Information technology — Coding of audio-visual objects — Part 17: Streaming text format

ISO/IEC 14496-17:2006 was developed in response to the need for a generic method for coding of text at very low bitrate as one of the multimedia components within an audiovisual presentation. ISO/IEC 14496-17:2006 allows for example subtitles and Karaoke song texts to be coded and transported as separate text streams at bitrates that are sufficently low for use in mobile services over IP. Target applications are in particular found in areas with severe transmission bandwidth constraints, such as mobile services over IP. However, services over broadband IP, over broadcast channels and over optical media may also benefit from the low bandwidth of ISO/IEC 14496-17:2006 text streams. ISO/IEC 14496-17:2006 specifies a text stream as a concatenation of text access units. A text access unit contains text information of a specific format. For each text stream, the format of the text information within the text access units is signalled. Each text access unit within a text stream is intended to be displayed at a specific instance in time. Note that ISO/IEC 14496-17:2006 signals the format of the text contained in text access units, but that it does not contain any text format specification. Within ISO/IEC 14496-17:2006, a particular instance of text streams are 3GPP text streams. Each text access unit contained in a 3GPP text stream carries 3GPP Timed Text data, as specified in 3GPP TS 26.245. For 3GPP text streams, ISO/IEC 14496-17:2006 defines a generic framing structure suitable for transport of 3GPP text streams across a variety of networks, such as RTP for transport over IP, and MPEG-2 Systems for use in media such as broadcast and optical discs. The flexible framing structure for text streams, consisting of so-called TTUs (Timed Text Units), allows for easy and convenient adaptation to the various transport layers. By using TTUs, multiple small text access units can be aggregated into one transport packet, but TTUs can also be used to fragment large text access units across multiple transport packets, while providing a reasonable level of error resilience in case of packet loss or non-recoverable packet errors. For each transport layer, the most suitable TTU structure can be chosen. If so desired, the text access units can be re-partitioned into TTUs for most effective adaptation to other transport systems.

Technologies de l'information — Codage des objets audiovisuels — Partie 17: Format de texte en flux

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INTERNATIONAL ISO/IEC
STANDARD 14496-17
First edition
2006-04-01
Information technology — Coding of
audio-visual objects —
Part 17:
Streaming text format
Technologies de l'information — Codage des objets audiovisuels —
Partie 17: Format de texte en flux
Reference number
ISO/IEC 14496-17:2006(E)
ISO/IEC 2006
---------------------- Page: 1 ----------------------
ISO/IEC 14496-17:2006(E)
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© ISO/IEC 2006

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ii © ISO/IEC 2006 – All rights reserved
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ISO/IEC 14496-17:2006(E)
Contents Page

Foreword............................................................................................................................................................ iv

Introduction ....................................................................................................................................................... vi

1 Scope ..................................................................................................................................................... 1

2 Normative references ........................................................................................................................... 1

3 Terms and definitions........................................................................................................................... 1

4 Text stream format................................................................................................................................ 2

5 Text access units.................................................................................................................................. 2

5.1 Timing and decoding of text access units ......................................................................................... 2

5.2 Format of text access units ................................................................................................................. 2

5.3 TextConfig ............................................................................................................................................. 3

5.4 Semantics .............................................................................................................................................. 3

6 Usage of a text stream within an MPEG-4 system context............................................................... 3

6.1 Signaling of a text stream .................................................................................................................... 3

6.2 Usage in the scene description........................................................................................................... 3

7 Text data format for 3GPP text streams ............................................................................................. 3

7.1 Introduction ........................................................................................................................................... 3

7.2 Carriage of text samples and sample descriptions in 3GPP text access units.............................. 4

7.3 Transport of 3GPP text access units in TTUs.................................................................................... 4

7.4 TTU Syntax and Semantics.................................................................................................................. 9

7.5 Positioning of 3GPP text streams..................................................................................................... 13

