ETSI ETS 300 743 ed.1 (1997-09)

Digital Video Broadcasting (DVB); Subtitling systems

DE/JTC-DVB-17

Digitalna videoradiodifuzija (DVB) – Sistemi za podnaslove

General Information

• Status: Published
• Publication Date: 17-Sep-1997
• Technical Committee: BROADCAS - EBU/CENELEC/ETSI on Broadcasting
• Current Stage: 12 - Completion
• Due Date: 26-Sep-1997
• Completion Date: 18-Sep-1997
• Ref Project: SIST ETS 300 743 E1:2005 - Digital Video Broadcasting (DVB); Subtitling systems

Standards Content (Sample)

SLOVENSKI STANDARD
SIST ETS 300 743 E1:2005
01-november-2005
Digitalna videoradiodifuzija (DVB) – Sistemi za podnaslove
Digital Video Broadcasting (DVB); Subtitling systems
Ta slovenski standard je istoveten z: ETS 300 743 Edition 1
ICS:
33.170 Televizijska in radijska Television and radio
difuzija broadcasting
SIST ETS 300 743 E1:2005 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST ETS 300 743 E1:2005
EUROPEAN ETS 300 743
TELECOMMUNICATION September 1997
STANDARD
Source: EBU/CENELEC/ETSI-JTC Reference: DE/JTC-DVB-17
ICS: 33.020
Key words: DVB, digital, video, broadcasting, TV
European Broadcasting Union Union Européenne de Radio-Télévision
Digital Video Broadcasting (DVB);
Subtitling systems
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
Postal address: F-06921 Sophia Antipolis CEDEX - FRANCE
Office address: 650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
X.400: c=fr, a=atlas, p=etsi, s=secretariat - Internet: [e-mail]
Tel.: [phone] - Fax: [phone]
Copyright Notification:
No part may be reproduced except as authorized by written permission. The copyright and the
foregoing restriction extend to reproduction in all media.
© European Telecommunications Standards Institute 1997.
© European Broadcasting Union 1997.
All rights reserved.

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Whilst every care has been taken in the preparation and publication of this document, errors in content,
typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to
"ETSI Editing and Committee Support Dept." at the address shown on the title page.

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Contents
Foreword .5
1 Scope .7
2 Normative references.7
3 Definitions and abbreviations .7
3.1 Definitions .7
3.2 Abbreviations .8
4 Introduction to DVB subtitling system.9
4.1 Overview .9
4.2 Data hierarchy and terminology .10
4.3 Temporal hierarchy and terminology .10
5 Subtitle decoder model.11
5.1 Decoder temporal model .11
5.1.1 Service acquisition .11
5.1.2 Presentation Time Stamps (PTS) .12
5.1.3 Page composition.12
5.1.4 Region composition.12
5.1.5 Points to note .13
5.2 Buffer memory model .13
5.2.1 Pixel display buffer memory .13
5.2.2 Region memory.14
5.2.3 Composition buffer memory.14
5.3 Cumulative display construction .14
5.4 Decoder rendering bandwidth model.14
5.4.1 Page erasure.14
5.4.2 Region move or change in visibility .14
5.4.3 Region fill.15
5.4.4 CLUT modification.15
5.4.5 Graphic object decoding.15
5.4.6 Character object decoding .15
6 PES packet format .16
7 The PES packet data for subtitling.16
7.1 Syntax and semantics of the PES data field for subtitling .16
7.2 Syntax and semantics of the subtitling segment.16
7.2.1 Page composition segment.17
7.2.2 Region composition segment.19
7.2.3 CLUT definition segment.21
7.2.4 Object data segment .22
7.2.4.1 Pixel-data sub-block .24
7.2.4.2 Syntax and semantics of the pixel code strings.25
8 Requirements for the subtitling data .27
8.1 Scope of Identifiers .27
8.2 Scope of dependencies .27
8.2.1 Composition page .27
8.2.2 Ancillary page.27
8.3 Order of delivery .28
8.3.1 PTS field.28
8.4 Positioning of regions and objects .28
8.4.1 Regions .28

