2015年7月17日
By 1月时特约编辑
专题文章

HTML5时代的到来:是时候和Flash说再见了

We’ve been hearing that Flash is dead since Apple introduced the iPad back in 2010. 然而，它仍然是高级网站使用的主要技术, 尤其是那些有广告支持的. 这是因为第一代基于html5的视频播放技术, 本质上就是著名的视频标签, 没有启用对优质内容分发至关重要的功能, 比如自适应流, 数字版权管理, 在线直播, 甚至是真正的流媒体，而不是渐进式下载.

终于, 三项HTML5新技术, 媒体源扩展(MSE), 加密媒体扩展(EME), 和动态自适应HTTP流(DASH), 将帮助优质内容生产商克服这些障碍吗, and transition to HTML5 for the adaptive delivery of live and VOD content with DRM. It won’t be easy, and it won’t be particularly clean, but it will definitely be possible.

定义

让我们从这三种技术的一些高级定义开始. MSE is a W3C HTML Working Group specification for a JavaScript interface to play back media data within a browser. 它比视频标签更实用, and browsers and devices that support MSE can play chunks of video (or byte-range requests within a single file), 使直播和视频点播内容的自适应交付成为可能.

DASH是一种标准化的文件格式, 就像苹果的HTTP 在线直播 (HLS)或微软的Smooth Streaming. 像所有基于http的自适应流格式一样, there are two elements: fragmented video 文件 (or byte-range requests within a single file), 清单文件, which identify the location of the various 文件 within an adaptive group and the location of the chunks or byte-range requests of the individual segments. 在使用, 大多数DASH内容包含在MP4文件中, 而清单文件是MPD, 哪个代表媒体演示描述, 文件.

MSE和DASH携手并进. 也就是说，要播放DASH文件，浏览器或设备必须支持MSE. 因此，MSE是回放规范，而DASH是指定的文件格式.

EME is another JavaScript API that enables HTML5-based DRM by extending MSE with application programming interfaces (APIs) to control the playback of protected content. 之前的高速, DRM由Flash等插件处理, Silverlight, 或都, 已与license服务器通信解锁受保护内容. EME eliminates the middleman and allows the browser or device to communicate directly with the license server.

MSE和EME

创建加密文件时, a standard called the common encryption scheme (CENC) details the standard encryption and key mapping techniques used to store the DRM-related data. 在播放端, EME通过整合所谓的内容解密模块来工作, 或清洁发展机制, 进入浏览器或移动操作系统. 例如，谷歌在Chrome和Android中包含了都 CDM. 按照目前的实现，每个浏览器或平台只包含一个或两个cdm. So where most producers previously supported only a single DRM for distribution to all or most of their target platforms, EME’s one platform-one CDM dynamic will force most producers to support multiple DRM technologies.

Note that not all HTML5-based technologies seeking to supplant Flash and Silverlight rely on MSE/DASH/EME. 特别是, OpenTelly’s THEOplayer enables the playback of encrypted HLS streams in an HTML5 browser without Flash or Silverlight plug-ins. 你会看到, 这很重要, because while the penetration of HTML5-compatible browsers is closing in on 98 percent, 支持MSE和EME的浏览器的百分比很高, 低得多.

支持MSE和EME

As with all things HTML5, MSE和EME only work in browsers that support the new specifications. 而桌面浏览器的确切数字则难以捉摸, 一般原则和一些粗略估计见表1，数据来源于StatCounter网站2015年5月提供的数据.

Google Chrome从版本23开始支持MSE/EME, so most of the installed base of Chrome browsers (now at version 43) supports these specs. Microsoft Internet Explorer 11, with a 15 percent total share, supports MSE, but only on Windows 8.微软目前在所有个人电脑和台式电脑市场上的份额约为11%. 如果你假设所有的Windows 8.计算机也运行IE 11, 这是合理的, 也就是说，“是”一列增加了11%, “No”一栏占4%. IE版本8-10, 根据StatCounter的数据，这些浏览器占所有浏览器的16%, 不支持任何一个规范.

关于火狐, Mozilla早在版本21就发布了对DASH的支持, YouTube现在在火狐38版中使用DASH. EME支持是新生的，据报道有很多bug, Windows只支持32位版本的浏览器, 而不是64位版本. Though it will be a while before all 火狐 browsers on all platforms support EME, 在表1中，我们仍然将其视为是.

