---
title: "Speed ramps"
sidebarTitle: "Speed ramps"
description: "Change a clip's playback speed over time with a rate lane in data-automation. One curve drives the preview, the render and the browser editor."
---

A speed ramp changes how fast a `<video>` or `<audio>` clip plays while it
plays: slow into a beat, fast out of it. It is a `rate` lane in
[`data-automation`](/reference/audio-effects#the-four-attributes), the same
attribute that carries volume envelopes.

```html
<video
  src="clip.mp4"
  data-start="0"
  data-duration="4"
  data-track-index="0"
  data-automation='{"version":1,"lanes":[{"target":"rate","points":[{"t":0,"v":1},{"t":2,"v":4},{"t":4,"v":0.5}]}]}'
></video>
```

## The lane

| Field | Meaning |
| --- | --- |
| `target` | `"rate"` |
| `points[].t` | Seconds from the start of the clip, not from the start of the composition |
| `points[].v` | Speed multiplier. `1` is normal, `2` is twice as fast. Clamped to `0.1` to `10` |
| `points[].curve` | Optional, `-1` to `1`. Bends the segment leaving this point |

Speed moves between points on a logarithmic scale, so going from `0.5` to `2`
passes through `1` at the midpoint. A lane wins over the constant
`data-playback-rate` on the same element. Without a lane, the constant applies.

## What stays the same everywhere

The source frame shown at each moment comes from one function, `sourceTimeAt`,
which the preview and the renderer both use, so the render shows the frames the
preview showed. Set `data-duration` on the clip to fix how long it holds the
timeline; without it the length follows from the source and the speed.

If a browser probe discovers a rate lane on an existing clip, export preserves
the scene's trim in both the ramp's position and the source media in-point.

Pitch is kept. The preview leaves the browser's default pitch correction on.
The render time-stretches audio in short constant-speed slices (a quarter
second, coarser on clips longer than a minute), which approximates the curve
while keeping pitch, so the sound follows the ramp closely rather than exactly.

## Presets

the browser editor's **Speed preset** menu draws a ready curve over the clip: Montage,
Hero, Bullet, Jump cut, Flash in and Flash out. An agent can write the same
points by hand.

## Editing in the browser editor

Select a clip and open its Media section. The **Speed** slider sets the
constant rate. The **A** toggle next to it automates speed, which shows a rate
lane on the clip in the timeline that you edit like the volume lane.

## Limits

- Only `<video>` and `<audio>` clips carry a rate lane.
- Speeds outside `0.1` to `10` are clamped.
- The viewer's own transport speed is a separate control that multiplies with
  the clip's speed, and it is narrower: the browser editor speed menu offers `0.25` to
  `2`, the player's menu offers up to `4`, and the runtime clamps a transport
  request to `0.1` to `5`.