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8.4.2 Objects sharing a PTS. 28
8.4.3 Objects added to a region. 28
8.5 Avoiding excess pixel-data capacity. 28
9 Translation to colour components . 28
9.1 4- to 2-bit reduction . 29
9.2 8- to 2-bit reduction . 29
9.3 8- to 4-bit reduction . 29
10 Default CLUTs and map-tables contents. 30
10.1 256-entry CLUT default contents . 30
10.2 16-entry CLUT default contents . 31
10.3 4-entry CLUT default contents . 31
10.4 2_to_4-bit_map-table default contents. 32
10.5 2_to_8-bit_map-table default contents. 32
10.6 4_to_8-bit_map-table default contents. 32
11 Structure of the pixel code strings (informative) . 33
Annex A (informative): How the DVB subtitling system works . 34
A.1 Data hierarchy and terminology. 34
A.2 Temporal hierarchy and terminology . 35
A.3 Decoder temporal model . 35
A.3.1 Presentation Time Stamps (PTS) . 35
A.3.2 Page composition. 35
A.3.3 Region composition. 36
A.3.4 Points to note . 36
A.4 Decoder display technology model. 36
A.4.1 Region based with indexed colours. 36
A.4.2 Colour quantization . 37
A.5 Decoder rendering bandwidth model. 37
A.5.1 Page erasure. 37
A.5.2 Region move or change in visibility . 38
A.5.3 Region erasure. 38
A.5.4 CLUT modification. 38
A.5.5 Graphic object decoding . 38
A.5.6 Character object decoding . 38
A.6 Examples of the subtitling system in operation . 39
A.6.1 Double buffering. 39
A.6.1.1 Instant graphics . 39
A.6.1.2 Stenographic subtitles . 42
A.7 Glossary. 44
History. 45

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Foreword
This European Telecommunication Standard (ETS) has been produced by the Joint Technical Committee
(JTC) of the European Broadcasting Union (EBU), Comité Européen de Normalisation ELECtrotechnique
(CENELEC) and the European Telecommunications Standards Institute (ETSI).
NOTE: The EBU/ETSI JTC was established in 1990 to co-ordinate the drafting of ETSs in the
specific field of broadcasting and related fields. Since 1995 the JTC became a tripartite
body by including in the Memorandum of Understanding also CENELEC, which is
responsible for the standardization of radio and television receivers. The EBU is a
professional association of broadcasting organizations whose work includes the
co-ordination of its Members' activities in the technical, legal, programme-making and
programme-exchange domains. The EBU has active members in about 60 countries in
the European Broadcasting Area; its headquarters is in Geneva*.
* European Broadcasting Union
Case Postale 67
CH-1218 GRAND SACONNEX (Geneva)
Switzerland
Tel: [phone]
Fax: [phone]
Digital Video Broadcasting (DVB) Project
Founded in September 1993, the DVB Project is a market-led consortium of public and private sector
organizations in the television industry. Its aim is to establish the framework for the introduction of
MPEG-2 based digital television services. Now comprising over 200 organizations from more than
25 countries around the world, DVB fosters market-led systems, which meet the real needs, and
economic circumstances, of the consumer electronics and the broadcast industry.
Transposition dates
Date of adoption: 5 September 1997
Date of latest announcement of this ETS (doa): 31 December 1997
Date of latest publication of new National Standard
or endorsement of this ETS (dop/e): 30 June 1998
Date of withdrawal of any conflicting National Standard (dow): 30 June 1998

