A Buyer's Guide to Multiformat 流媒体 Servers

2012 saw significant progress on several fronts for media servers. Some changes were small but important, such as naming conventions -- Adobe dropped Flash from its media server names, 例如，虽然其他人对行业未来的影响更大，但几乎每个人都同意基于HTTP的动态自适应流, 简称破折号, 值得支持.

In this year's Buyer's Guide, 我们来看看你需要了解的一些关键特征，以便做出明智的决定.

破折号

毫无疑问, 向破折号的转变是行业的一个关键分水岭:它不仅解决了自适应比特率(ABR)问题，以处理间歇性网络传输和带宽不确定性, 但它也利用“普通的”HTTP服务器发送段-块-片段到符合dash的播放器.

因此，如果我们都转向HTTP，第一个显而易见的问题是:为什么要使用媒体服务器? 答案主要与我们所处的过渡期有关:因为许多传统设备和玩家无法玩破折号内容, 而且在可预见的未来，许多设备(如iOS设备)都不支持破折号, there is a need for servers to convert between formats.

Note that most servers are not converting between codecs, as H.264 is the dominant codec today, 在最近的一次媒体峰会上，一位小组成员宣称，2012年是WebM死亡的一年. 仍然, even for those who are using elementary MP4 files, 在各种分段HTTP格式之间对内容进行分段的需求正在增加而不是减少.

A good media server will hold on-demand files in a mezzanine format, 它可以被分成一个或几个Adobe的HTTP动态流(HDS), Apple's HTTP 在线直播 (HLS), Microsoft Smooth Streaming, 还有一些其他的格式. The idea is to keep the mezzanine file intact, 这在现在所有的HTTP格式中都是可能的——HLS现在允许比特范围分割——这样服务器就不需要处理存储大量的小比特了.

除了, several media servers are taking advantage of the 破折号 Live Profile, allowing content to be streamed live to 破折号 and HLS simultaneously, through the use of transmultiplexing (transmuxing). 2012年底和2013年初上市的一些解决方案将允许将单个最高公分母文件转换为几个不同的比特率.

字幕

美国最近出台了一项法律，要求媒体服务器公司提供封闭式字幕功能. The solutions range from timed-text, 比如SAMI, SMIL, Timed Text Markup Language (TTML, and its less-robust TTML Lite version), and traditional CEA-608/CEA-708 compliance.

最初的九月. 30, 2012, 的最后期限, imposed by the Federal Communications Commission (FCC), does not make it mandatory for all online content to provide closed captioning, but it does set the ball in motion for the overall trend toward closed captioning.

媒体服务器公司和消费电子产品(CE)制造商将面临的主要问题之一是制定通用的时间-文本标准. 好消息是，绝大多数支持流媒体的设备也能够显示字幕 网飞公司 notes that some of the devices that it serves videos to, such as a few older BD players, 电视, 还有机顶盒, 没有能力, and no upgradeable firmware exists -- and all new devices, plus a number of smartphone and tablet apps, are also captioning- and subtitling-aware.

对所有这些不同的设备进行寻址正是一个好的媒体服务器应该提供的服务, so check out the options on your media server of choice before you buy, 因为从现在开始，隐藏字幕将继续成为在线视频的重要要求.

数字版权管理和加密

随着我们越来越接近通用交付协议(HTTP)和通用文件格式(CFF), based on ISO Base Media File Format), there is also a move toward a common encryption scheme (CES). The CES will provide the ability to use one of five common DRM schemes.

除了, 一些媒体服务器提供了将DRM“沿着堆栈向下”移动到编码器的功能，以便在源离开编码器发送到媒体服务器之前应用DRM和加密. 以这种方式, 内容可以通过工作流的摄取和交付部分保持加密, only being decoded and decrypted at the client player.

Apple's limitations on developers, banning them from using the unique device identifier (UDID), 意味着HLS不允许像fMP4(碎片化MP4)那样集成DRM. 因此，在考虑媒体服务器时，要考虑它将如何处理基于hls的设备的DRM.

And in the Darkness Bind Them

If there's not "one ring to rule them all" when it comes to DRM, 在交付过程中也没有必要过早地复用(或复用)内容. 一些媒体服务器提供——或者即将提供——执行“后期绑定”功能的能力.

术语“后期绑定”主要指的是将“在线DVD”或流媒体的音频和视频“轨道”彼此分开的能力，直到最后可能的时刻. In some instances this binding occurs at the client player, with the media server keeping track of which audio track (e.g., language) will be paired with a particular video stream.

以HLS为例, late binding is not really possible, thanks in no small part to the MPEG-2 Transport Stream (M2TS) heritage of HLS. The requirement to multiplex M2TS into single, 交错音视频包是从发送主音轨和次级音轨用于广播的需求而发展起来的, the original industry to use M2TS. The use of DVDs with multiplex audio and video solidified this approach, but the more flexible fragmented MP4/CFF approach has made late binding possible.

期望看到基于ISOBMFF的公共流格式(CSF)使延迟绑定成为媒体服务器中的一个常见特性, 因为这意味着不需要为每个音轨复制和重新包装视频, 允许内容提供商轻松地为给定的视频资产提供多种语言轨道，而无需同时发送所有音频替代方案.

结论

We've just scratched the surface of today's multi-format media server options. 其他需要考虑的领域是在应用中支持补充性播放器功能(游戏邦注:例如在Android设备上播放HLS内容的能力，其操作系统版本低于Google官方支持的版本)，以及拥有HTTP缓存源服务器功能或媒体服务器执行动态重新编码的能力.

Media servers will continue to expand functionality, including access to the upcoming 破折号 264 subset specification. Stay tuned to StreamingMedia.com to stay abreast of these exciting enhancements.

Five Questions to Ask When Selecting a Media Server

• Do I need multiple streaming formats and adaptive bitrate, or just one of the two?
• Will I use two or more adaptive bitrate formats?
• Is 破折号 important for my current media server workflow?
• Will HLS be a critical delivery requirement?
• 那么RTMP呢??

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