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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, the same attribute that carries volume envelopes.

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The lane

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.

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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.

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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.

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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.

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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.