什么是流媒体?

在最基本的层面上, streaming media is the delivery of audio and video files from a server to a client over the internet or a cellular data network. 第一个流媒体音频于1995年发布, while the first streaming video followed two years later; you can read more about the early history of the technology in the first version of "什么是流媒体，我们在2011年发表了这篇文章. 不用说，自那以后，情况发生了很大变化.

2019年的流媒体会是什么样子? 如果你是这个领域的新手, 它可能看起来像一个难以理解的标准集合, products, and technologies, and in truth, it is. But it can be broken down into a small set of decisions that streaming producers must make when defining their service. This guide identifies those decisions and points you to other content to help you make them.

目标流设备

首先选择你想要发送视频的平台. Virtually all producers want to reach computers and mobile devices, but you have to dig a bit deeper. For example, which browsers do you want to support on computers, and how far back do you want to go? If your viewers work in government or educations you may need to support browsers like Internet Explorer 11 or earlier, 这意味着对Flash等传统格式的支持. 如果你的目标是年轻观众， Flash fallback 可能不是问题吗.

对于手机平台，你需要决定是通过应用还是浏览器来传递信息. Apps enable more features and design flexibility but increase development cost and time. 浏览器支持更快、更便宜，但提供的功能更少(参见“视频:浏览器vs. 内容分发应用").

The next major set of platforms tackled by streaming producers are OTT (over-the-top) devices like Roku, Apple TV, Chromecast, 和亚马逊Fire TV. 您必须为每个设备创建一个通道或等效的通道, but they represent an opportunity to support huge swaths of viewers with each development effort. 如果你的目标观众更年轻, 你可能还想支持PlayStation或Xbox等游戏平台.

The final set of target platforms typically undertaken by only the largest of streaming producers are smart TV platforms. 虽然有一些标准化的组织，比如 SmartTV Alliance (飞利浦、LG、松下、东芝)或 HbbTV，每个平台可能都需要单独的努力. 有关支持这些平台的概述, download the presentation handout from this Streaming Media West workshop entitled "编码2018:编解码器 & pc、移动、 & 奥特/机顶盒/智能电视."

自适应比特率(ABR)格式

上面确定的每个平台都支持某些自适应比特率(ABR) 规定如何对视频文件进行编码和打包的格式. 如果你在Safari浏览器中向iOS设备分发视频, 你必须将你的视频打包成HTTP直播(HLS)格式(参见“What is HLS"). 如果你是通过浏览器向Android设备发行游戏, 基于HTTP的动态自适应流, or DASH, 是首选(参见“什么是MPEG DASH"). As noted, 如果你通过应用程序将视频分发到移动设备, 您通常可以选择您喜欢的ABR格式.

For computers, your format decision will most likely depend upon the off-the-shelf player you select; an excellent starting point for technical readers is this video from Robert Reinhardt at Streaming Media West entitled "选择最好的现成的视频播放器." All OTT boxes and smart TVs support one or more formats except Apple TV (of course) which only supports HLS. Some older gaming platforms are similarly inflexible and only support older formats like Microsoft’s Smooth Streaming.

In the end, 以达到他们的目标受众, 大多数制作人最终都支持至少两种格式, HLS and DASH, 对其他格式的支持很少. 我们将讨论如何在打包中支持多种格式 & 下面的编码模式部分.

Feature Sets

自适应比特率流的基本概念是每个输入文件, 无论是直播还是视频点播(VOD), is encoded to a different set of files with different resolutions and bitrates to optimize the playback experience for all viewers, whether watching from a mobile phone via 3G or on a 4K smart TV connected via 100Mbps broadband. The configuration for these different files is called an encoding ladder; the chart below is a suggested encoding ladder from Apple’s HLS创作规范适用于苹果设备.

苹果建议的编码阶梯 HLS创作规范适用于苹果设备.

您的编码阶梯将根据您的视频输入而改变压缩技术你使用的，你的目标平台，甚至你的地理位置. 这里有两个有用的视频来创建你的编码阶梯-一个覆盖 bitrates，还有一个覆盖物 resolution. Here are some observations 您可以使用它来微调编码阶梯.

数码版权管理

如果你正在分发敏感或优质内容, 您可能需要使用数字版权管理技术来保护它, 或DRM(这里有一个有用的 primer DRM，而这个 article 描述了好莱坞电影公司如何使用DRM). 与ABR技术一样，不同的平台支持不同的DRM技术. For example, Chrome and Chromecast support Google Widevine; Apple TV, iOS, and MacOS support Apple FairPlay; and Edge supports Microsoft PlayReady. Fortunately, deploying multiple DRMs is simpler than it sounds from both a technology and an administrative perspective.

