DVB数字视频广播字幕系统详解：段落结构与标准介绍

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DVB（Digital Video Broadcasting）字幕系统详解是针对欧洲广播联盟（EBU）和欧洲电信标准化协会（ETSI）联合制定的标准，具体参考文档为DE/JTC-DVB-17，该标准适用于数字视频广播环境中的字幕处理。ETS300743发布于1997年9月，它定义了在DVB传输中如何构建和管理字幕，确保电视广播中音频与视觉信息的同步。 DVB字幕系统的核心关注点在于实现高质量的数字电视字幕，这包括以下几个关键环节： 1. **Segment结构**：DVB字幕被划分为一系列小段（segments），每个segment通常包含一段连续的时间线上的文本信息，比如一句或几句话。这些segments按照特定的格式存储，以便在接收端解码并显示。每个segment可能有其开始和结束时间，以及编码格式（如UTF-8或ISO-8859-1）。 2. **同步与嵌入**：为了保证字幕与视频图像的同步，DVB系统采用精确的时间戳，使得字幕能在正确的时间出现在屏幕上，不会与画面内容脱节。字幕可以嵌入到视频流中，也可以作为独立的数据流传输，由接收设备根据需要选择显示。 3. **多语言支持**：考虑到欧洲的多元文化，DVB字幕系统支持多种语言，允许同时呈现不同语言的字幕，这对于国际广播和跨语言服务至关重要。 4. **格式兼容性**：DVB标准确保了字幕系统的全球互操作性，使得不同国家和地区的接收设备能够无缝处理来自不同制作中心的字幕内容。 5. **版权保护**：标准中强调了版权的重要性，规定了复制和再分发字幕内容时必须获得书面许可，以尊重知识产权。 6. **错误处理**：尽管标准制定者已尽力确保内容准确无误，但文档中还是提到了可能出现的错误，包括内容、排版或其他类型的错误，并鼓励用户对任何发现的问题提出反馈。 DVB字幕系统是现代电视广播技术中的一个关键组成部分，它通过标准化的方法确保了字幕的质量、同步性和国际化，从而提高了全球观众的观看体验。对于IT专业人员来说，理解这个标准对于开发和维护数字电视平台，尤其是涉及到字幕功能的系统至关重要。

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ETS 300 743: September 1997

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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ETS 300 743: September 1997

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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