Sustainability of Digital Formats: Planning for Library of Congress Collections

Digital Video Broadcasting; Subtitling systems (DVB-SUB) standard defines "the method by which subtitles, logos and other graphical elements are coded and carried in DVB bitstreams." The draft document is produced by the European Standard (EN)'sJoint Technical Committee and published by the European Telecommunications Standards Institute (ETSI). The Digital Video Broadcasting Project (DVB) is an industry-led consortium defining open technical specifications for digital broadcasting and media. ETSI EN 300 743 was first published in 1997, specifying subtitles for SDTV, since the standard has had many revisions introducing new features and updating to new services. The most current standard is ETSI EN 300 743 V1.6.1 2018-7 (referenced throughout this document). See Notes for more information about previous versions.

DVB-SUB Subtitling System:

The DVB-SUB Subtitling system "provides a syntax for delivering subtitle streams." Streams transmit one or more subtitle services, the textual or graphical subtitle information, to display subtitle information sequences of pages that are overlayed on the intended video/image. Pages contain a rectangular area(s) with specified attributes called regions. Attributes include region id, resolution, pixel depth, and background colour. Regions are used as the background to place objects, such as a character, line of text, logo, or icon.

Subtitling Segment - as defined by the standard, the "basic syntactical element of a subtitle stream" or "building block." Segments are defined for various subtitling data and functions.

DVB-SUB standard defines several types (listed in order as they would appear if present in a display set):

• Display definition segment (DDS) - optional, defines the display width and height of the TV image into which the associated DVB subtitles are to be rendered. If there is no DDS, it is implied that the stream is coded in accordance with ETSI EN 300 743.
• Page composition segment (PCS) - carries information on the page composition, such as the list of included regions, the spatial position of each region, some time-out information for the page, and the state of the page.
• Region composition segment (RCS) - carries information on the region composition and attributes, such as the horizontal and vertical resolution, the background colour, the pixel depth of the region, CLUT family is used, and a list of included objects with their position within the region.
• Disparity signalling segment (DSS) - supports the subtitling of plano-stereoscopic 3DTV content.
• CLUT definition segment (CDS) - contains information on a specific CLUT family.
  • DVB subtitle streams use CLUTs (Colour Look-Up Tables) to define colours. The use of CLUTs allows colour schemes to be dynamic.
• Alternative CLUT segment (ACS) - conveys the subtitling CLUT used for colour systems other than the CDS/Recommendation ITU-R BT.601.
• Object data segment (ODS) - carries information on a specific subtitle object. 3 types of objects:
  • Graphical object - interlaced format, containing run-length bitmap colours.
  • Graphical object - progressive format, compressed using zlib compression and applied DEFLATE compression.
  • Text object - string of character codes, text object not defined in present standard document.
• End of display set segment (EDS) - contains no internal information, used to signal end of segments, start decoding of current display.

DVB-SUB Subtitle Steam Transport:

As stated in the standard, "a DVB subtitle stream is carried within Packetized Elementary Stream (PES) packets in the MPEG-2 Transport Stream (TS) according to ISO/IEC 13818-1, in transport packets identified by the same Packet IDentifier (PID)." See MPEG-2_family

• A single stream can carry several different services (i.e., different languages) but all the subtitle data required for a subtitle service is carried within a single subtitle stream
• Presentation time stamp (PTS) in the PES header provides presentation timing information and is associated with subtitle data in all segments carried in that PES packet
• PTS defines the time at which decoded segments are presented

According to Katie Terrell Hanna in What is Digital Video Broadcasting (DVB)?, updated March 2022, "A fundamental decision of the DVB Project was the selection of MPEG-2, one of a series of MPEG Standards for compression of audio and video signals. MPEG-2 reduces a single signal bandwidth from 166 Mbps to 5 MBOS, allowing broadcasters to transmit digital signals using an existing cable, satellite, and terrestrial systems.

Milos Pesic of Medium.com, wrote in DVB Bitmap Subtitles Processing at Zattoo, 2020, "Another quite popular format, especially in Europe, is the DVB bitmap subtitling format. Instead of a textual representation, a subtitle entry of this type is represented as a graphical bitmap (image) with timing and screen positioning metadata. Players receiving these images overlay them on top of the video at the time and on the screen, position indicated by associated metadata."

