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The Essential Guide to SCTE-35 

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What is SCTE?

The acronym SCTE is short for The Society of Cable and Telecommunications Engineers. SCTE is a non-profit professional organization that creates technical standards and educational resources for the advancement of cable telecommunications engineering and the wider video industry. When talking about it, you may hear people abbreviate SCTE with the shorthand slang “Scutty”.

SCTE was founded in 1969 as The Society of Cable Television Engineers, but changed its name in 1995 to reflect a broader scope as fiber optics and high-speed data applications began playing a bigger role in the cable tv industry and became the responsibility of its engineers. Currently there are over 19,000 individual SCTE members and nearly 300 technical standards in their catalog, including SCTE-35, which will be the focus of this post.

What is SCTE-35?

SCTE-35 was first published in 2001 and is the core signaling standard for advertising and program control for content providers and content distributors. It was initially titled “Digital Program Insertion Cueing Message for Cable” but recent revisions have dropped “for Cable” as it has proven useful and versatile enough to be extended to OTT workflows and streaming applications. There have been several revisions and updates published to incorporate member feedback and adapt to advancements in the industry, most recently on Nov, 30 2023.

SCTE-35 signals are used to identify national and local ad breaks as well as program content like intro/outro credits, chapters, blackouts, and extensions when a live program like a sporting event runs long. Initially, these messages were embedded as cue tones that dedicated cable tv hardware or equipment could pick up and enable downstream systems to act on. For modern streaming applications, they are usually included within an MPEG-2 transport stream PID and then converted into metadata that is embedded in HLS and MPEG-DASH manifests.

SCTE-35 markers and their applications for streaming video

While SCTE-35 markers are primarily used for ad insertion in OTT workflows, they can also signal many other events that allow an automation system to tailor the program output for compliance with local restrictions or to improve the viewing experience. Let’s take a look at some common use cases and benefits of using SCTE-35 markers.

Use cases and benefits of SCTE-35

Ad Insertion – As mentioned above, inserting advertisements into a video stream is the main use case for SCTE-35 markers. They provide seamless splice points for national, local and individually targeted dynamic ad replacement. This allows for increased monetization opportunities for broadcasters and content providers by enabling segmenting of viewers into specific demographics and geographic locations. When ad content can be tailored for a particular audience, advertisers are willing to pay more, leading to higher revenue for content providers and distributors.

Program boundary markers – Another common use case is to signal a variety of program boundaries. This includes the start and end of programs, chapters, ad breaks and unexpected interruptions or extensions. Many of these become particularly useful in Live-to-VOD scenarios. Ad break start/end markers can be used as edit points in a post-production workflow to automate the removal of ads for viewers with ad-free subscriptions. A program end marker can be used to trigger the next episode in a series for binge viewing sessions if so desired. All of these markers open new possibilities for improving the user experience and keeping your audience happy and engaged.

Blackouts and alternate content – Another less common, but important use case is to signal blackouts, when a piece of content should be replaced or omitted from a broadcast. This often applies to regional blackouts for sporting events. Respecting blackout restrictions is crucial for avoiding fines and loss of access to future events. Using SCTE-35 allows your automation system to take control and ensure you are compliant.

Types of SCTE-35 markers

SCTE-35 markers are delivered in-band, meaning they are embedded or interleaved with the audio and video signals. There are five different command types defined in the specification. The first 3 are legacy commands: splice_null(), splice_schedule() and splice_insert(), but splice_insert() is still used quite often. The bandwidth_reservation() command may be needed in some satellite transmissions, but the most commonly used command with modern workflows is time_signal(). Let’s take a closer look at the 2 most important command types, splice_insert and time_signal.

splice_insert commands

Splice_insert commands are used to mark splice events, when some new piece of content like an ad should be inserted in place of the program or a switch from an ad break back into the main program. Presentation time stamps are used to note the exact timing of the splice, enabling seamless, frame accurate switching.

time_signal commands

Time_signal commands can also be used to insert new content at a splice point, but together with segmentation descriptors, they can handle other use cases like the program boundary markers mentioned above. This enables the segmenting and labeling of content sections for use by downstream systems.

Using SCTE-35 markers in streaming workflows

MPEG-2 transport streams

In MPEG-2 transport streams, SCTE markers are carried in-band on their own PID within the transport stream mux. These streams are usually used as contribution or backhaul feeds and in most cases are not directly played by the consumer. They may be delivered over dedicated satellite or fiber paths or via the public internet through the use of streaming protocols like SRT or proprietary solutions like Zixi.


SCTE messages are visible inline in the HLS manifest. They may use the EXT-X-DATERANGE tag which relies on UTC timecode, the EXT-X-SCTE35 tag with a variety of attributes, EXT-X-CUE-OUT and EXT-X-CUE-IN with a duration or the EXT-OATCLS-SCTE35 tag with the raw base64 encoded SCTE-35 payload.

Example HLS manifest with Cue Out, duration and Cue In tags:


Example HLS manifest using EXT-OATCLS-SCTE35 tag with base64 encoded marker:



In MPEG-DASH streams, SCTE-35 defined breaks and segments are added as new periods to the .mpd file.

<Period start="PT0S" id="1">
   <!-- Content Period -->

<Period start="PT32S" id="2">
    <!-- Ad Break Period -->
   <EventStream timescale="90000"
     <Event duration="2520000" id="1">
       <Signal xmlns="urn:scte:scte35:2013:xml">
         <Binary>      /DAlAAAAAAAAAP/wFAUAAAAEf+/+kybGyP4BSvaQAAEBAQAArky/3g==

<Period start="PT60S" id="3"> 
   <!-- Content Period -->

With SCTE-35 messages embedded in the stream, various forms of automation can be triggered, whether it’s server or client-side ad insertion, content switching, interactive elements in the application or post-production processing.

Bitmovin Live Encoding SCTE Support

SCTE-35 message pass-through and processing

Bitmovin now officially supports SCTE-35 message processing and pass-through from mpeg transport streams delivered via Zixi and SRT inputs. For the most commonly used splice_insert and time_signal commands, we will segment the stream at the splice point and where applicable, #EXT-X-CUE-OUT and #EXT-X-CUE-IN tags will be added to the HLS manifest to trigger dynamic ad insertion workflows downstream.

Live cue point insertion API

In addition to the SCTE-35 pass-through mode, Bitmovin customers can insert new ad break cue points in real-time, using live controls in the user dashboard or via API. These can be inserted independently of existing SCTE-35 markers in the input stream and may be useful for live events when the time between ads is variable depending on breaks in the action. This allows streamers that don’t have SCTE-35 markers embedded in their source to take advantage of the same downstream ad insertion systems for increased monetization.

API Call:

POST /encoding/encodings/{encoding_id}/live/scte-35-cue
      "cueDuration": 60, // duration in seconds between cue tags (ad break length)

The #EXT-X-CUE-OUT tag will be inserted into the HLS playlist, signaling the start and duration of a placement opportunity to the DAI provider. Based on the cueDuration and the segment length, the #EXT-X-CUE-IN tag will be inserted after the configured duration and the ad opportunity will end, continuing the live stream.

HLS manifest with Cue Out, duration and Cue In tags inserted via the API call above:


Want to get started using SCTE-35 in your streaming workflow? Get in touch to let us know how we can help.

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