7.6 Format specific decoder configuration for 3GPP text streams ..................................................... 14

7.7 Hypothetical Text Decoder for 3GPP text stream decoding........................................................... 17

7.8 Profile and Level parameters for 3GPP text streams...................................................................... 18

Annex A (normative) Font Referencing ......................................................................................................... 19

Annex B (informative) Transport of Text Streams ........................................................................................ 20

Bibliography ..................................................................................................................................................... 21

© ISO/IEC 2006 – All rights reserved iii
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ISO/IEC 14496-17:2006(E)
Foreword

ISO (the International Organization for Standardization) and IEC (the International Electrotechnical

Commission) form the specialized system for worldwide standardization. National bodies that are members of

ISO or IEC participate in the development of International Standards through technical committees

established by the respective organization to deal with particular fields of technical activity. ISO and IEC

technical committees collaborate in fields of mutual interest. Other international organizations, governmental

and non-governmental, in liaison with ISO and IEC, also take part in the work. In the field of information

technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of the joint technical committee is to prepare International Standards. Draft International

Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as

an International Standard requires approval by at least 75 % of the national bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent

rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.

ISO/IEC 14496-17 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,

Subcommittee SC 29, Coding of audio, picture, multimedia and hypermedia information.

ISO/IEC 14496 consists of the following parts, under the general title Information technology — Coding of

audio-visual objects:
— Part 1: Systems
— Part 2: Visual
— Part 3: Audio
— Part 4: Conformance testing
— Part 5: Reference software
— Part 6: Delivery Multimedia Integration Framework (DMIF)

— Part 7: Optimized reference software for coding of audio-visual objects [Technical Report]

— Part 8: Carriage of ISO/IEC 14496 contents over IP networks
— Part 9: Reference hardware description [Technical Report]
— Part 10: Advanced Video Coding (AVC)
— Part 11: Scene description and application engine
— Part 12: ISO base media file format
— Part 13: Intellectual Property Management and Protection (IPMP) extensions
— Part 14: MP4 file format
— Part 15: Advanced Video Coding (AVC) file format
iv © ISO/IEC 2006 – All rights reserved
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ISO/IEC 14496-17:2006(E)
— Part 16: Animation Framework eXtension (AFX)
— Part 17: Streaming text format
— Part 18: Font compression and streaming
— Part 19: Synthesized texture stream

— Part 20: Lightweight Application Scene Representation (LASeR) and Simple Aggregation Format (SAF)

— Part 21: MPEG-J GFX
— Part 22: Open Font Format
© ISO/IEC 2006 – All rights reserved v
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ISO/IEC 14496-17:2006(E)
Introduction

This International Standard was developed in response to the need for a generic method for coding of text at

very low bitrate as one of the multimedia components within audiovisual presentations. This International

Standard allows for example subtitles and Karaoke song texts to be coded and transported as separate text

streams for presentation jointly with other components of an audiovisual presentation at bitrates that are

sufficently low for use in mobile services over IP.
vi © ISO/IEC 2006 – All rights reserved
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INTERNATIONAL STANDARD ISO/IEC 14496-17:2006(E)
Information technology — Coding of audio-visual objects —
Part 17:
Streaming text format
1 Scope

This International Standard specifies the coded representation of textual information for timed presentation on

screens. The text may be streamed in association with video and audio, in which case the text may represent

subtitles e.g. with translations of the associated audio in another language, or as an aid to the hard of hearing;

another example is the text of a song in a Karaoke application. However, the text may also be streamed as a

stand-alone application without any associated video and audio. The streaming text format is specified in a

transport agnostic manner, so as to allow transport over a large variety of transport means, while providing a

reasonable level of random access and error robustness.

The text streams are defined as byte streams that are capable of carrying text access units of a specified

format, optionally interleaved with data needed for the decoding of the text stream. The format of text streams

and text access units is specified, as well as signaling and decoding of text streams.