In the article, FFMPEG - From Zero to Hero, 2022, Nick Ferrando states, "In the broadcast world, the 'Embed Captions' are often contained (muxed) into an MPEG-TS file (.ts). The 'MPEG-Transport-Stream' (MPEG-TS, MTS), or simply transport stream (TS) is a standard digital container (which is a file format that allows multiple data streams to be embedded -or muxed- into a single file, usually along with metadata for identifying and further detailing those streams format) for transmission and storage of audio, video, and Program and System Information Protocol (PSIP) data. MPEG-TS it's a standard format used in broadcast systems, such as DVB (Digital-Video-Broadcasting)."

Advantages of DVB Subtitling:

As stated in Using DVB/Teletext Screen Subtitling with FlipFactory, 2010, "Advantages of the DVB Subtitling include: well-proven and reliable technology; viewer on/off control of subtitle display; subtitle style, size, and position controlled by broadcaster; all languages and alphabets are supported; efficient use of bandwidth; and HD compliant."

Limitations of DVB Subtitling:

According to Milos Pesic in the article, DVB Bitmap Subtitles Processing at Zattoo, 2020, There are some very limiting concerns when it comes to DVB bitmap subtitles:

• Limited support on client devices supporting standard HTTP protocols (DASH, HLS)
• Do not meet standard captioning requirements in some countries (visual customization)
• Bitmap images are not scalable by the end-user, unless with compatible IDR

Uses of DVB Subtitling:

Per BroadPeak.tv's blog, Simplifying the Management of Subtitles in an IP Video World, August 2020, DVB subtitles "became popular in the broadcast world of live TV as they are images (literally bitmap images) with the subtitles already 'burned.' These images can be decoded easily by broadcast set-top boxed used by cable and satellite operators."

Open standard. Produced by the European Telecommunications Standards Institute (ETSI), an independent, not-for-profit, standardization organization.

ETSI EN 300 743 V1.6.1 standard states, "this draft European Standard (EN) produced by Joint Technical Committee (JTC)Broadcast of the European Broadcasting Union (EBU), Comité Européen de Normalisation ELECtrotechnique (CENELEC) and the European Telecommunications Standards Institute (ETSI), and is now submitted for the combined Public Enquiry and Vote phase of the ETSI standards EN Approval Procedure."

ETSI EN 300 743 V1.6.1 (2018-07) | Digital Video Broadcasting (DVB); Subtitle systems
(https://www.etsi.org/deliver/etsi_en/300700_300799/300743/01.06.01_20/en_300743v010601a.pdf)

DVB BlueBook A009
(https://dvb.org/wp-content/uploads/2019/12/a009_dvb_bitmap_subtitles_nov_2017.pdf)
(https://dvb.org/?standard=dvb-subtitling-systems)

DBV Subtitling method produces "high-quality selectable subtitling on a wide variety of platforms and has been adopted as a world-wide standard," as stated on EdgeWare.tv'sDVB Bitmap Subtitling page.

According to Paul Szucs in the DVB Webinar on Subtitling Standards (part 1), 2018, "Despite the ongoing work on text-based next-generation subtitles, bitmap-based subtitles will continues to be a relevant feature in DVB broadcast systems...Bitmap-based subtitles offer a more pragmatic and efficient approach in DVB deployment regions where support is needed for subtitle in many languages with different fonts, some of which are extremely complex." Video Link: (https://dvb.org/webinar/subtitling-systems-january- 2018/).

Per DVB.org Standards & BlueBooks, European Standards are "the highest ranking ETSI publication approved by the national standards organisations of Europe. An EN is a publication which can be and often is included in European and national legislation."

Digital TV Status Map (http://en.dtvstatus.net/).

Comments welcome.

Information found on DVB registration, licenses, and patents, but no specific information found on DVB-SUB licensing and patents.

Per DVB.org, DVB Services Sàrl, formed in 2003 under Swiss law, acts as a registration authority for all DVB identifiers. "All DVB broadcasts, whether over satellite, cable, terrestrial or IP networks, use DVB's Service Information (DVB-SI, EN 300 468) standard to help receivers automatically tune to available broadcasts."

