April 1, 2015
By Jan Ozer Contributing Editor
Featured Articles

如何为多屏传送打包视频

Most producers aim for their videos to play on every available platform. 这些平台呈现在右边的金字塔中 Figure 1. To achieve this goal, 制作人可以使用多种格式, 如图1中左边的表所示. The color coding in the table shows the suitability of each format for each target platform in the familiar traffic light format: red means stop, green means go, yellow means caution.

Also shown in the pyramid are the major standards or players in each market. As you can see, 在电脑和笔记本中, 有两个主要的标准——Flash是事实上的, but waning, standard, 而HTML5是后起之秀，但功能有限, 至少目前是这样. 没有显示媒体源扩展(MSE)的影响, 我将在下面的单独一节中介绍.

但是你可以用一个非常广泛的画笔来描绘电脑市场, you have to approach the other markets on a platform-by-platform basis. 例如，在移动端，HTML5被H普遍支持.264 playback, 但这只能提供单列文件, not adaptive, playback, 并且不支持直播或DRM. For these features, you have to address the two major platforms separately. Apple is simple—all iOS versions support HLS, simplifying adaptive delivery to that platform. Android is not. Android 3.0及以后的版本支持HLS, though there are various issues that make this support problematic at best, unusable at worst. Android version 5, or Lollipop, supposedly addresses these issues, but it hasn’t shipped yet. So there isn’t one adaptive approach that achieves native, browser-based playback on iOS and Android.

当你爬上金字塔, 不同的市场变得更加分裂, and the company behind each platform chooses the format or formats that it supports. For example, in the Retail OTT market, Apple TV supports only HLS, and not DASH. Roku is the same. Chromecast支持DASH, 亚马逊Fire TV也是如此, but only if you create your Fire TV media player with the VisualOn OnStream MediaPlayer+ SDK.

目标平台和适用性

How Much Can You Afford?

The reference to the VisualOn OnStream MediaPlayer+ SDK brings up another key point. At most points in the pyramid, there are multiple ways to implement any specific technology. 在PC/笔记本领域, native browser-based playback is always easiest and cheapest because all you need are MP4/WebM files and the video tag and you’re in. 像Flash这样普及的插件是第二简单的, because you can use any number of free or inexpensive off-the-shelf (OTS) players, or build your own.

However, other plug-ins or players can extend format support beyond that enabled in the Flash Player. For example, 如果您部署了JW Player, a leading OTS player, 你可以把HLS传送到电脑和笔记本电脑上, 它能让你适应计算机的传输, notebooks, 和iOS设备的单一格式. Other alternatives, such as castLabs DASH Everywhere, enable DASH-based playback from a browser. Or you can create your own DASH- or HLS-compatible player from scratch.

移动端有自己的设计选择. Again, native playback is easiest because it doesn’t require any custom development, 但是你只能使用本地支持的格式. If you create an app, however, 你几乎可以实现任何技术:DASH, HLS, Flash, 甚至平滑流.

再进一步, if you create an app, 您还可以替换特定平台的功能失调组件, 比如前面提到的Android上有缺陷的HLS播放. Some well-heeled developers have taken this approach, as has Adobe in Primetime.

The bottom line is that you have enough cash, you can use virtually any technology in any platform. 排列太多了，无法解决, so I’ll focus primarily on browser-based playback in the scenarios presented below, 本机或通过OTS播放器或sdk.

Let’s start with single file delivery to desktops, notebooks, and mobile devices.

通过HTML5与Flash回退的单一文件交付

The simplest approach to reaching the bottom two levels of the pyramid is to use HTML5 delivery with Flash fallback. At this point, 绝大多数浏览器都兼容HTML5, 不过一些消息来源估计是ie8, which is not, still had a 20 percent share or more as of July 2014. For this reason, most producers that deliver via HTML5 also fall back to Flash in legacy browsers.

Of course, not all HTML5-compatible browsers play the same codec; Apple Safari and Microsoft IE play H.264, while Mozilla Firefox and the Opera browser play WebM, and Google Chrome can play either. Some producers encode in both formats for HTML5 delivery; some encode in H.264发送到Safari, IE, and Chrome and fall back to Flash for Opera and Firefox since Flash can play the same H.264-encoded file.

Figure 2 显示了H所需的HTML编码.264/ WebM传输与Flash回退. 这个代码是在Sorenson Squeeze中创建的，它可以产生H.264 and WebM output, plus the HTML5 code, a nice convenience for nontechnical users.

HTML5交付编码

This approach delivers virtually all viewers in the bottom two levels of the pyramid, 但也有一些非常严重的限制. Specifically, it’s VOD-only (no live), 仅支持单个文件(无自适应), 渐进式下载(无流媒体). 此外，DRM没有标准模式. All that said, it’s enough for many web producers, and it’s simple and inexpensive to implement.

从专有技术到标准(疣和全部)

OK, so HTML5 with Flash fallback gives you single file progressive delivery of VOD files without DRM. 如果您需要自适应、实时、真实的流媒体或DRM，该怎么办? 你选择一种不同的技术. Since we’re undergoing a fundamental shift in these technologies, let’s take a quick overview.

The streaming industry was founded on proprietary technologies that ebbed and flowed as different technologies achieved dominant market share—RealNetworks yielded to Microsoft Windows Media, 最终让位于Adobe Flash. In the adaptive space, Microsoft produced Smooth Streaming for Silverlight and Adobe produced adaptive technologies for Flash, RTMP, 以及基于http的动态流, 而苹果则为iOS和Safari开发了HTTP Live Streaming.

专有技术有利有弊. On the plus side, 因为一个组织控制着格式和播放器, 为这项技术添加新功能很简单, whether it’s adaptive, or DRM, or captions. On the negative side, since many proprietary technologies must integrate with the browsers used to find and play the videos, they require plug-ins, 哪些会导致安全问题和不稳定.

标准也有优点和缺点. The key benefit of DASH is that you can deliver a single set of files to all compatible players, 从金字塔的底部到顶端. Standards also come with negatives, chief among them that you can’t force companies to adopt them. 苹果就是这一理念的典型代表, 它还没有表明是否以及何时会在iOS中支持DASH. 微软也没有提供帮助, only releasing support for the Media Source Extensions (MSE) required for DASH playback on Internet Explorer in Windows 8.1, which, at 12.2%的份额(每个StatCounter), only recently surpassed Windows XP in worldwide installed base and is dwarfed by Windows 7 (43.9 percent). True, as mentioned above, you can implement DASH without browser support via plug-ins or custom development, but pervasive support simplifies matters and reduces development costs.