2 Normative references

The following referenced documents are indispensable for the application of this document. For dated

references, only the edition cited applies. For undated references, the latest edition of the referenced

document (including any amendments) applies.

ISO/IEC 14496-18:2004, Information technology — Coding of audio-visual objects — Part 18: Font

compression and streaming
3GPP TS 26.245: 2003, Timed text format (Release 6)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
text stream

byte stream capable of carrying text access units of a specified format, optionally interleaved with data needed

for the decoding of the text stream
3.2
text access unit
individually accessible portion of data within a text stream

NOTE Each text access unit contains the coded representation of text data. For presentation, the text access unit

can be associated with a single time stamp.
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ISO/IEC 14496-17:2006(E)
3.3
3GPP text stream
text stream carrying 3GPP text access units
3.4
3GPP text access unit
text access unit carrying data from a text sample specified by 3GPP
3.5
text sample

when used in the context of a 3GPP text stream, a text sample, as specified in 3GPP TS 26.245, consisting of

a text string, optionally followed by one or more text modifiers
3.6
text string

when used in the context of a 3GPP text stream, data within a text sample, representing a string of characters

encoded using UTF-8 or UTF-16, as specified in 3GPP TS 26.245
3.7
text modifier

when used in the context of a 3GPP text stream, data within a text sample, specifying a modification to the

presentation of the text string within that text sample, as specified in 3GPP TS 26.245

3.8
sample description

when used in the context of a 3GPP text stream, descriptive text data, providing global information about one

or more text samples, such as font(s) to be used and positioning of the text, as specified in 3GPP TS 26.245

3.9
Timed Text Unit
TTU

syntactical structure within a 3GPP text stream for carriage of text access units, whereby its index j identifies

which type of data (such as a complete text access unit, a fragment thereof or a sample description) is carried

4 Text stream format

A text stream is a byte stream that is capable of carrying text access units of a specified format, optionally

interleaved with data needed for the decoding of the text stream. The text stream format is defined so as to

conveniently allow for carriage in transport packets and files. However, such carriage itself depends on

specific formats for file storage and transport and is beyond the scope of this standard.

5 Text access units
5.1 Timing and decoding of text access units

Each text access unit contains text data of a specified format; to each text access unit a single time stamp

applies. The time stamp assigned to a text access unit indicates the time at which the text access unit is to be

presented on the display. To decode a text stream, a receiver needs information on the text stream, as

defined by the so-called decoder configuration in TextConfig. The TextConfig signals the format of the text

data and may provide information specific to the format of the text data.
5.2 Format of text access units
text access unit{
textData; // the format of the textData is textFormat specific
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ISO/IEC 14496-17:2006(E)
5.3 TextConfig
TextConfig(){
bit(8) textFormat;
bit(16) textConfigLength;
formatSpecificTextConfig();
5.4 Semantics

textFormat – one byte signaling the format of the text data. The value 0x01 signals that the text data

carries timed text data as defined in 3GPP TS 26.245, in a manner defined in clause 7.

Table 1 — textFormat
0x00 Reserved
0x01 Timed Text as specified in 3GPP TS 26.245
0x02 – 0xEF Reserved
0xF0 – 0xFE User-private
0xFF Reserved

textConfigLength – unsigned integer that specifies the size in number of bytes of

formatSpecificTextConfig().
6 Usage of a text stream within an MPEG-4 system context
6.1 Signaling of a text stream

When used in an MPEG-4 system context, a text stream shall be signaled by a streamType value 0x0D and

by an objectTypeIndication value of 0x08.
6.2 Usage in the scene description

When used within an MPEG-4 Scene description, a text stream object is used as follows:

• If the text stream object is used by an AnimationStream node, it shall be presented in the scene,

regardless of whether the AnimationStream node is active or not;

• If the text stream object is not used by any AnimationStream node, it shall not be presented in the

scene.

Spatial presentation of the text data is specified by the underlying streaming text format. In case the scene

description has display size indication, results are undefined if the positioning of text data covers an out-of-

display area.

