Video speed — 0.25x slow-motion through 4x fast-forward
Change video playback speed with five presets from 0.25x (slow-motion) up to 4x (fast-forward). Audio is time-stretched to match while preserving pitch, or it can be muted entirely. Runs ffmpeg.wasm in your browser — videos never leave your device.
How to use
Drop a video file (mp4 / mov / webm / mkv etc.) or pick one via Choose file. Select one of the five playback-speed presets (0.25x / 0.5x / 1.5x / 2x / 4x) and optionally uncheck Keep audio to export silent video. Click Change speed; ffmpeg.wasm re-encodes the video in your browser and shows a preview plus a download button. With audio kept the atempo filter time-stretches the soundtrack while preserving pitch — voices stay at their original tone.
FAQ
- Which video formats are supported?
- Anything ffmpeg.wasm can demux: mp4 / mov / m4v / webm / mkv / avi / flv / wmv / 3gp and so on. Output is always H.264 + AAC inside an MP4 container.
- Does the audio pitch shift?
- No. The atempo filter time-stretches the audio while preserving pitch. For 0.25x and 4x — outside atempo's single-pass range — the filter is chained internally to keep quality intact.
- Why only 0.25x through 4x?
- Below 0.25x frame duplication becomes too obvious; above 4x dropped frames make playback choppy. The five presets cover the practically useful range.
- Can I skip re-encoding?
- Speed change implies a timebase rewrite, so `-c copy` is not an option. The tool re-encodes with libx264 (CRF 23, medium preset) to minimise quality loss.
- Is my input uploaded?
- No. ffmpeg.wasm runs entirely in your browser, so the video stays on your device.
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