames)

### Conversion 'interpolate': 25 → 24fps

• The total duration `60 sec` stays the same.
• Frames are recalculated/interpolated based on the original frames
• # frames of output = `1440` frames
• Naive conversion: drop a frame every 25 frames
• Video conversion:
`ffmpeg -i [input] -r 24 -y [output]`
• Audio conversion: nothing to do, sound stays as-is

### 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.