All rules regarding time control and segmentation of stream objects apply to the text stream object.

7 Text data format for 3GPP text streams
7.1 Introduction
This clause specifies the text data format for 3GPP text streams.
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ISO/IEC 14496-17:2006(E)
7.2 Carriage of text samples and sample descriptions in 3GPP text access units

3GPP TS 26.245 defines timed text data to consist of text samples and sample descriptions, and that each

text sample consists of one text string, optionally followed by one or more text modifiers. Each text string

represents the characters that form the text to be displayed, while the text modifiers carry the changes that are

to be applied to the text string during the time that the text is to be displayed within a text box, such as text

colour changes synchronized with a song for a Karaoke application.

A sample description provides global information about a text sample, for example about font(s) to be used,

about the positioning of the text within the text box, the background colour of that text box, etc. Multiple

sample descriptions are allowed; to each sample description (SD) an index is assigned and to each text

sample the index of the applicable sample description is associated. While a sample description will typically

apply to multiple text samples, to each text sample exactly one sample description applies.

The relationship between sample descriptions, text samples, a text string and text modifiers is depicted in

Figure 1.
Sample Descriptions
SD-a
….…. SD-x
Each Text Sample
associated to one SD
……..
Text Sample 1 Text Sample 2 Text Sample 3 Text Sample n
Text Samples
text string text modifiers
…….. ……..

Figure 1 — Sample Descriptions, text samples, text strings and text modifiers in 3GPP text streams

A 3GPP text access unit contains data from exactly one text sample. Consequently, a 3GPP text access unit

shall not contain data from more than one text sample. Each 3GPP text access unit is to be presented during

a certain period of time, specified by duration information. In addition to a text sample, a 3GPP text access

unit may contain zero or more complete sample descriptions. By allowing interleaving of text samples with

sample descriptions in 3GPP text access units, 3GPP text streams are capable of carrying sample

descriptions in-band .
7.3 Transport of 3GPP text access units in TTUs

Typically, a 3GPP text access unit is small, around 100 – 200 bytes, and often much smaller than the size of

the packets that carry the text data across a transport network. It is therefore expected that transport systems

will often aggregate multiple 3GPP text access units into one transport packet. On the other hand, 3GPP text

access units can also be large, for example when scrolling horizontal text at the bottom of the screen, in which

case fragmentation of 3GPP text access units may be required prior to transport. In conclusion, transport of

3GPP text access units often requires aggregation and sometimes fragmentation. So as to conveniently

1) Note however that 3GPP sample descriptions may also be provided out-of-band.
4 © ISO/IEC 2006 – All rights reserved
Character 1
Character 2
Character 3
Character n
Text modifier 1
Text modifier 2
Text modifier 3
Text modifier k
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ISO/IEC 14496-17:2006(E)

aggregate and fragment 3GPP text access units in a transport independent manner, this Specification defines

a flexible framing structure consisting of so-called TTUs, Timed Text Units.

Five different types of TTUs are defined; one for carriage of a complete 3GPP text access unit, three for

carriage of text sample fragments, and one for carriage of a complete sample description, while three types

are reserved for future use. Because sample descriptions are small, there is no support for carriage of sample

description fragments.

The flexible framing structure provided by TTUs allows for easy and convenient adaptation to the various

transport layers, while performing TTU alignment with the applied transport packets. For each transport layer

the most suitable TTU structure can be chosen. For example, by using TTUs, small text samples can be

aggregated into one transport packet, but TTUs can also be used to fragment text samples across multiple

transport packets, while providing a reasonable level of error resilience in case of packet loss or non-

recoverable packet errors. If so desired, the text data within a text access units can be re-partitioned into

TTUs for most effective adaptation to other transport systems. See Figure 2. TTUs are defined for 3GPP text

streams only, but comparable structures may be defined for other text streams in future versions of this

