DVB Subtitle标准文档EN300743 V1.5.1更新

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"DVB Subtitle标准文档，版本EN_300743V1.5.1(2014-01)，由欧洲电信标准协会(ETSI)发布，是数字视频广播（DVB）字幕系统的欧洲标准。此文档详细规定了DVB Subtitle的相关技术规范和要求，旨在支持电视和视频广播中的字幕服务。" DVB Subtitle标准是针对数字电视和视频广播领域中字幕系统的一项重要规范。它由ETSI制定，目的是确保不同设备和服务提供商之间字幕的互操作性和一致性，从而提高观众体验。EN 300 743 V1.5.1是这个标准的一个特定版本，发布于2014年1月，可能包含了之前版本的改进和更新。标准的关键点可能包括以下几个方面： 1. **格式规范**：定义了字幕数据的编码格式，可能涵盖了如CEN/ISO/IEC 13818-6 (MPEG-2 Transport Stream) 中的字幕包结构，以及如何在不同的传输环境中有效地嵌入和解码字幕信息。 2. **字幕编码**：可能规定了字符集、字体、颜色、位置、滚动和淡入淡出等视觉效果的编码方式，以实现多样化的字幕样式和表现形式。 3. **同步机制**：描述了字幕与视频内容的同步方法，确保字幕与音频和视频内容精确匹配，提高观看体验。 4. **字幕服务管理**：可能涉及如何在节目和频道中选择和切换字幕，以及如何通过电子节目指南（EPG）和其他交互式功能提供字幕信息。 5. **兼容性**：确保新标准与现有的DVB系统和设备兼容，同时也考虑未来的技术发展趋势。 6. **错误处理和恢复**：规定了在传输过程中出现错误时，如何进行恢复以保持字幕服务的连续性。 7. **实施和测试指南**：为制造商和运营商提供详细的实施指导，以及测试用例和验证标准，以确保产品和服务符合DVB Subtitle的标准。要获取这个标准的最新版本或更多详细信息，用户可以访问ETSI的官方网站 http://www.etsi.org。值得注意的是，尽管此文档可能有多个电子版本或打印版，任何争议的解决将以ETSI秘书处网络驱动器上保存的PDF版本为准。由于DVB Subtitle标准不断演进，用户应定期检查ETSI的门户网站 http://portal.etsi.org，以获取文档的最新状态和任何可能的修订或状态变更。

ETSI

ETSI EN 300 743 V1.5.1 (2014

01)

Subtitle data signalled

by page-id = 1

(spanish)

Subtitle data signalled

by page-id = 2

(italian)

Subtitle data signalled

by page-id = 3

(shared)

PES packet data

language = spanish

composition-page_id = 1

ancillary-page _id = 3

language = italian

composition-page_id = 2

ancillary-page_id = 3

Subtitle descriptors

associated with this

subtitle stream in the

PMT

….

PID = X

a: Example of use of different page_ids to distinguish between different subtitle languages

for the same service (shown with a shared ancillary page)

language = spanish

composition-page_id = 1

ancillary-page_id = 1

language = italian

composition-page_id = 1

ancillary-page_id = 1

Subtitle descriptors

associated with these

subtitle streams in the

….

PID = X

Subtitle data signalled

by page_id = 1

(spanish)

PES packet data

PID = X

PID=Y

PES packet data

PID = Y

Subtitle data

signalled by page_id = 1

(italian)

b: Example of use of PIDs to distinguish between different subtitle languages

for the same service (shown with no ancillary page)

Figure 2: Example of two ways of conveying dual language subtitles (one using shared data)

In summary, the DVB subtitling system provides a number of techniques that allow efficient transmission of the

subtitling data:

• objects that occur more than once within a region need only be transmitted once, and then positioned multiple

times within the region;

• objects used in more than one subtitle service need only be transmitted once;

• pixel data within objects are 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;

ETSI

ETSI EN 300 743 V1.5.1 (2014

01)

• 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;

• only those CLUT values to be used need be transmitted.

The above features are fully supported within the DVB subtitling system.

Subtitle streams intended for HDTV may include an optional data structure called the display_definition_segment

which explicitly defines the display size for which that stream has been created.

Subtitle streams associated with standard definition TV services need not include a display_definition_segment and

may be encoded in accordance with EN 300 743 (V1.2.1) [5]. Such streams will be nevertheless be decodable by

decoders compliant with this and any later versions of EN 300 743.

Subtitle streams associated with standard definition TV services and intended to be decoded by decoders designed to

EN 300 743 (V1.2.1) [5] shall not include a display_definition_segment.

In addition, functionality is provided to allow more efficient operation where there are private agreements between the

data provider and the manufacturer of the decoder:

• objects resident in ROM in the decoder can be referenced;

• character codes, or strings of character codes, can be used instead of objects with the graphical representation

of the character(s). This requires the decoder to be able to generate glyphs for these codes.

The private agreements required to enable these features are beyond the scope of the present document.

