Retiming video is two operations, not one
A video file carries a picture track and a sound track, each with its own timestamps. Changing speed means adjusting both — and adjusting them in different ways, because the two media do not respond to retiming alike.
The video side is straightforward: presentation timestamps are rewritten so frames are shown closer together or further apart. Nothing is added or removed; the same frames simply arrive at a different rate.
The audio side cannot work that way. Playing samples faster raises pitch, which turns a 2× clip into chipmunk speech. So the audio is time-stretched instead — cut into overlapping windows that are spaced out or packed together, leaving the frequencies inside each window untouched. Voices stay recognisable at speed.
What slow motion can and cannot do
This is the expectation worth setting properly. Slowing a clip down does not invent new frames. A 30fps video at 0.5× has the same 30 frames per second of content spread over twice the duration, so each frame is displayed twice as long.
For a slow pan or a talking head, that looks fine. For anything moving quickly it looks stepped, because your eye can now see the gaps between positions that were previously blurred together by motion.
Smooth slow motion has to be shot for it — at 120fps or 240fps, so that slowing to a quarter speed still leaves 30 or 60 frames every second. Software that appears to do better is generating intermediate frames with motion interpolation, which is a substantially heavier operation and produces its own distinctive artefacts around edges.
Timelapse from ordinary footage
Speeding up is the easier direction, because there is more information than needed rather than less. Frames are dropped and the rest are shown faster, which is exactly what a timelapse is.
The main thing to watch is camera shake. Handheld wobble that reads as natural at normal speed becomes violent at 4×, because the same movement now happens four times as fast. Footage shot on a tripod speeds up far better than footage shot by hand.
Audio is retimed too, but sped-up speech stops being useful well before 4×. For a pure timelapse, consider stripping the sound with the Mute Video tool first.
Common uses
- Turning a long recording into a short timelapse
- Slowing a clip down to see exactly what happened
- Compressing a screen recording or tutorial into less time
- Making a video fit an exact runtime for a platform limit
- Reviewing sports or technique footage frame by frame