DVB Services Sàrl, owned by the DVB project, provides services related to digital broadcasting, such as licenses of trademarks, intellectual property, and delivery of identification numbers.

• ETSI EN 300 468 V1.16.1 | Digital Video Broadcasting (DVB); Specification for Service Information (SI) in DVB systems
  (https://www.etsi.org/deliver/etsi_en/300400_300499/300468/01.16.01_20/en_300468v011601a.pdf)
• TS 101 211 V1.12.1 | Digital Video Broadcasting (DVB); Guidelines on implementation and usage of Service Information (SI)
  (https://www.etsi.org/deliver/etsi_ts/101200_101299/101211/01.12.01_60/ts_101211v011201p.pdf)
• ETSI SR 000 314 V2.26.2 | Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in respect of ETSI standards
  (https://www.etsi.org/deliver/etsi_sr/000300_000399/000314/02.26.02_60/sr_000314v022602p.pdf)
• TR 101 200 V1.1.1 | Digital Video Broadcasting (DVB); A guideline for the use of DVB specifications and standards
  (https://www.etsi.org/deliver/etsi_tr/101200_101299/101200/01.01.01_60/tr_101200v010101p.pdf)

Comments welcome.

Per the standard, codec usage is specified in ETSI TS 101 154, Digital Video Broadcasting (DVB); Specification for the use of Video and Audio Coding in Broadcast and Broadband Applications, providing guidelines for coding and decoding MPEG-2 system layer.

ETSI TS 101 154, Private Data Bytes Detailed Specification, "Transport stream (TS) packets coded according to Recommendation ITU-T H.222.0 / ISO/IEC 13818-1."

ISO/IEC 13818-1:2022 | Information technology — Generic coding of moving pictures and associated audio information — Part 1: Systems
(https://www.iso.org/standard/83239.html)

Bitmap subtitles can be difficult to edit without special software to convert them. There are free open-source software available that can open and run Optical Character Recognition (OCR) on embedded TS/M2T files, including Subtitle Edit and Subtitle Composer.

Comments welcome.

Timing and screen position metadata within MPEG-2 TS. Limited information found on specific DVB-SUB metadata.

Comments welcome.

DVB-compliant broadcasting, receivers, and equipment is needed, distinguished the DVB logo, for encoding and decoding DVB subtitle streams.

Per the standard, "the specification makes demands which are assumed to be reasonable in a region based, indexed colour, graphics system. Implementers are free to implement the graphics system in other ways. However, it is their responsibility to compensate for the implications of using an architecture that is different from that envisaged in the subtitle decoder model."

Katie Terrell Hanna explains in What is Digital Video Broadcasting (DVB)?, updated March 2022, "DVB uses conditional access (CA) systems to prevent external piracy...Each CA system provides a security module that scramble and encrypts data. This security module is embedded within the receiver or is detachable in the form of a PC card. Inside the receiver, there is a smart card that contains the user's access information...Detachable PC cards allow subscribers to use DVB services anywhere DVB technology is supported."

Comments welcome.

Intellectual Property Rights, per DVB.org, "Its most significant features are the commitment of each member to grant licenses to essential patents unless it gives notice that its patents are unavailable, and DVB's active encouragement to the formation of patent pools covering DVB standards. These measures protect the interests of those with IPRs to license, and those who are licensing the IPRs, to deploy DVB products and services in the marketplace."

ETSI SR 000 314 V2.26.2 | Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in respect of ETSI standards
(https://www.etsi.org/deliver/etsi_sr/000300_000399/000314/02.26.02_60/sr_000314v022602p.pdf)

Comments welcome.

Some support. DVB-SUB are Bitmap images, a simple raster format, but are intended to be rendered by a compliable Integrated Receiver Decoder, which may or may not have the ability to scale, zoom, or pan.

Per standard, "HDTV and UHDTV IRDs that offer a means of scaling or positioning the subtitles under user control (e.g. to make them larger or smaller) can use the information conveyed in the display definition segment to determine safe strategies for zooming and/or positioning that will ensure that windowed subtitles can remain visible. However, scaling operations are not recommended for subtitles that have been anti-aliased for their original graphical resolution. Any scaling applied to such subtitles could degrade them significantly and thereby impact their readability."

