What Is SCTE-35? Ad Break Signaling in Streams, Explained
What is SCTE-35? SCTE-35 is the standard for signaling ad breaks inside a video stream. Learn how splice markers work and how SSAI uses them to insert ads.
SCTE-35 is the industry standard for signaling ad breaks and other splice points inside a video stream. It defines small data messages, called cues or markers, that ride along with the video and say "an ad break starts here, and it lasts this long." Downstream systems read those markers to insert ads, trigger blackouts, or mark program boundaries, all without a human touching the stream. The standard comes from SCTE, the Society of Cable Telecommunications Engineers, and it moved from cable into streaming almost unchanged.
For a FAST channel, SCTE-35 is the connective tissue of monetization. The playout system decides where breaks belong; the markers carry that decision; the ad insertion system acts on it. This guide explains what the markers say, where they live in a stream, and how server-side ad insertion uses them.
What does an SCTE-35 marker actually say?
A marker is a compact binary message with a few key facts. Conceptually, each cue answers four questions:
- What kind of event is this? An ad break starting (cue-out), an ad break ending (cue-in), or a program boundary.
- When exactly does it happen? A precise timestamp tied to the video timeline, so the splice lands on the right frame.
- How long is the break? A planned duration, such as 120 seconds, so the inserter knows how much ad time to fill.
- What identifies it? An event ID, so systems can match a cue-out with its cue-in and avoid acting twice.
The standard defines two main command types. A splice_insert is the direct instruction: splice here. A time_signal is more descriptive: it carries segmentation descriptors that label what the moment is, such as "chapter end" or "provider ad start." Modern streaming workflows lean on time_signal because the richer labels support more automation.
Where do SCTE-35 markers live in a stream?
The marker travels differently depending on the packaging, but the meaning stays identical.
| Layer | How SCTE-35 appears | Who reads it |
|---|---|---|
| MPEG transport stream | Binary cue messages on their own PID inside the broadcast feed | Encoders, packagers, cable splicers |
| HLS manifest | Tags such as EXT-X-CUE-OUT, EXT-X-CUE-IN, or EXT-X-DATERANGE carrying the cue | SSAI services, players, validators |
| DASH manifest | Event streams embedded in the MPD with the cue payload | SSAI services, players |
In HLS, the packager translates the binary cue into manifest tags at the right segment boundary. If you open a FAST channel's playlist, you can often spot the break markers in plain text; our guide to HLS manifests walks through a real example. Placement discipline matters: markers must land on segment boundaries, and the signaled durations must be honest, or every downstream system inherits the error.
How does SSAI use SCTE-35 markers?
Server-side ad insertion is the system that turns markers into revenue. The sequence runs like this:
- Playout emits the cue. The channel's playout engine inserts an SCTE-35 cue-out at the scheduled break, with a duration, and a cue-in when content resumes.
- The SSAI service detects it. Sitting between origin and viewer, the ad server watches the manifest for markers on every session.
- Ads are fetched and stitched. The service requests ads (typically via VAST), transcodes or selects renditions that match the content, and replaces the break window with ad segments, per viewer.
- Playback never leaves the stream. The viewer's player sees one continuous video, which is why stream-stitched ads resist ad blockers and feel like TV.
Because the ads fill exactly the window the marker declared, break planning is an editorial and revenue decision encoded in data. How many breaks per hour, and how long each pod runs, is your ad load strategy; FAST ad loads typically run 4 to 8 minutes per hour versus cable's 12 to 16. You can model pod structures with the free ad pod calculator.
On Vidiyo, this whole chain is handled for you: the platform's playout inserts the markers and runs SSAI, so uploading and scheduling a channel is enough to get monetized ad breaks. You never write a cue by hand, but knowing what the markers say makes ad reports and break placement far easier to reason about.
Quick answers
What does SCTE-35 stand for? It is standard number 35 from SCTE, the Society of Cable Telecommunications Engineers. The document's formal title is "Digital Program Insertion Cueing Message."
What is the difference between cue-out and cue-in? Cue-out signals the start of an ad break, usually with a duration. Cue-in signals the return to program content. An SSAI service fills the window between them with ads.
Do FAST channels need SCTE-35? Effectively yes. Distribution platforms and SSAI vendors expect marker-driven breaks; without markers, ads either cannot be inserted or get forced in at arbitrary points.
Is SCTE-35 only for ads? No. Its segmentation descriptors also mark program starts and ends, chapters, and regional blackout windows. Ad insertion is simply the most common streaming use.
What's next
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