Buyers' Guide to Media Servers 2017
2016 saw the rebirth of several older ideas regarding media servers. 无论是将多通道编码器和媒体服务器集成到同一机箱中，还是将传统格式流式传输到传统设备和现代设备的方法, 媒体服务器软件的核心功能继续扩展，以处理主流和边缘用例.
以下是过去一年的亮点，涵盖了在购买媒体服务器软件时需要考虑的几个关键问题. And if you’re looking to host your media service in the cloud, 或者您希望使用托管服务，而不是直接购买媒体服务器软件, 你会很高兴地知道，本买家指南中提到的所有公司都可以根据你的预算提供灵活的选择.
Manifest Density
如果过去几年的问题集中在将一种流协议从RTMP转换为另一种流协议, say, 到Apple HTTP Live Streaming (HLS)或Smooth Streaming——新的挑战是如何处理可伸缩性挑战和不断变化的网络基础设施环境所要求的清单文件的频繁更改.
清单文件充当一个声明的播放列表，供最终用户的视频播放器从一段内容转到另一段. 文件本身通常是文本或可扩展标记语言(XML)，以方便人类工程师和提供清单文件中列出的内容所需的媒体服务器的可读性.
In its simplest form, 清单——在MPEG术语中也称为媒体演示描述(MPD)——类似于Spotify的播放列表, where the end user picks which songs she would like to listen to, and in what order.
The manifest file, though, 对用户是不可见的，并且包含的不仅仅是要播放的歌曲. 清单和mpd都包含有关媒体段的信息——构成Apple HTTP Live Streaming (HLS)或MPEG Dynamic Adaptive Streaming over HTTP (MPEG- dash)的数据块——以及在作为清单文件一部分提供给设备的多种数据速率和分辨率之间进行选择所需的信息.
In addition, as ad-based over-the-top (OTT) video continues to grow as an industry offering, the need to manipulate manifest files grows. As such, 随着内容发布者希望自定义AD播放，清单文件的密度也会增加, multiple data rates, various screen resolutions, 以及其他必要的相关元数据，以将广告服务选项细化到高度细化的级别.
Why are multiple data rates offered, not just for the primary content, 但也包括在她的个性化播放列表中，与用户选择的优质内容一起播放的广告?
主要原因是用户反馈:消费者宁愿以较低的比特率观看或收听流媒体内容，也不愿以较高的数据率等待内容缓冲(或断断续续地播放). 适应当前网络拥塞或间歇性传输的能力被称为自适应比特率(ABR)流, 它为任何特定的清单文件增加了高度的复杂性和密度.
From Encode Once to Mix Once
即使有清单密度的复杂性，媒体服务器也已经成为一个挑战. Most media servers can handle robust manifest files, 包括单个高分辨率流的转码(编解码器转换)或翻译(分辨率或数据速率降阶). This combination is necessary for proper ABR delivery, 特别是对于从获取点开始流式传输的高比特率/高分辨率的直播内容，例如4K，需要缩小到1080p甚至720p以进行流式传输.
这方面的行业口号是“一次编码，到处流”已经有好几年了. But at least one media server company is mixing up that slogan, replacing it with “mix once, play everywhere.”
“清单操作需要对编码器和原始服务器进行定制更改，以及对每种交付格式进行清单修改, says Arjen Wagenaar, CTO of Unified Streaming他补充说，这些更改通常使用维护起来很复杂的客户端sdk.
“There is always that nagging uncertainty: Will players, web browsers, and TVs be able to play the stream from the manipulated manifest?” he asks.
This approach, which the company calls Unified Remix, stitches content upstream from the origin server, 创建一个参考MP4文件，供公司的源服务器(统一源)作为源访问, which is then streamed as a single stream.
This approach to integrating disparate pieces of content, including primary show content and ads, has been around for some time, 但直到最近，它才进入媒体服务器应用程序，普通大众可以购买这些应用程序，并将其传输到各种各样的设备上. In essence, 统一流处理的是流而不是清单文件，这样“流的功能就好像它有一个单一的起源和一个单一的时间轴。.”
Old Formats Are New Again
The move toward MP4 files isn’t just limited to one media server company. The ISO Base Media File Format (ISO-BMFF) is the MP4 file container, 这是能够使用字节范围寻址流碎片MP4 (fMP4)的关键，而不需要创建数百, thousands, 甚至在流媒体开始之前成千上万的独立片段或块.
但是，如何以一种允许传统设备接收流媒体的方式来传输实际的MP4文件呢? It turns out that DDVTECH, makers of the MistServer, 在视频点播(VOD)和直播方面，是否已经采用了这种方式来处理传统设备的流媒体.
“Back at the beginning of 2016 when we released Live MP4, it had a latency of about 2 keyframe intervals,” says Jaron Viëtor, CTO of DDVTECH.
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