Video Speed Changer

Change how fast a video plays: speed up a long tutorial or a cooking clip, slow down a sports moment, or turn an hour of sunset, traffic or a 3D print into a short time-lapse. The sound follows the speed with the voice pitch kept natural (no chipmunk effect), changes pitch like a record if you prefer, or is removed. The result is an H.264 + AAC MP4 made in your browser with the ffmpeg engine (about 30 MB, downloaded the first time); your video is not uploaded.

How to use this tool

1. Choose the speed — or "Custom" and any value from 0.1× to 100×.
2. Choose what happens to the sound and, for slow motion, whether frames are blended.
3. Choose one or more videos; processing starts right away.
4. Check the new length, play the result and download it.

Formula

New length = original length ÷ speed

The picture timestamps are divided by the speed and the original frame rate is kept: speeding up drops frames, slowing down repeats them (or blends neighbours with the smoothing option). The sound is stretched with the atempo filter, which works between 0.5× and 2× per step, so 4× runs as 2 × 2 and 0.25× as 0.5 × 0.5. Sound is kept from 0.25× to 16×; beyond that it would be unrecognisable and is left out.

Example

A 12-minute recording at 4× becomes 12 ÷ 4 = 3 minutes, with speech still at its normal pitch. A 10-second jump at 0.25× becomes 10 ÷ 0.25 = 40 seconds of slow motion. A 45-minute sunset at 32× becomes about 1 minute 24 seconds without sound.

How long it takes

Every frame of the original is decoded and the result is encoded at the original frame rate, on one processor core inside the tab: roughly 27 output frames per second at 480p, 12–13 at 720p and 6 at 1080p on a recent computer. A time-lapse is quick because few frames are written; slow motion takes longest because many are.