Video Speed and Reverse

📖 Tool Introduction

Change how fast a video plays, or play it backwards, entirely in your browser. Time lapses, slow motion, beat-matched edits and reversed clips for effect are all handled here.

Speed runs in two parts. The picture is handled by changing how long each frame is shown, so half speed means every frame stays twice as long. Audio cannot simply be stretched or the pitch would shift with it, so a time-stretch that preserves pitch is used: at 2x it still sounds like the same voice, just faster, without that squeaky fast-forward quality.

Speeds run from 0.25x to 4x. Slower suits slow motion and showing detail; faster suits time lapses and compressing long material. Outside the 0.5x to 2x band the change is split into chained stages automatically, because the audio filter only accepts that range in one pass. The result is the same, just arrived at in steps.

Reverse plays the clip backwards, picture and sound together. It has to hold the whole clip in memory, so a long video can fail outright. Cutting it into segments under ten seconds first raises the success rate considerably.

Processing uses a WebAssembly build of FFmpeg: about 24 MB downloads the first time and is cached after. The video is never uploaded and stays on your device.

One more thing worth knowing before you start: the audio filter used here keeps pitch steady, but it works best within a moderate range. At the extremes, artefacts creep in as faint warbling, which is a property of the algorithm rather than something a setting can fix.

One more thing worth knowing before you start: speeding a clip up shortens it, which shifts anything timed to the original, such as burnt-in captions or cuts matched to music. Plan the speed change before adding those, not after.

✨ Key Features

• Eight speeds from 0.25x to 4x, covering slow motion and time lapse
• Pitch preserved through time stretching, so faster does not mean squeaky
• Reverse playback for both picture and sound
• Automatic chaining when a speed falls outside the single-pass range
• Batch mode: add several videos and each is processed in turn
• Local processing, so the video never leaves your device

🚀 How to Use

1. Drag a video into the drop area, or click to choose one
2. Pick a speed: 0.5x for slow motion, 2x or 4x for time lapse
3. Tick reverse for a backwards effect, cutting long videos into segments under ten seconds first
4. Press start and watch the progress bar
5. Press download when it finishes

⚠️ Things to Note

• Engine size: The first run downloads about 24 MB of WebAssembly engine, cached for later visits
• Reverse and memory: Reversing holds the whole clip in memory, so long videos can fail. Cut them into segments under ten seconds first
• Speed range: 0.25x to 4x is available. Anything outside 0.5x to 2x is chained across stages automatically with the same result
• Pitch stays put: Audio is time stretched rather than resampled, so faster does not go squeaky and slower does not go muddy. Extreme speeds still sound artificial, because the algorithm is working well outside the range speech normally occupies
• Privacy: The video is processed locally and never uploaded. Reloading the page clears it completely