DRAFT
Author: Keith Millar
Date: 01/05/2018
Version: 0.1
- Introduction
- High Level Requirements
- Marker Solution Overview
- Generic capabilities
- SCTE-104/35 Message Types
This document outlines the requirements and solution for the signaling of markers and other commands within both the Broadcast contribution feed and the downstream broadcast feed.
Within a distribution system a single broadcast contribution feed can be used to support multiple broadcast feeds. For example a single feed could support a simulcast IPTV service, DTT Service and a DTH service, where each broadcast service requires a different set of markers.
The document describes how this can be supported in a flexible way. The primary focus of DVB is the signalling of Break and Spot markers, however a more holistic view needs to be considered as there is a desire by broadcaster to also signaling boundaries such as Programmes and Chapters, along with other creative uses.
Within the broadcast industry SCTE-104 is today widely used for the signalling of markers to the inserters (encoders) and SCTE-35 for signalling of markers to end devices. ITV sees no reason for this to change and therefore the document concentrates on how these existing standards can be used to meet the needs of the broadcasters.
The following are considered the main high level requirements for signalling boundary markers.
- The ability to insert/update a marker independently of another marker. I.e. should not require the complete set of markers for a given time point to be sent again as a single message. This can also be seen as supporting separate workflows for the different marker requirements.
- Support for existing devices that make use of SCTE-35 Splice Insert messages.
- Support for existing inserters (encoders) that make use of SCTE-104 Splice Insert messages.
- The ability for the inserter (encoder) via configuration to ignore markers within the contribution feed. This is required as within some downstream delivery chains they are not required and may be undesirable.
- The ability to signal Break markers within the contribution feed.
- The ability to signal Spot markers within the contribution feed.
- The ability to signal Programme markers within the contribution feed.
- The ability to signal Chapter markers within the contribution feed.
- The ability to signal Break markers to the inserter (over TCP/IP).
- The ability to signal Spot markers to the inserter.(over TCP/IP)
- The ability to signal Programme markers to the inserter (over TCP/IP).
- The ability to signal Chapter markers within the contribution feed (over TCP/IP).
- Carriage of markers of different types (e.g. Splice Insert, Programme, Chapter, etc), on separate MPEG-PIDs within a service. Using PDI_PID_index of SCTE-104 to map to MPEG-PIDs.
- Support for signalling if inserter (encoder) should perform stream conditioning for a given SCTE-104 messages.
- Support for signalling if inserter (encoder) should process an SCTE message.
As can be seen from the diagram above there can be an number of markers with the same time point. However this should not force them all to be signalled as a single message (SCTE-104 or SCTE-35).
SCTE-104 is the industry standard mechanism for signalling markers to an inserter (encoder), and these SCTE-104 messages can be delivered to the inserter using SDI-VANC or TCP/IP. The following sections describe the use of these 2 delivery mechanisms.
To support the requirement for inserting/updating a marker independently of another marker, it is required to have separate message flows for each Use Case. Within SCTE-104 this is supported by the DPI_PDI_Index field, where each flow has a unique DPI_PDI_index.
Today a Splice Insert message is by far the most common method used for signalling Spot and Break positions within a A/V stream. Typically a Splice Insert for a “from_network” (i.e start of a Spot or Break) is sent and the inserter/device uses the duration declared within the Splice Insert message to determine the “to_network” point.
It is imperative that Splice Insert messages are supported within a DVB-TA standard, as they are widely supported within inserters (encoders) and end devices.
The SCTE-104/35 specifications support the use of Segmentation Descriptors which provide an alternative way of signaling Spot and Break positions, and also allow the signalling of non advertising markers such as Programme and Chapter boundaries. It is possible to include multiple segmentation descriptors within a single SCTE 104/35 message however this has a number of potential operational/deployment issues as follows:
- The playout events that signal the insertion of an SCTE-104 message may come from multiple sources and therefore its potentially problematic to combine them (if they have the same time point) to generate a single SCTE-104 message.
- End devices are likely to be more resilient if the SCTE-35 resulting from the
- SCTE-104 fits within a single MPEG-TS packet (or a small number of packets).