注意，Safari支持MSE/EME, 但仅限于OS X Yosemite, 所以不太可能达到2%的安装量. 最后, 而Opera支持MSE(不支持EME), 它只适用于WebM编解码器, 大多数生产商计划使用H.264. 这些数据让我们获得了86%的总浏览器份额, 其余14%的绝大部分不支持MSE或EME.

In mobile markets, iOS doesn’t support MSE or EME, but Android has supported MSE since version 4.版本1和版本4以来的EME.3. 根据谷歌开发者仪表盘, 这意味着接近90%的Android设备应该支持MSE, 大约55%的人支持EME. Windows Phone 8.1支持MSE，不支持EME.

高速分散

As mentioned, most browsers and mobile operating systems will support only one, or at most, two DRMs. 这显示在表2. 在这个动态中, producers have two options; they can support multiple DRMs, 或者强迫他们的观众只在选定的平台或设备上观看, 在Chrome中, 但不是IE, 火狐, 或旅行.

支持多个drm不应该是一个逻辑上或合同上的问题. That is, the CENC standard can include key-related 信息 for multiple DRMs in a single file. 在合同方面, 许多数字版权管理提供商, 比如BuyDRM, DRM今天, EZ-DRM, 和Verimatrix, 现在支持多种DRM技术. 所以我没有直接和微软签约, Adobe, 都, 和苹果, a video distributor should be able to find one or two DRM providers to access all necessary DRMs.

DRM技术支持

表2. 浏览器/平台支持的DRM技术

目前尚不清楚苹果对FairPlay的计划. 具体地说, 而苹果似乎已经让Netflix和Hulu也可以使用FairPlay, 苹果尚未宣布是否会更广泛地授权FairPlay, 如果是这样的话, 是否会通过第三方发行, 或者只是直接.

已知的已知

With this 信息 as background, several realities about supporting HTML5 become evident. 让我们来回顾一下.

Mse / eme / dash在短期内不会成为通用的解决方案

今天, most producers output 文件 for Flash or Silverlight playback for desktop distribution and HLS for iOS. 由于苹果不愿将MSE/EME应用于iOS, 即使DASH可能会在桌面上取代Flash或Silverlight, 在苹果设备上就不会, 至少对于基于浏览器的播放来说是这样. 与Flash和silverlight兼容的格式一样, 经销商或许可以开发一款播放DASH文件的应用程序.

另一方面, many video distributors create one set of 文件 and transmux as necessary with tools such as the Wowza Streaming Engine, 它已经支持DASH了. 短暂的, Transmuxing是轻量级的, real-time operation that involves changing the container format of the encoded file and creating the necessary manifest 文件. It does not require transcoding or re-encoding 文件 and can be performed by the server without introducing significant latency or additional load. So while switching from Flash or Silverlight to MSE/EME might not reduce the file creation requirements, 它可能也不会扩大它们.

后退对mse / eme来说是必不可少的

鉴于目前对MSE/EME的支持徘徊在65%左右, it’s clear that producers can’t cut over to the new standards without addressing noncompliant browsers and platforms. 通常, 这涉及到一种叫做后退的技术, 或者返回到浏览器支持的任何技术. 在操作, the player queries the browser to determine its capabilities; those that support MSE/EME receive the DASH 文件. Browsers that don’t support MSE/EME receive 文件 in whichever format they do support, 这通常是

Flash或平滑流Silverlight. 有趣的是, several of the off-the-shelf players discussed below can transmux DASH 文件 into the required format in real time within the browser. 这包括转换内容和返回到所需的DRM. 缺少此功能, the distributor would have to transmux at the server or create multiple packages of 文件 and DRM for MSE/EME and Flash or Silverlight.

制作人应该考虑现成的玩家

Many Flash and HTML5 producers use off-the-shelf players to reduce development cost and time to market. With capabilities such as player-side transmuxing and multiple DRM support key to implementing HTML5-based video playback, 那些考虑转向HTML5的开发者也应该考虑这么做. There are several players available; let’s start with those that participated in a 流媒体 East panel last May titled 取代Flash: HTML5中的自适应流媒体和DRM、Bitmovin、CastLabs和OpenTelly.

Bitmovin

奥地利公司 Bitmovin 提供bitdash播放器和bitcodin云转码服务. The player schema deploys DASH on compatible browsers with fallback to Flash or HLS on legacy platforms and iOS. 该方案预期同时提供DASH和HLS编码文件, 通过播放器内传输支持Flash回退. 为安卓, 该公司在mse兼容版本上支持DASH播放, 或通过基于WebView或ExoPlayer的应用程序.

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