多格式流媒体服务器买方指南

2012年，媒体服务器在几个方面取得了重大进展. 有些变化虽小但很重要, 比如命名约定——Adobe将Flash从媒体服务器名称中去掉了, for instance -- while others were much more impactful for the industry going forward -- almost everyone agreeing that Dynamic Adaptive Streaming over HTTP, 简称破折号, 值得支持.

在今年的买家指南中, we'll take a look at a few key features you'll need to know about to make an informed decision.

破折号

毫无疑问, the move toward 破折号 is a key watershed for the industry: Not only does it address adaptive bitrate (ABR) to handle intermittent network delivery and bandwidth uncertainty, but it also makes use of "plain vanilla" HTTP servers to send segments-chunks-fragments to 破折号-compliant players.

So, the obvious first question if we're all moving to HTTP: Why use a media server? The answer primarily has to do with the transition period we are in: Since many legacy devices and players are incapable of playing 破折号 content, and since many devices -- such as iOS devices -- won't support 破折号 in the foreseeable future, 服务器需要在格式之间进行转换.

请注意，大多数服务器不会在编解码器之间进行转换，如H.264是目前占主导地位的编解码器, and one panelist at a recent media summit declared that 2012 was the year that WebM died. 仍然, 即使对于那些使用基本MP4文件的人也是如此, the need to segment content between various segmented HTTP formats is growing rather than declining.

一个好的媒体服务器将以夹层格式保存按需文件, one that can be segmented variously into one or several of Adobe's HTTP Dynamic Streaming (HDS), 苹果的HTTP直播(HLS), 微软流畅流, 还有一些其他的格式. 这样做的目的是为了保持夹层文件的完整, which is possible now in all HTTP formats -- HLS now allows for bit range segmentation -- so that the server doesn't need to deal with storing a vast sea of tiny bits.

除了, 一些媒体服务器正在利用破折号 Live Profile, 允许内容同时流媒体直播到破折号和HLS, 通过使用多路复用(transmuxing). Some solutions reaching the market in late 2012 and early 2013 will allow for the transrating of a single highest-common-denominator file into several varying bitrates.

字幕

A recent law in the United States has pushed media server companies to offer closed-captioning functionality. 解决方案包括时间-文本, 比如SAMI, SMIL, 定时文本标记语言(TTML), 以及不那么健壮的TTML Lite版本), 和传统的CEA-608/CEA-708合规性.

最初的九月. 30, 2012, 的最后期限, 美国联邦通信委员会(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.

One of the main issues that will be faced by media server companies and consumer electronics (CE) manufacturers alike is coming up with common timed-text standards. The good news is that the vast majority of streaming-capable devices are also capable of displaying subtitles -- 网飞公司需要注意的是，它提供视频服务的一些设备, 比如一些老式的BD播放器, 电视, 还有机顶盒, 没有能力, 而且不存在可升级的固件——所有新设备都不存在, 另外还有一些智能手机和平板电脑应用程序, 你是否也知道字幕和字幕.

Addressing all of these various devices is just the kind of service a good media server will provide, 因此，在购买之前，请查看媒体服务器上的选项, since closed captioning will continue to be an important requirement for online video from now on.

数字版权管理和加密

As we've moved closer to a common delivery protocol (HTTP) and a Common File Format (CFF, 基于ISO基础媒体文件格式), 还有一个向通用加密方案(CES)迈进的趋势。. CES将提供使用五种常见DRM方案之一的能力.

除了, several media servers are offering the ability to move DRM "down the stack" to reside at the encoder so that DRM and encryption can be applied before the source ever leaves the encoder to be sent to the media server. 以这种方式, content can remain encrypted through the ingest and delivery portions of the workflow, 只能在客户端播放器上解码和解密.

苹果对开发者的限制, 禁止他们使用唯一设备标识符(UDID), means that HLS doesn't allow DRM integration the way that fMP4 (fragmented MP4) does. As such, when considering a media server, consider how it will handle DRM for HLS-based devices.

在黑暗中束缚他们

当涉及到DRM时，如果没有“一个戒指统治所有人”, there's also not a necessity to multiplex (or mux) content too early in the delivery process. Several media servers offer -- or will soon offer -- the ability to perform "late binding" functionality.

The term "late binding" primarily refers to the ability to keep the audio and video "tracks" of an "online DVD" or stream separate from one another until the last possible moment. 在某些情况下，这种绑定发生在客户端播放器上, 与媒体服务器保持跟踪哪个音频轨道(e.g.(语言)将与特定的视频流配对.

以HLS为例, 延迟绑定实际上是不可能的, 这在很大程度上要归功于HLS的MPEG-2传输流(M2TS)遗产. 要求M2TS多路成单路, interleaved audiovideo packets grew from the requirement to send primary and secondary audio tracks for broadcast, 原行业使用M2TS. dvd与多路音频和视频的使用巩固了这种方法, but the more flexible fragmented MP4/CFF approach has made late binding possible.

Expect to see the Common Streaming Format (CSF) that's based on the ISOBMFF make late binding a common feature in media servers, as it means there will be no required duplication and repackaging of the video for each audio track, allowing content providers to easily provide multiple language tracks for a given video asset without having to send all audio alternatives at the same time.

结论

我们只是触及了当今多格式媒体服务器选项的表面. Other areas to consider are the support of complementary player functionality in apps -- such as the ability to play HLS content in Android devices at operating system versions lower than what Google officially supports -- as well as the ability to have an HTTP cache origin server functionality or for the media server to perform dynamic re-encoding.

媒体服务器将继续扩展功能, 包括访问即将到来的破折号 264子集规范. 敬请关注StreamingMedia.以跟上这些令人兴奋的增强.