Specification.
3GPP domain
Text Samples Sample Descriptions
MPEG domain
3GPP Text Access Units
Timed Text Units (TTUs) Re-partitioning
Text
stream
Transport System

Figure 2 — Carriage of text samples and sample descriptions in 3GPP text access units and the use of

TTUs for creating a 3GPP text stream

The format of TTUs is defined in this Specification, as well as some general requirements for their use, while

the actual usage of TTUs is transport specific and beyond the scope of this Specification. However, as TTUs

support text access unit aggregation and fragmentation in a generic and error resilient manner, transport

systems are strongly recommended to use TTUs for aggregation and fragmentation instead of transport

specific tools.

Each text access unit with a 3GPP text stream is defined to consist of one or more TTUs. Each TTU type is

identified by its index j and referred to as TTU[j]. The following TTU types are defined:

• TTU[0]; this TTU type is reserved for future use;

• TTU[1]; this TTU type is capable of carrying one complete 3GPP text access unit;

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ISO/IEC 14496-17:2006(E)

• TTU[2]; this TTU type is capable of carrying one fragment of a text string from a text sample;

• TTU[3]; this TTU type is capable of carrying the first fragment of a text modifier from a text sample;

• TTU[4]; this TTU type is capable of carrying a non-first fragment of a text modifier from a text sample;

• TTU[5]; this TTU type is capable of carrying one complete sample description;
• TTU[6] and TTU[7]; these TTU types are reserved for future use.

Figure 3 depicts the options to construct TTUs from text samples and sample descriptions. Each complete

sample description is contained in one TTU[5]. Each text sample that is not fragmented, is contained in one

TTU[1]. When a text sample is fragmented, then the text string is separated from the text modifiers for

carriage in different TTUs. Each text string fragment is contained in one TTU[2], so that there are as many

TTU[2]s as text string fragments. Note however that the text string itself may not be fragmented, in which case

there is only a single text string fragment carried in one TTU[2]. The first text modifier fragment is contained in

one TTU[3]. Hence, TTU[3] signals the start of the modifiers. Each subsequent text modifier fragment, if any,

is contained in one TTU[4].
Sample
Descrip- Small Text Sample, not
tion requiring fragmentation Large Text Sample requiring fragmentation
text string modifiers text string modifiers
TTU[5] TTU[1] ... ...
TTU[2] TTU[2] TTU[3] TTU[4] TTU[4]
Contains Contains Contains text Contains first Contains
sample complete string modifier non-first
description sample fragment fragment modifier
fragment
Text Access Units
[5] [5] [5] [5]
TTU[1] or TTU[2] TTU[2] TTU[3] TTU[4] TTU[4]
AU: AU:
Note:
• one TTU[1] • one or more TTU[2], zero or one TTU[3]
[5] denotes
• optionally one or more • if one TTU[3], then zero or more TTU[4]
one TTU[5]
TTU[5] • TTUs in random order
• optionally one or more TTU[5], randomly
interleaved with other TTUs

Figure 3 — Carriage of Sample Descriptions and Text Samples in TTU[j]s, and construction of Text

Access Units by means of TTUs

In Figure 3, also the basics of the construction of text access units by means of TTUs are depicted. If a text

sample is not fragmented, then a text access unit contains the TTU[1] that carries the entire text sample. If the

text sample is fragmented, then the text access unit contains one or more TTU[2]s, each with a fragment of

the text string. If the text sample also carries modifiers, then the text access unit carries one TTU[3] and,

depending on the size of the modifiers, zero or more TTU[4]s. In addition, each text access unit contains

optionally one or more TTU[5]s, each carrying a sample description that may apply to this or future text

samples in the text stream.

An empty text sample, that is a text sample without data (hence without a text string and modifiers), may be

used to terminate the display of a text string prior to the start of the display of the next text sample in the

stream. Such empty text sample shall be carried in a TTU[1] as a separate access unit.