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1 Scope
This European Telecommunication Standard (ETS) specifies the method by which subtitles, logos and
other graphical elements may be coded and carried in DVB bitstreams. The system applies Colour
Look-Up Tables (CLUTs) to define the colours of the graphical elements. The transport of the coded
graphical elements is based on the MPEG-2 system described in ISO/IEC 13818-1 [1].
2 Normative references
This ETS incorporates by dated and undated reference, provisions from other publications. These
normative references are cited at the appropriate places in the text and the publications are listed
hereafter. For dated references, subsequent amendments to or revisions of any of these publications
apply to this ETS only when incorporated in it by amendment or revision. For undated references the latest
edition of the publication referred to applies.
[1] ISO/IEC 13818-1: "Coding of moving pictures and associated audio".
[2] ETS 300 468: "Digital Video Broadcasting (DVB); Service Information (SI) in
DVB systems".
[3] ISO/IEC 10646-1 (1993): "Information Technology - Universal Multiple Octet
Coded Character Set (UCS) - Part 1: Architecture and Basic Multilingual Plane".
[4] ITU-R Recommendation 601-3 (1992): "Encoding parameters of digital
television for studios".
3 Definitions and abbreviations
3.1 Definitions
For the purposes of this ETS, the following definitions apply:
ancillary page: An optional page that can be used to carry CLUT definition and object data segments that
can be shared by more than one subtitle stream. For example, the ancillary page can be used to carry
logos or character glyphs.
Colour Look-Up Table (CLUT): A look-up table applied in each region for translating the objects' pseudo-
colours into the correct colours on the screen. In most cases, one CLUT is sufficient to present correctly
the colours of all objects in a region, but if it is not enough, then the objects can be split horizontally into
smaller objects that, combined in separate regions, need not more than one CLUT per region.
CLUT-family: A family of CLUTs which consists of:
- one CLUT with 4 entries;
- one CLUT with 16 entries;
- one CLUT with 256 entries.
NOTE 1: Three CLUTs are defined to allow flexibility in the decoder design. Not all decoders
may support a CLUT with 256 entries, some may provide sixteen or even only four
entries. A palette of four colours would be enough for graphics that are basically
monochrome, like subtitles, while a palette of sixteen colours allows for cartoon-like
coloured objects. Having a CLUT of only four entries does not imply that only a rigid
colour scheme can be used. The colours that correspond to the four entries can be
redefined, for instance from a black-grey-white scheme to a blue-grey-yellow scheme.
Furthermore, a graphical unit may be divided into several regions that are linked to
different CLUTs, i.e. a different colour scheme may be applied in each of the regions.
composition page: The page which carries the page composition. This page may contain graphical
elements as well. Those elements that may be shared by different screen layouts are carried in an
"ancillary page".

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NOTE 2: Thus, alternative screen layouts, defined as different page compositions, may use the
same CLUTs and objects. There is no need to convey the common information for
each screen layout separately. This sharing is particularly useful when subtitles are
provided in several languages, all combined with the same logo. To retain flexibility,
the position at which a region is shown on the screen is not a property of that region
itself, but defined in the page composition, so that a shared region may be shown in
different locations on different screen layouts.
decoder state: Pixel and composition buffer memory allocations and values.
display: A completed set of graphics.
display set: The set of segments that operate on the decoder state between page composition segments
to produce a new display.
display sequence: A sequence of one or more displays.
epoch: The period between resets to the decoder state caused by page composition segments with page
state = "mode change".
object: Anything that can be presented on a TV screen, e.g. a subtitle, a logo, a map, etc. An object can
be regarded as a graphical unit. Each has its own unique ID-number.
packet identifier: See ISO/IEC 13818-1 [1].
page composition: The top-level definition of a screen layout. Several regions may be shown
simultaneously on the screen; those regions are listed in the page composition. At any one time, only one
page composition can be active for displaying, but many may be carried simultaneously in the bitstream.
PES packet: See ISO/IEC 13818-1 [1].
pixel-data: A string of data bytes that contains, in coded form, the representation of a graphical object.
region: A rectangular area on the screen in which objects are shown. Objects that share one or more
horizontal scan lines on the screen are included in the same region.
NOTE 3: A region therefore monopolizes the scan lines of which it occupies any part; no two
regions can be presented horizontally next to each other.
transport packet: See ISO/IEC 13818-1 [1].
transport packet stream: A sub-set of the transport packets in a transport stream sharing a common
Packet Identifier (PID).
transport stream: See ISO/IEC 13818-1 [1]. A data stream carrying one or more MPEG programs.
subtitle stream: A stream of subtitling segments that when decoded will provide a sequence of subtitling
graphics meeting a single communication requirement (e.g. the graphics to provide subtitles in one
language for a one program). A subtitling stream may contain data from a single page (the composition
page) or from two pages (the composition page and the ancillary page).
3.2 Abbreviations
For the purposes of this ETS, the following abbreviations apply:
bslbf bit string, left bit first
Cb as defined in ITU-R Recommendation 601-3 [4] (see subclause 7.2.3)
CLUT Colour Look-Up Table
Cr as defined in ITU-R Recommendation 601-3 [4] (see subclause 7.2.3)
DVB Digital Video Broadcasting
IRD Integrated Receiver Decoder
MPEG Moving Pictures Experts Group

