VidTech

The Essential Guide to SCTE-35 

Andy Francis
. 7 min read
scte-35 - Bitmovin

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. 

scte-35 - Bitmovin
Ad Break Example.
source: SCTE-35 specification

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.

scte-35 - Bitmovin
Workflow example with Program Boundaries and Blackouts.
source: SCTE-35 specification

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.

HLS 

The Bitmovin Live Encoder supports a range of different HLS tags that are written when SCTE-35 triggers are parsed from the MPEG-TS input stream. Multiple marker types can be enabled for each HLS manifest. Which marker types to use depends on the consumer of the HLS manifest. An example consumer would be a Server Side Ad Insertion (SSAI) service. They usually state in their documentation which HLS tags they support for signaling SCTE-35 triggers.

  • EXT_X_CUE_OUT_IN: Ad markers will be inserted using #EXT-X-CUE-OUT and #EXT-X-CUE-IN tags.
  • EXT_OATCLS_SCTE35: Ad markers will be inserted using #EXT-OATCLS-SCTE35 tags. They contain the base64 encoded raw bytes of the original SCTE-35 trigger.
  • EXT_X_SPLICEPOINT_SCTE35: Ad markers will be inserted using #EXT-X-SPLICEPOINT-SCTE35 tags. They contain the base64 encoded raw bytes of the original SCTE-35 trigger.
  • EXT_X_SCTE35: Ad markers will be inserted using #EXT-X-SCTE35 tags. They contain the base64 encoded raw bytes of the original SCTE-35 trigger.
  • EXT_X_DATERANGE: Ad markers will be inserted using #EXT-X-DATERANGE tags as specified in the HLS specification. They contain the ID, start timestamp, and hex-encoded raw bytes of the original SCTE-35 trigger.

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

#EXTINF:4.0,
2021-07/video/hls/360/seg_18188.ts
#EXT-X-CUE-OUT:120.000
…
#EXTINF:4.0,
2021-07/video/hls/360/seg18218.ts
#EXT-X-CUE-IN

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

#EXTINF:4.0,
2021-07/video/hls/360/seg_18190.ts
#EXT-OATCLS-
SCTE35:/DBcAAAAAAAAAP/wBQb//ciI8QBGAh1DVUVJXQk9EX+fAQ5FUDAxODAzODQwMDY2NiEEZAIZQ1VFSV0JPRF/3wABLit7AQVDMTQ2NDABAQEKQ1VFSQCAMTUwKnPhdcU=

Note: You can copy the base64 encoded marker above, (beginning with the first / after SCTE35: ) and paste it into this payload parser to see the full message structure.

MPEG-DASH 

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

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

<Period start="PT32S" id="2">
    <!-- Ad Break Period -->
   <EventStream timescale="90000"
    schemeIdUri="urn:scte:scte35:2014:xml+bin">
     <Event duration="2520000" id="1">
       <Signal xmlns="urn:scte:scte35:2013:xml">
         <Binary>      /DAlAAAAAAAAAP/wFAUAAAAEf+/+kybGyP4BSvaQAAEBAQAArky/3g==
         <Binary>
       </Signal>
      </Event>
    </EventStream> 
</Period>

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

With SCTE 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 message pass-through and processing 

Bitmovin supports the parsing of SCTE-35 triggers from MPEG-TS input streams for Live Encodings, the triggers are shown below as splice decisions. The triggers are then mapped to HLS manifest tags.

Splice Decisions

Certain SCTE-35 triggers signal that an advertisement or break (to from the original content starts or ends. The following table describes how the Bitmovin Live Encoder treats SCTE-35 trigger types and SCTE-35 Segmentation Descriptor types as splice decision points, and the compatibility of those types with the different command types, Spice Insert and Time Signal.

✓= Supported

✖ = Not currently supported

Segmentation UPID Type (Start/End)Descriptor Type NameSPLICE_INSERTTIME_SIGNAL
0x10, 0x11PROGRAM
0x20, 0x21CHAPTER
0x22, 0x23BREAK
0x30, 0x31PROVIDER_ADVERTISEMENT
0x32, 0x33DISTRIBUTOR_ADVERTISEMENT
0x34, 0x35PROVIDER_PLACEMENT_OPPORTUNITY
0x36, 0x37DISTRIBUTOR_PLACEMENT_OPPORTUNITY
0x40, 0x41UNSCHEDULED_EVENT
0x42, 0x43ALTERNATE_CONTENT_OPPORTUNITY
0x44, 0x45PROVIDER_AD_BLOCK
0x46, 0x47DISTRIBUTOR_AD_BLOCK
0x50, 0x51NETWORK

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:

#EXTINF:4.0,
    2021-07-09-13-18-34/video/hls/360_500/segment_18188.ts
    #EXT-X-CUE-OUT:60.000
    #EXTINF:4.0,
    2021-07-09-13-18-34/video/hls/360_500/segment_18189.ts
    ...
    #EXTINF:4.0,
    2021-07-09-13-18-34/video/hls/360_500/segment_18203.ts
    #EXT-X-CUE-IN
    #EXTINF:4.0,
    2021-07-09-13-18-34/video/hls/360_500/segment_18204.ts


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

Resources

Tutorial: Bitmovin Live Encoding with SCTE-35, HLS and SSAI

Guide: Bitmovin Live Encoding and AWS MediaTailor for SSAI

Guide: Bitmovin Live Encoding with Broadpeak.io for SSAI

SCTE website 

SCTE-35 specification

SCTE-35 payload parser

Bitmovin Live Encoding data sheet

Andy Francis

Andy Francis

Technical Content Lead

Andy Francis is an A/V nerd at heart who has spent the last 15+ years sending audio and video over the internet. He has extensive experience working with everything from video conferencing to live event streaming to large-scale OTT platforms and is passionate about spreading knowledge and access to streaming technologies.


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