Why Do This in Your Browser?
Changing video playback speed is more common than it looks. Tutorials and lectures benefit from 1.25× or 1.5× to cut watch time. Sports footage and product demos benefit from 0.5× slow motion. Speed-ups up to 16× also turn long screen recordings into quick overviews. Until recently, all of this required a paid editor or a desktop FFmpeg install.
This tool handles speed changes in browser, with optional audio pitch preservation. The video PTS (presentation timestamps) are scaled by the chosen factor; the audio is independently re-tempo'd with the `atempo` filter, which preserves pitch — voices don't sound like chipmunks at 2× or like baritone whales at 0.5×.
How It Works
Speed changes are conceptually simple: multiply all timestamps by 1/speed_factor. A 1-minute video at 2× becomes a 30-second video; at 0.5× becomes a 2-minute video. Changing timestamps alone would leave a video at an odd frame rate, so the tool re-encodes it to H.264 (CRF 23) at the source frame rate: speeding up drops frames, slowing down repeats them. Audio is the tricky part, because naively speeding up audio also raises pitch (the chipmunks effect). The fix is the `atempo` filter, which preserves pitch by intelligently overlap-adding short audio segments.
Optional second-order tweaks: drop the audio entirely if pitch preservation isn't enough (silent slow-motion is a common cinematic effect), or mute it for big speed-ups, where even pitch-preserved audio turns choppy.
Tip: If you only want a portion of the video at a different speed, trim it first, then change speed. After a speed change, convert to WebM or another format for web embedding.
Common Use Cases
How We Compare
Honest read on free, paid, and self-hosted options for this kind of job:
See also: Pair reversal with the speed changer; the Video Reverser runs FFmpeg’s reverse filter in-browser.