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PCR Programme Clock Reference
PCS Page Composition Segments
PES Packetized Elementary Stream
PID Packet IDentifier
PMT Program Map Table
PTS Presentation Time Stamp
RCS Region Composition Segments
ROM Read-Only Memory
TS Transport Stream
uimsbf unsigned integer, most significant bit first
Y as defined in ITU-R Recommendation 601-3 [4] (see subclause 7.2.3)
4 Introduction to DVB subtitling system
This ETS specifies the transport and coding of graphical elements in the DVB subtitling system.
4.1 Overview
To provide efficient use of the display memory in the decoder this subtitling system uses region based
graphics with indexed pixel colours. Each display is composed of a number of regions with specified
position. A region is a rectangular area with a horizontal and vertical size, pixel depth. A region can have a
defined background colour and graphical objects can be positioned within the region.
Pixel depths of 2, 4 and 8-bits are supported allowing up to 4, 16 or 256 different pixel codes to be used in
each region. Each region is associated with a CLUT which defines the colour and transparency for each of
the pixel codes.
At the discretion of the encoder, objects designed for displays supporting 16 or 256 colours can be
decoded into displays supporting fewer colours. A quantization algorithm is defined to ensure that this
process is predictable by the originator. This feature allows a single data stream to be decoded by a
population of decoders with mixed, and possibly evolving, capabilities.
This subtitling system provides a number of techniques that allow efficient transmission of the graphic
data:
- pixel structures that occur more than once within a bitmap can be transmitted only once, and then
positioned multiple times within the bitmap;
- pixel structures used in more than one subtitle stream shall only be transmitted once;
- pixel data is compressed using run-length coding;
- where the gamut of colours required for part of a graphical object is suitably limited, that part can be
coded using a smaller number of bits per pixel and a map table. For example, an 8-bit per pixel
graphical object may contain areas coded as 4 or 2-bits per pixel each preceded by a map table to
map the 16 or 4 colours used onto the 256 colour set of the region. Similarly, a 4-bit per pixel object
may contain areas coded as 2-bits per pixel;
- colour definitions can be coded using either 16 or 32-bits per CLUT entry. This provides a trade off
between colour accuracy and transmission bandwidth.
The above features require only compliance with this ETS. Additional features are provided that allow
more efficient operation where there are additional agreements between the data provider and the
manufacturer of the decoder:
- graphic objects resident in ROM in the decoder can be referenced;
- character codes, or strings of character codes, can be used in place of graphic object references.
This requires the decoder to be able to generates glyphs for these codes.
This ETS is not concerned with the private agreements required to make these features operate.