4.2 Data hierarchy and terminology

The basic "building block" of a DVB subtitle stream is the subtitling segment. These segments are carried in PES

packets, which are in turn carried by transport packets. The number of segments carried in a PES packet is only

limited by the maximum length of a PES packet, as defined by ISO/IEC 13818-1 [1].

A subtitle stream shall be carried in transport packets identified by the same PID. A single subtitle stream can carry

several different subtitle services. All the subtitling data required for a subtitle service shall be carried by a single

subtitle stream. The different subtitle services can be subtitles in different languages for a common program.

Alternatively, they could in principle be for different programs (provided that the programs share a common PCR).

However a single subtitle stream shall not convey both a subtitle service which includes a display_definition_segment

and one that does not; in this case the subtitle services shall be carried in separate streams and on separate PIDs.

Subtitle streams intended for HDTV services and which include a display_definition_segment are distinguished from

those which are intended for standard definition services and have been coded in accordance with

EN 300 743 (V1.2.1) [5] by the use of HDTV-specific stream_content & component_type values in the DVB

component descriptor signalled in the SDT and EIT for that service [2]. This provides a means whereby legacy SD-only

decoders should ignore streams which include display_definition_segments.

Different subtitle services can also be supplied to address different display characteristics or to address special needs.

For instance:

• different subtitle services might be provided for 4:3 and 16:9 aspect ratio displays;

• subtitle services might be provided specifically for viewers with impaired hearing. These may include

graphical representations of sounds.

Within a subtitle stream, a page id value is assigned to each segment. Segments can either contain data specific for one

subtitle service, or data that is to be shared by more than one subtitle service. The data for a subtitle service shall be

carried in segments identified by at most two different page id values:

• one page id value signalling segments with data specific for that subtitle service; the use of this type of data is

mandatory;

• one page id value signalling segments with data that may be shared by multiple subtitle services; the use of

this type of data is optional.

ETSI

ETSI EN 300 743 V1.5.1 (2014

01)

For each subtitle service a subtitling_descriptor as defined in EN 300 468 [2] signals the page id values of the segments

needed to decode that subtitle service. The subtitling descriptor shall be included in the PMT of the program and shall

be associated to the PID that conveys the subtitle stream. In the subtitling descriptor the page id of segments with data

specific to that service is referred to as the composition page id, while the page id of segments with shared data is

referred to as the ancillary page id. For example, the ancillary page id might signal segments carrying a logo that is

common to subtitles in several different languages.

The PTS in the PES packet header provides presentation timing information for the subtitling data, and is associated

with the subtitle data in all segments carried in that PES packet. The PTS defines the time at which the associated

decoded segments should be presented. This may include removal of subtitles, for example when an entire region is

removed or when all objects in a region are removed. There may be two or more PES packets with the same PTS value,

for example when it is not possible or desirable to include all segments associated to the same PTS in one PES packet.

The complete set of segments of a subtitle service that are associated to the same PTS is referred to as a display set.

The last segment of a display set shall be followed by an "end_of-display-set segment", which signals that no more

subtitling data associated to a certain PTS is needed for that service before decoding can commence. The display sets

shall be delivered in their correct presentation-order, and the PTSs of subsequent display sets shall differ by more than

one video frame period.

For carriage of multiple types of subtitling data, several segment types are defined, in particular:

• display definition segment; a subtitle service may be intended or have been prepared for display sizes other

than full standard definition television (i.e. other than 720 pixels by 576 lines – e.g. for HDTV). The optional

display definition segment explicitly defines the display size for which that service has been created;

• page composition segment; the decoding of a subtitle service will typically result in the display of subsequent

pages, each consisting of one or more regions; the page composition segment 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; in each region typically one or more objects are positioned, while using one

specific CLUT, identified by a CLUT-id; the region composition segment carries information on the region

composition and on region attributes, such as the horizontal and vertical size, the background colour, the pixel

depth of the region, which CLUT is used and a list of included objects with their position within the region;

• CLUT definition segment; the CLUT definition segment contains information on a specific CLUT, identified

by a CLUT-id, such as the colours used for a CLUT entry;

• object data segment; the object data segment carries information on a specific object; there are two types of

objects, graphical objects and text objects. An object data segment with a graphical object contains run-length

encoded bitmap colours, while a text object carries a string of one character codes;

• end of display set segment; the end of display set segment contains no internal information, but is used to

signal explicitly that no more segments need to be received before the decoding of the current display set can

commence.

The page id value of a segment containing data for a subtitle service shall be equal either to the value of the

composition_page_id or the ancillary_page_id provided in the subtitle descriptor. Page compositions are not shared by

multiple subtitle services; consequently, the page id of each page composition segment shall be equal to the

composition_page_id value.

In summary, the data hierarchy is:

• Transport Stream (TS);

• transport packets with the same PID;

• PES packets, with PTSs providing timing information;

• subtitle service;

• segments signalled by the composition page id and optionally the ancillary page id;

• where appropriate, a display definition segment;

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