Change video speed

What happens to the sound

Naively speeding up audio makes everyone sound like a chipmunk. The atempo filter used here stretches time without shifting pitch, so a 2× speed-up sounds like someone talking quickly rather than someone inhaling helium. Above 2× the filter is chained, which is why 4× still sounds reasonable.

Slowing down

Slow motion below 1× repeats frames rather than inventing new ones, so a 30 fps clip at quarter speed still shows 30 distinct pictures per second of original footage. It will look stepped. Genuinely smooth slow motion needs footage shot at a high frame rate to begin with.

How speed is actually applied

The picture is retimed with setpts, which rewrites the timestamp on every frame rather than throwing frames away or making new ones. At 2× each frame is told to appear at half its original moment, so the clip is half as long and the frame rate of the output doubles in effect. At 0.5× the timestamps are spread out, and because there is nothing to put between them, frames are held on screen twice as long. This is why speeding up looks smooth and slowing down looks stepped: one has more frames than it needs, the other has fewer.

The consequence worth planning around is that a 4× speed-up of 30 fps footage is showing you 120 distinct frames per second of source, which is more motion detail than the output frame rate can carry, so some is simply discarded. Nothing is wrong when that happens; it is the arithmetic.

Picking a speed on purpose

• 1.25× to 1.5× is invisible on a talking head and is the cheapest way to tighten a recording that drags.
• 2× is the practical limit for anything where the speech still needs to be followed.
• 3× and 4× are for time-lapse effects: a journey, a build, a queue. Untick the audio at these speeds, it contributes nothing.
• 0.5× is the useful slow speed. It doubles each frame's time on screen, which reads as deliberate rather than broken.
• 0.25× only works on footage shot at 60 fps or higher. On 30 fps material it stutters visibly.

Length, size and sound

Speeding up shortens the file and usually shrinks it, though not proportionally: fast motion is harder to compress, so a 2× clip is generally more than half the size of the original. Slowing down lengthens it and inflates it, since the repeated frames still have to be encoded.

If your source has no audio track, leave keep the audio track unticked. Asking to retime an audio stream that does not exist is the most common reason this tool fails. For sound alone, change audio speed applies the same pitch-preserving filter with no video work at all, and is far quicker.

Common questions

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