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4.2 Data hierarchy and terminology
The "building block" of the subtitling information is the subtitling_segment. These segments are carried in
PES packets which are in-turn carried by Transport Packets.
All the broadcast data required for a subtitle stream will be carried by a single transport packet stream (i.e.
on a single PID). A single transport packet stream can carry several different streams of subtitles. The
different subtitle streams can be subtitles in different languages for a common program. Alternatively, they
can be for different programs (provided that the programs share a common PCR).
Different subtitle streams can also be supplied to address different display characteristics or to address
special needs. For instance:
- different subtitle streams can be provided for 4:3 and 16:9 aspect ratio displays;
- subtitle streams can be provided for viewers with impaired hearing. These may include graphical
representations of sounds.
Within a transport packet stream the segments for different subtitling streams are identified by their page
identifiers. One or more subtitling_descriptors ETS 300 468 [2] in the PMT for a program describe the
available subtitling streams and specify the PID and page ids that shall be decoded for each subtitling
stream.
A subtitling stream may contain data from a single page (the composition page) or from two pages (the
composition page and the ancillary page). The ancillary page can be used to carry objects that are
common to 2 or more subtitle streams. For example, the ancillary page can carry a logo that is common to
subtitle streams for several different languages.
The PTS in the PES packet provides presentation timing information for the subtitling data. The number of
segments carried by each PES packet is only limited by the maximum length of a PES packet defined by
MPEG.
In summary the data hierarchy is:
- Transport Stream (TS);
- transport packet stream (common PID);
- PES (provides timing);
- subtitle stream (composition or composition and ancillary pages);
- page;
- segment.
4.3 Temporal hierarchy and terminology
At the segment level in the data hierarchy there is temporal hierarchy. The highest level is the epoch. This
is analogous to the MPEG video sequence. No decoder state is preserved from one epoch to the next.
An epoch is a sequence of one or more displays. Each display is a completed screen of graphics.
Consecutive displays may differ little (e.g. by a single word when stenographic subtitling is being used) or
may be completely different. The set of segments that form each display is called a display set.
Within a display set the sequence of segments (when present) is:
- page composition;
- region composition;
- CLUT definition;
- object data.
All segments associated with composition page shall be delivered before any segments from the optional
ancillary page. The ancillary page may only carry CLUT definition or object data segments.

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5 Subtitle decoder model
The subtitle decoder model is an abstraction of the processing required for the interpretation of subtitling
streams. The main purpose of this model is to define a number of constraints which can be used to verify
the validity of subtitling streams. The following figure shows a typical implementation of a subtitling
decoding process in a receiver.
192 kbit/s Subtitle Decoder 512 kbit/s
Transport Pre- Coded data Subtitle Pixel
MPEG-2
PID filter
buffer processor buffer processing buffer
TS packets
kByte kByte
24 80
512 byte and filters
Composition
buffer
4 kByte
Figure 1: Subtitle decoder model
The input to the subtitling decoding process is an MPEG-2 Transport Stream (TS). After a selection
process based on PID value, complete MPEG-2 Transport Stream packets enter into a transport buffer
with a size of 512 byte. When there is data in the transport buffer, data is removed from this buffer with a
rate of 192 kbit/s. When no data is present, the data rate equals zero.
The MPEG-2 transport stream packets from the transport buffer are processed by stripping off the packet
headers of TS packets and of Packetized Elementary Stream (PES) packets with the proper
data_identifier value. The Presentation Time Stamp (PTS) fields shall be passed on to the next stages of
the subtitling processing. The output of the pre-processor is a stream of subtitling segments which are
filtered based on their page_id values.
The selected segments enter into a coded data buffer which has a size of 24 kbyte. Only complete
segments are removed from this buffer by the subtitle decoder. The removal and decoding of the
segments is instantaneous (i.e. it takes zero time). If a segment produces pixel data, the subtitle decoder
stops removing segments from the coded data b
...