2012年12月4日
By 1月时特约编辑
专题文章

珀斯Wijering谈HLS, DASH和JW Player

长尾的视频最近宣布发布 JW Player 6, a major upgrade to the company's popular free and low-cost video player. 一个关键的新功能是支持Apple HTTP 在线直播 (HLS)。, 使台式电脑能够播放HLS流, and for video publishers to support desktop and mobile viewers with one set of adaptive files. It also raises questions about the importance of the Dynamic Adaptive Streaming over HTTP (DASH) spec, 哪个应该提供相同的能力, 但目前还没有上市.

我们采访了杰伦·维杰林, JW播放器的创造者, who was kind enough to answer questions about how the JW Player works and its potential impact on DASH.

社交媒体: 玩家是否提供原生H.264播放功能还是依赖于H.264 playback provided by other modules, like Internet Explorer or Safari, or the Flash Player?

JW: The player relies on the Adobe Flash plug-in to provide HLS支持 on desktops and on the HTML5

社交媒体: So, if a Firefox user doesn't have the Flash Player installed, they can't play HLS (or H.264(一般).

JW: 这是正确的. 浏览器(或Flash)必须为H提供支持.264.

社交媒体: You've commented publicly that the installed base for Flash was about 99 percent. 这是估计还是你有实际数据? With all the buzz about HTML5, why do you think that number is so high?

JW: 这是一些轶事. 我们在自己的网站上看到了这一点，加上分析提供商，比如 Statowl 显示汇总细目. The piece of the pie without Flash is almost exclusively mobile devices.

Flash Player的安装基础一直很好, Flash的升级周期比以往任何时候都好, 支持跨Win和Mac的自动更新, 以及Chrome的内置支持. Flash is also still required for several key web use cases, one of which is streaming video. Hulu, 英国广播公司, HBO——一切都还是闪电侠, 出于质量和安全的考虑, 加上生态系统支持.

社交媒体: What's your sense of the progress the browser vendors are making vis-à-vis DASH support?

JW: 据我所知，没有浏览器供应商在其路线图上支持DASH. Chrome在媒体源API上的工作是最接近它的. This API allows publishers to build adaptive streaming using JavaScript. Chrome会做H.264 decoding, but parsing of the manifests and bitrate selection are up to the publisher.

Flash中的类似API允许JW Player支持HLS. 当它到达时, we'll likely leverage the Chrome Media Source API for HLS支持 in HTML5 too, 这意味着HLS将在Chrome浏览器中播放没有安装Flash.

社交媒体: 你如何看待DASH对市场的支持? I've heard through plug-ins, and then perhaps players like the Flash Player. 你是这么看的吗? 也许是通过像你这样的玩家? What's your sense of when general purpose publishers (as opposed to OTT and other closed system vendors) can actually start thinking about using DASH in HTML5?

JW: The only way right now to support DASH is indeed to build it using the Flash Plug-in. It can be easily done, just like JW Player supports HLS through the Flash plug-in today. 我认为DASH在HTML5中的实现还需要数年时间.

I do also see publishers building DASH into apps, on the Android, iOS and Windows 8 platforms. This is only possible for large publishers though; those that have the dev resources to build and reach to distribute the app. 较小的酒吧将被排除在外.

社交媒体: 你觉得DASH vs .的优缺点是什么. HLS支持? What are the critical features that DASH provides that HLS doesn't (if any)?

JW: 关键特性是DRM. Apple's HLS provides AES128-based encryption support, but not full-on DRM with licensing. DASH支持各种协议(如PlayReady)。. 幸运的是，在我们的市场细分(SMB)中，DRM不是必需的. Protection from hotlinking is much more important, and AES in HLS provides that.

There are several other features in DASH that HLS used to miss (like separate audio/video fragments or external closed captions), 但最近版本的HLS协议增加了许多这样的内容. 从某种意义上说，HLS正变得越来越“时髦”。. 也许苹果这样做是为了慢慢实现DASH兼容性? 目前，HLS是向iOS设备传输流媒体的唯一途径.

社交媒体: Android平台上的HLS播放状态如何?

JW: 根据官方说法，Android从Honeycomb开始就支持HLS. 在实践中, we see a number of critical bugs in the current iterations of Android that break in-browser HLS playback:

在Honeycomb上，HLS播放总是会导致平板电脑崩溃. The version is of little concern though, since it has 2 percent market share and is shrinking.
On Ice Cream Sandwich (26 percent share), HLS plays, but VOD streams cannot be seeked. 宽高比也没有检测到，导致图像变形. When going full-screen, a video is re-started from the beginning (again with no support for seeking).
在Jelly Bean(3%的市场份额), 但越来越多的), 宽高比问题已修复，但无寻道问题仍然存在. 另外, 新的默认浏览器(Chrome)不支持HLS, leading to broken mimetype detection and an error message plus crash of the stream when taking it full-screen.

Taking into account that Gingerbread (54 percent share) does not support HLS, the only solution to streaming HLS on Android is building your own app.

社交媒体: 为玩家添加HLS支持有什么困难? 为什么不早点呢??

JW: Adding HLS meant we had to do the unpacking (de-muxing) of the TS streams ourselves, so we had to learn the nitty-gritty details of video containers first. 这包括供应商之间的所有细微差异. Wowza sets a certain byte this way, Akamai that way, and Sorenson yet in another way. We had to do a lot of testing and reverse engineering to get our HLS支持 compatible with all major solutions and performing well on old devices.

In addition to that, publishers were (and are) quite content with RTMP. 协议运行得很好，所以为什么要改变呢? Only in this year, we've really seen an uptake in demand for HTTP streaming. 发布者对HTTP流的主要需求是总是支持iOS，这就是为什么我们选择HLS而不是Adobe的HDS.

社交媒体: Would you expect that Adobe will add HLS playback to the Flash Player sometime soon? 如果没有，为什么没有呢?

JW: 我不认为Adobe会把这个添加到Flash Player中. HLS is a format controlled by Apple, directly competing with HDS, Adobe's own format. 与HDS, Adobe控制着服务器, 协议, 和客户端, 所以它可以提供独特的功能, 例如DRM或多播. 与HLS, the Flash player is "just another" client and the Adobe Media Server is "just another" HLS compatible server.