See BMP_5.

Comments welcome.

Per standard, display resolutions assume screen resolution of SDTV - 720 by 576 pixels, the optional Display Definition Segment (DDS) defines display resolution for the service it was created for, i.e., HDTV - 920 by 1080 pixels.

See BMP_5.

Comments welcome.

Per standard, DVB subtitles use CLUTs (Colour Look-Up Tables) to define colours. ETSI defines CLUT as a look-up table applied in each region for translating the objects' pseudo-colours into the correct colours to be displayed.

There are 3 segment types that can be present in a display set that pertain to color: RCS, CDS, and ACS. See Description for more detail.

DVB-SUB format directly supports IRDs that can present 4 colours, 16 colours and 256 colours. The format allows 256 colour graphics to be broadcast but then provides a model by which the whole spectrum of 256 colours can be quantized to 16 or 4 colours. The intention is to offer broadcasters and equipment manufacturers both a route and an incentive to move to 256 colour systems while allowing the introduction of subtitling services at a time when many systems will not be able to implement 256 colours.

See BMP_5.

Comments welcome.

Low support. Per standard, Character Object Decoding: Information inside the subtitling streams is not sufficient for character object decoding, the subtitle system delivers character references rather than graphical objects. "A local agreement between broadcasters and equipment manufacturers may be an appropriate way to ensure reliable operation of character coded subtitles. A local agreement would probably define the characteristics of the font (character size and other metrics). It should also define a model for rendering of the characters."

See BMP_5.

Comments welcome.

Little to no information found on support for multispectral bands.

Comments welcome.

Some support. Per standard, Version 1.4.1 introduced support for subtitles for 3DTV, defined by ETSI TS 101 154 and ETSI EN 300 468. An optional Disparity Signalling Segment (DDS) was specified, enabling a region or a part of a region to facilitate rendering subtitle over 3DTV content.

EN 300 743 - V1.6.1 - Annex D: Guidelines on the use of ETSI EN 300 743 for 3D content.

TS 101 154 - V2.4.1 defines 3DTV as "DVB frame compatible plano-stereoscopic three- dimensional television" and Plano-Stereoscopic as "three-dimensional picture that uses two single pictures, left and right, displayed on a single place surface (the TV screen in the case of 3DTV)."

See BMP_5.

Comments welcome.

Low support. DVB-SUB contains textual information, but it is displayed as bitmap image as its final output.

Comments welcome.

Versions and Compatibility

Edition 1, 1997, subtitles for SDTV.

Version 2, 2002, general maintenance.

Version 3, 2006, subtitles for HDTV, new optional segment - display definition segment (DDS), defines the display resolution for which that stream has been created, it allows subtitles with resolutions other than SDTV to be provided, and optionally allows subtitles to be positioned with a window that constitutes only a part of the full display resolution.

• Subtitle encoded using g Recommendation ITU-R BT.601 [3] colorimetry, i.e., the same as that used in video components of SDTV.
• HDTV systems use Recommendation ITU-R BT.709 [10] colorimetry.

Version 4, 2011, added support for subtitle for 3DTV, new optional segment - disparity signaling segment (DSS), enables a region or part of a region to be attributed with a disparity value, to facilitate the optimal rendering of subtitle over 3DTV content.

Version 5, 2013, general maintenance.

The Digital Video Broadcasting Project (DVB) is an industry-led consortium of broadcasters, manufacturers, network operators, software developers, and others committed to designing global standards for the delivery of digital television and data services.

Per DVB.org History, The DVB-S system was agreed in 1994, and the first DVB broadcast services in Europe started in spring 1995 by pay TV operator Canal plus in France. The DVB-T system was agreed later, in 1997. The first DVB-T broadcasts began in Sweden and the UK in 1998. DVB-T services started in parts of Germany in 2002, and in 2003, there was Europe's first analogue switch off in Berlin.

By 1997 the development of the DVB Project had successfully followed the initial plans, and the project had entered its next phase, promoting its open standards globally, and making digital television a reality. DVB standards were adopted worldwide and became the benchmark for digital television worldwide.