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ISO/IEC 14496-17:2006(E)

The order of TTUs within a text access unit is random within some constraints. When text samples are

fragmented, it is allowed to repeat a fragment that is considered more important than others, and to transmit

modifiers prior to text string fragments, which may be useful for large text strings. TTU[5]s may be interleaved

randomly with other TTUs. For ordering TTUs within a text access unit, only the following constraint applies:

• When a sample description is provided in-band, then a sample description shall be included in the

stream prior to any text sample that utilizes that sample description. This means that an in-band

sample description, carried in a TTU[5], shall precede any TTU[1], TTU[2], TTU[3] and TTU[4] that

references that sample description. Note that such TTU[5] may be carried in the same text access

unit, or in one or more preceding text access unit(s).
7.3.1 Constraints for fragmentation

A number of constraints shall be applied when fragmenting text samples for carriage in TTU[2], TTU[3] and

TTU[4]. The following constraints shall be applied:

• Each text string shall be fragmented on character boundary, so that each fragment contains

complete character codes only, so as to ensure that each received text fragment can be decoded

even in the case that a previous fragment gets lost. As a consequence, text string fragmentation

requires knowledge of UTF-8/-16 formats to determine character boundaries.

It is not required to fragment text modifiers on modifier boundary; consequently, when a modifier fragment is

lost, receivers will typically not be capable of decoding any remaining fragments.

7.3.2 TTU structure

Each TTU starts with an 8 bit TTU header; this header carries the type of TTU, signals whether characters in

the text string of the text sample are encoded using UTF-8 or UTF-16, and signals the length (size) of the TTU

in bytes. The remaining fields after the TTU header are TTU type specific and described in the following

subclauses. Note that in addition to the transport agnostic requirements defined in this Specification, transport

specific requirements not defined in this Specification may apply.
7.3.2.1 TTU[1] signaling

A TTU[1] carries a complete text sample; the TTU header and the TTU data length field are followed by fields

that provide the following information:
• the index of the applicable sample description;
• the duration of the text sample, and

• the length in bytes of the text string in the text sample. Note that the length of the text string in bytes

does not give the character count, as a character can vary in length from, in general, 1 to 6 bytes.

The length of the text modifiers is not specified, as it is assumed that this information is derived from the total

text sample length also provided by the TTU header.

Carriage of an empty text sample in a TTU[1] is signaled by the presence of the TTU data length field

encoded with a specific value (see subclause 7.4.2).

NOTE 1 When the transported text string was stored in a 3GP file, it is to be ensured that the values of the two fields

that precede the text string in a 3GP file, the 16 bit string length field and the byte order mark are correctly encoded in the

equivalent fields of the TTU header.

NOTE 2 If a timed text stream is to be stored in a 3GP file, then the byte order mark and the text string length need to

be recovered from the TTU headers in order to correctly store the stream in the 3GP file.

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ISO/IEC 14496-17:2006(E)
7.3.2.2 TTU[2] signaling

A TTU[2] carries a text string fragment from a text sample. The TTU header and the TTU data length field are

followed by fields that signal:

• the total number of text sample fragments; this number shall include each text string fragment and

each modifier fragment.

• the sequence number of the fragment contained in this TTU[2]; the sequence number of the first text

sample fragment is equal to 0, and for each subsequent fragment the sequence number is

incremented by one, so that in case of N fragments, the sequence number of the last fragment is

equal to N-1. A text sample can be fragmented in at most 16 different fragments;
• the duration of the text sample;
• the index of the applicable sample description;

• the total length of the original non-fragmented text sample; this information allows the receiver to

allocate buffer space for the decoding of the text sample; this information is provided for each text

string fragment so as to also allow for buffer space allocation after losing one or more text string

fragments.
7.3.2.3 TTU[3] and TTU[4] signaling

In a TTU[3] and TTU[4], the first and a non-first modifiers fragment is carried, respectively; the TTU header

and the TTU data length field are followed by fields that signal:
• th
...

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