Technology-wise, the transition from Flash to HTML5 was enabled by a specification called the 加密媒体扩展 (EME)，它允许单个文件包含多种DRM技术. At the same time, multiple vendors offer licenses to all relevant DRMs simplifying the commercial side.

Closed Captions

对于某些类型的视频，可能会有封闭的字幕 required, while for others they may be desirable to reach the hearing impaired or for playback in loud or public places where audio may not be discernible. 您可以在这篇题为“流媒体的封闭字幕."

流媒体编解码器

Codecs are the technologies that compress audio and video and allow you to deliver your content to viewers via a range of connections (see "What is a Codec," here). Codecs are absolutely critical to streaming video; no codecs, no streaming video.

在过去的十年里，有一种视频编解码器叫做 H.264 with AAC audio compression has been the technology of choice for almost all streaming producers. 然而，在过去的五年里，有两种视频编解码器， HEVC and VP9, have been deployed to reduce bandwidth costs and increase video quality over lower bitrate connections. This latter point is key; for example, where H.264可以以2mbps的速度传输高质量的720p流, HEVC和VP9可以以相同的数据速率传输高质量的1080p流, 哪个对大多数观众来说更好看. 2018年，一项名为 AV1 用一种叫做多功能视频编码到2020年左右.

选择和部署编解码器是一项艰巨的任务复杂的分析涉及编码效率等因素, 平台的兼容性, 以及ABR技术的支持. For an overview of these considerations, check out this video from Streaming Media East, "操作方法:比较AV1, VP9, HEVC & H.264."

流式封装和编码模式

如上所述, to reach all of your target platforms you’ll likely have to support multiple ABR formats, 通常是DASH和HLS. There are two approaches; static and 动态打包.

带静态包装, you encode and package all files necessary to deliver both ABR formats and upload them to an origin server for distribution. 这取决于你如何编码你的视频, this may double your encoding cost and will certainly increase online storage costs.

另一种方法叫做动态打包. Here, you encode all the rungs of your encoding ladder and upload those to an origin server. 当浏览者点击你的链接, a separate server detects which format the player requires and automatically creates the required packaging in real time.

Dynamic packaging minimizes storage and encoding costs but requires a server running 24/7 to package the content. Typically, 动态打包 is cheaper than static packaging when considering all associated costs, 但这因应用而异. 要了解有关动态打包工作原理的更多信息，请查看此 how-to article.

在2020年左右的某个时候，一种叫做通用媒体申请格式 (CMAF) will enable a single set of files to support both HLS and DASH for most newer platforms, 但并不是所有的传统平台. 对于那些可以忽略传统观众的制片人来说, CMAF will slash the storage costs associated with static packaging and make this the most affordable option.

内容分发网络

与大多数其他形式的网络内容相比, like text, images, and PDF files, 流媒体视频要大得多，因此更难传送. For this reason, 大多数流媒体生产商都部署了内容交付网络, or CDN, 发布他们的视频(见“什么是内容交付网络").

较大的组织可能希望部署多个cdn, 既可以实现冗余，又可以优化不同区域的交付. 了解如何以及为什么支持多个cdn的更多信息, 看看来自流媒体西部的小组讨论题为“CDN优化:朝着广播经济方向努力 & 规模化质量."

服务质量(QoS)和体验质量(QoE)

当流媒体视频对您的组织至关重要时, measuring how effectively your content is delivered becomes equally mission-critical. There are two basic technologies here; quality of service (QoS), 衡量视频基础设施的技术有效性的方法是什么, 体验质量(QoE), 哪一个衡量的是实际的观看体验.

虽然明显相关，但概念绝对是分开的. For example, 如果你的视频包装有缺陷, QoS可以是完美的, 但是观看体验会很糟糕. 出于这个原因，大多数大型生产商使用不同的服务来监控两者. 有关QoE和QoS技术的概述，请查看“衡量并改进:对于视频发行商来说，QoE和QoS至关重要.有关如何部署QoE技术的教程，请查看如何测量视频编码QoE ."

We’ve thrown a lot of concepts at you in this "What Is" guide, but also a lot of resources. 而这篇文章只是流媒体世界的一个小提示, 如果你能走到这一步，你已经有了一个很好的开始.