2014年9月18日
By 1月时特约编辑
专题文章

如何部署封闭字幕

Deploying closed captions involves three steps: creating the closed captions, 将它们转换成适当的格式, 并将它们与视频集成以供显示. 创建和格式化标题是相对独立的, largely non-technical activities that can be accomplished by most computer-literate people.

Integrating them with the audio/video stream is usually programming professional’s job, 有上百种, 如果不是成千上万的话, 排列和用例, 特别是在实时字幕中. This makes the integration step hard to usefully address in anything short of a tome the length of 战争与和平. 本文主要关注前两个活动, while taking on a few examples of the actual deployment stage for demonstration purposes. 此外，我将专门关注美国的字幕.S., since incorporating other regions would be impossible in the space provided.

地形

There are multiple caption formats used by various distribution 和 playback technologies, 所示图1, which shows the requirements for added captions to videos displayed via the JW球员 (note that JW球员 can also display captions already embedded in HLS 和 Flash streams). So the first step you should take in all caption-related exercises is to identify the caption format or formats that you’ll need to supply for your chosen distribution method or methods.

图1. If you’re using the JW球员, you have to supply your captions in one of these three formats.

Let’s start with a quick look at caption formats before we get started on the creation process. On the origin side, CEA-608 captions are the NTSC st和ard for analog TV in the U.S. 和加拿大，也称为第21行标题. CEA-708字幕是美国和加拿大数字电视的ATSC标准. If you’re working with a live television stream or a captioned video file from a TV station or other broadcaster, 标题很可能是这些格式中的一种.

If you’re working with previously captioned video that isn’t of broadcast origin, 可以以多种基于文本的格式提供标题. 其中最受欢迎的是场景封闭字幕格式(.，而其他常见的源格式包括SubRip (.srt)、SubViewer (.越南央行或 .分)、mp分(.mpsub)和LRC (.领头.

对分布, you can either embed the captions in the streaming file itself -- a technique primarily used for files bound for iOS devices 和 Safari, which means you’ll need an encoding tool that can embed the captions into the streams -- or distribute the captions in a separate file called a sidecar file, 哪一种更受欢迎，因为它增加了灵活性, 特别是在支持多种输出格式时.

下面是最常见的sidecar格式，如在Zencoder的博客上描述过g:

TTML(定时文本标记语言)是W3C推荐的格式.
DFXP (Distribution Format Exchange Profile) is a profile of TTML defined by the W3C which is often used synonymously with TTML.
SMPTE-TT (Society of Motion Picture 和 Television Engineers – Timed Text) is an extension of the DFXP profile recommended by SMPTE.
SAMI (Synchronized Accessible Media Interchange) is based on HTML 和 was developed by Microsoft.
WebVTT is a text format proposed as the st和ard by the Web Hypertext Application Technology Working Group (WHATWG).

注意，许多发行版格式可以使用不同的sidecar格式. 例如，HTML5的标题可以是TTML或WebVTT格式. 播放器技术也是如此，例如图1所示的JW player. 同样，一旦你知道了目标格式，你就可以开始工作了. Here the tasks vary depending upon whether the captions exist already or if you’re creating them from scratch.

使用现有的标题文件

在现场场景中, the easiest case is when you’re receiving a real-time stream that contains integrated captions, 通常是CEA-608或CEA-708标题. 在这些情况下, if your encoding or transcoding tool can input these captions 和 convert them as necessary for the various outputs, 你可以走了. 大多数高端编码器, 比如来自思科的那些, 数字急流, 电影《百家乐软件》英文名, Envivio, 和谐波, 具备这些能力, 像Wowza Transcoder这样的转码工具也是如此, 或iStreamPlanet的Aventus平台图2.

图2. iStreamPlanet Aventus平台的输入和输出能力

我在图中突出显示了与标题相关的特性. 如你所见, Aventus可以输入带有CEA-708标题的MPEG-2传输流, convert the captions to the formats required for Apple HTTP 在线直播, Adobe HTTP动态流, 微软平滑流和Adobe基于rtmp的动态流. 本质上, this is a transmux of the original CEA-708 captions into a variety of formats, 上述所有系统都可以执行的功能是什么.

If you don’t have a streaming encoder capable of ingesting 和 transmuxing the captions, you can use a hardware device such as the EEG DE285 HD Caption Decoder to separately capture the captions, 然后把它们发送到流媒体服务. There are also hybrid hardware/ software live captioning workflows based upon Telestream的MacCaption，或者与之对应的Windows， CaptionMaker起价1095美元. 请联系Telestream或EEG Enterprises了解更多信息.

如果您没有收到嵌入字幕的提要, or if you’re not using a live encoder than can convert embedded captions or another tool to extract the captions, 您必须从头开始创建标题, 我马上就会讲到.

就像视频直播一样, 视频点播(VOD)文件将已经有字幕, 要么嵌入在视频文件中, 或者作为sidecar文件. For example, you might get h和ed a broadcast file with embedded CEA-708 captions. 在这些情况下, 许多高端编码工具, 比如上面提到的那些, can ingest the captions from these files 和 convert them as needed for the required formats. 如果你没有这样的编码器, 一个软件工具，如Telestream的MacCaption或CaptionMaker, 哪些提供了一系列与标题相关的功能, 也许我能帮你提取字幕. 还有一些更便宜或免费的任务专用工具，例如 CCExtractor，也可以执行这些功能.

如果你被要求转换DVD或蓝光光盘, you may be able to extract subtitles from these sources using CCExtractor or SubRip (用于dvd)或其他工具. If you Google “extract subtitles from Blu-ray Disc” you’ll see a range of options. 服务提供者，例如 CaptionMax 和标题科罗拉多也有帮助.

如果你收到一份带有转录的视频文件, you can convert the transcription to captions with MacCaption/CaptionMaker using a feature called automatic time stamping, 使文字记录和视频同步. 使用内部算法, the software automatically detects any problem areas 和 highlights them in red, 这样你就可以在必要的时候复习和改正. 否则, you can use any of the caption creation tools discussed below to create the captions from the transcript, 虽然这显然要花更长的时间.

如果您从视频文件和标题文件开始, 但需要一个不同的标题格式交付, you can usually import these into a program such as MacCaption/CaptionMaker for export into the required delivery format. 或者，有许多免费的工具和服务，例如标题格式转换器从3 playmedia, 可以输入SRT或SBV字幕, 后者是YouTube使用的格式, 并以多种交付格式导出您的标题.

如果你想把字幕工作租出去, there are a number of companies who can create or convert the captions for you, 包括前面提到的CaptionMax和标题科罗拉多, 以及不过, 非公开字幕服务, 3 playmedia.

创建自己的标题

创建自己的标题, you have to know the purpose the captions are supposed to serve 和 some basic creation 和 formatting rules. Let’s start by exploring the differences between closed captions 和 subtitles. 封闭字幕是观众可以自己启用的字幕. 相反，总是显示打开的标题. 说明文字适用于耳聋和听力困难的观众, 也适用于在酒吧或健身俱乐部等嘈杂环境中观看电视的人, 谁可能听不到音频. 与此形成鲜明对比的是, 字幕用于将音频翻译成另一种语言, 一般假设音频是可以听到的. 这是一个微妙但关键的区别.

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