Speed Up or Slow Down Video in Avidemux (Slow Motion Guide)

Changing video speed in Avidemux means changing how many frames play per second. Slow a 30 fps clip down to 15 fps and it plays at half speed — instant slow motion. Speed it up to 60 fps and it plays twice as fast. Avidemux does this with its FPS filters, and this guide shows both routes plus how to keep audio in sync.

Key idea: Speed changes in Avidemux require re-encoding the video — there is no lossless “copy mode” shortcut for this. Plan for an encode pass.

Method 1: Change FPS filter (fastest)

  1. Open your video in Avidemux.
  2. Go to Video > Filters, click Transform, and add Change FPS.
  3. Set the new frame rate: lower than the source for slow motion (e.g. 30 fps → 15 fps = half speed), higher for fast motion (30 fps → 60 fps = double speed).
  4. Set Video Output to an encoder (e.g. x264), configure it, and save. Every frame is kept; they just play slower or faster.

Method 2: Resample FPS (smoother slow motion)

The Resample FPS filter can interpolate between frames, producing smoother slow motion from ordinary footage. Enable its motion-interpolation option for the best result — it costs more encoding time but visibly reduces choppiness.

Choppy slow motion? Footage shot at 30 fps only has 30 distinct moments per second. Stretching it to half speed without interpolation just holds each frame longer. For truly smooth slow motion you need high-frame-rate source footage (60/120 fps) or interpolation.

Keeping audio in sync

When the video duration changes, the audio must be stretched to match, or it will drift out of sync. In Audio > Filters, add the Stretch filter and set the same speed ratio you used for video (e.g. 0.5× for half-speed slow motion). Then encode the audio (AAC or MP3) and save.

Slow motion recipe (example)

  • Source: 30 fps clip, want half-speed slow motion
  • Video filter: Resample FPS → 15 fps with interpolation on
  • Audio filter: Stretch → 0.5×
  • Video Output: x264 (CRF ~20), Audio Output: AAC, save as MP4

Can Avidemux reverse a video?

No — Avidemux has no built-in reverse-video effect. For reversed playback, use a full editor like Shotcut, or ffmpeg:

ffmpeg -i input.mp4 -vf reverse -af areverse reversed.mp4

Frequently asked questions

Will the audio stay in sync?

Only if you stretch it to the same ratio. Use the Stretch audio filter with the matching speed factor, otherwise audio and video drift apart.

Does changing speed re-encode the video?

Yes. Speed changes cannot use copy mode — the frame timing itself changes, so the video must be re-encoded.

Can I reverse a video in Avidemux?

No. Avidemux cannot play a clip backwards. Use Shotcut or the ffmpeg command above for reversed video.

Why does my slow motion look choppy?

Your source footage probably has too few frames per second. Enable motion interpolation in Resample FPS, or shoot the original at 60/120 fps for smooth results.