Therefore to ease operational and deployment issues it is recommended that a SCTE-104 message shall contain one segmentation descriptor. Segmentation descriptors may be included within a Splice_insert, Splice_null and a time_signal message, and it is recommended that DVB constrains the use of the segmentation descriptor to the time_signal message.
In addition to the concerns described above when delivering segmentation descriptors over VANC the following should also be taken into consideration:
- Poor support within available SCTE-104 VANC inserters to include multiple segmentation descriptors within a single SCTE-104 message
- The maximum size of a single SCTE-104 message is 2000 bytes in length (SMPTE.ST2010.2008 - multi-operation message). This may in the long term be limiting, if all markers had to be in one SCTE-104 message.
When using VANC to deliver SCTE-104, a specific frame can only contain one payload i.e SCTE-104 message. This means that if there is a requirement to signal multiple markers at the same time point, then they must all have different pre-roll times which results in different SCTE-104 message insertion times.
Where the inserter (encoder) receives SCTE-104 commands over the TCP/IP connection, there are a couple of ways in which the management of SCTE- messages can be handled.
- The source of the SCTE-104 messages determines what messages to send based on the broadcast channel that the encoder services
- As per SCTE-104 over VANC
With the first option there is no need of standardisation, and therefore is not discussed further.
The following sections describe the required capabilities of an Inserter irrespective of how the SCTE-104 is delivered.
Today most inserter (encoder) implementations don’t support the use of the SCTE-104 DPI_PID_Index to determine the MPEG-PID that is used for the corresponding outgoing SCTE-35 component. Typically it is assumed that there is a single DPI_PID_index (with a value of 0) and they process all SCTE-104 message from that single DPI_PID_index.
This is no longer adequate and so we would like to encourage encoder vendors to fully implement the ability to support multiple SCTE-104 DPI_PID_Indexes, as follows:
- Enable mapping of PDI_PID_Index to MPEG-PIDs via configuration.
- Allow multiple PDI_PID_Indexes to map to the same MPEG-PID.
- Allow message aggregation as defined below.
As has been discussed it is desirable form an SCTE-104 perspective to support multiple SCTE-104 flows (using DPI_PID_index). However if there is considered a need to support an SCTE-35 message containing multiple segmentation descriptors, then the aggregation of the messages could be performed by the inserter (encoder).
The design shall allow an Inserter (encoder) to be configured with the set of SCTE-104 messages that it should process and also how it should process them.
| Ignore Signal | Generate SCTE-35 | Stream Condition | Comments |
|---|---|---|---|
| No | No | Yes | Encoder will perform stream conditioning but will not insert an SCTE-35 Splice_insert message. |
| Yes | NA | NA | No action by encoder |
| No | Yes | Yes | Encoder will perform stream conditioning and will insert an SCTE-35 Splice_insert message. |
| No | Yes | No | Encoder will not perform stream conditioning, but will insert an SCTE-104 message. |
| No | No | No | Encoder may perform some internal action, but will not generate an SCTE-35 message. (e.g. Modify Subtitle delay) |
Enabling the above allows a broadcaster to incrementally roll-out SCTE104/SCTE-35 messages across its broadcast channels, where a single source feed (containing SCTE-104s) is used as input into multiple inserters (encoders).
The following sections define the SCTE-104/35 messages that shall be used for specific marker Use Cases.
It should be noted that this is not an exhaustive list and the solution shall support sending messages of any type. In fact SCTE-104 messages can be used to send generic signalling to an Inserter by extending the SCTE-104 opcodes, and this should not be precluded (e.g. SubTitle delay signalling).
Spot and Break markers shall be signalled using SCTE-35 Splice Insert messages, with a duration field. Typically only a “from network” message will be sent, as the duration field within this message is sufficient to determine the “to network” point.
An optional Avail descriptor may be included within the Splice Insert message. In addition to the Splice Insert message an optional time_signal message with a segmentation descriptor signalling a Spot/break marker may be sent. However this is seen as secondary to the Splice Insert message.
To signal Programme and Chapter boundaries a SCTE-35 segmentation descriptor shall be used. This shall be carried within a time_signal message. Support of segmentation descriptors within the Splice_insert and Splice_Null message is not considered appropriate.
The inserter (Encoder) shall periodically generate an Splice Null message when there are no actual SCTE-35 messages to be delivered. This is used to provide a heartbeat to the mux, so that it does not alarm.