ISO/IEC 14496-17:2006 信息技术-视听目标代码 第1部分：流动文本格式

标准编号：ISO/IEC 14496-17:2006

中文名称：信息技术-视听目标代码 第1部分：流动文本格式

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

发布日期：2006-04

标准范围

ISO/IEC 14496-17:20 06是为了响应以非常低的比特率对文本进行编码的通用方法的需要而开发的，作为视听演示中的多媒体组件之一。ISO/IEC 14496-17:20 06允许例如字幕和卡拉OK歌曲文本被编码并作为单独的文本流以足够低的比特率传输以用于IP上的移动服务。目标应用尤其存在于具有严重传输带宽限制的领域，例如IP上的移动服务。然而，宽带IP、广播信道和光学介质上的服务也可以受益于ISO/IEC 14496-17:20 06文本流的低带宽。ISO/IEC 14496-17:20 06将文本流指定为文本访问单元的级联。文本存取单元包含特定格式的文本信息。对于每个文本流，用信号通知文本访问单元内的文本信息的格式。文本流内的每个文本访问单元旨在在特定时刻显示。请注意，ISO/IEC 14496-17:20 06表示文本访问单元中包含的文本的格式，但它不包含任何文本格式规范。在ISO/IEC 14496-17:20 06内，文本流的特定实例是3GPP文本流。包含在3GPP文本流中的每个文本访问单元携带3GPP定时文本数据，如3GPP TS 26.245中所指定的。对于3GPP文本流，ISO/IEC 14496-17:20 06定义了适合于跨各种网络传输3GPP文本流的通用成帧结构，诸如用于在IP上传输的RTP，以及用于在诸如广播和光盘的介质中使用的MPEG-2系统。由所谓的TTU（定时文本单元）组成的文本流的灵活成帧结构允许容易和方便地适应各种传输层。通过使用TTU，可以将多个小文本访问单元聚合到一个传输分组中，但是TTU也可以用于跨多个传输分组分段大文本访问单元，同时在分组丢失或不可恢复的分组错误的情况下提供合理级别的错误恢复。对于每个传输层，可以选择最合适的TTU结构。如果需要，可以将文本访问单元重新划分为TTU，以便最有效地适应其他传输系统。

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.

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