CFF:一种格式统治一切?
Since the early days of online video, various battles have been waged among competing video codecs and formats, but those battles were merely indicative of the larger struggle between those who support a 通用文件格式 for all online video delivery and those who support proprietary formats. The most recent push for a 通用文件格式 is coming from proponents of MPEG破折号, and if the standing-room-only crowd at an MPEG破折号 panel at 流媒体 West earlier this month is any indication, its time may have come.
紫外线, which uses both a 通用文件格式 and common encryption, is supported by six major studios and the seventy-member Digital Entertainment Content Ecosystem (DECE) as a way to deliver premium content such as movies on both physical media (discs) and online (digital downloads). As described in detail in a "是什么...?Jan Ozer的文章， MPEG破折号 is a way to standardize manifests (called Media Presentation Descriptions 或MPDs) that is moving through the ISO ratification process piece by piece.
微软，苹果和Adobe
Let's take a look at how MPEG破折号 and 紫外线 fit together with two currently used fragmented MP4 (fMP4) solutions, Adobe HTTP Dynamic Streaming and 微软 IIS Smooth Streaming. Understanding this is important to understanding why we might be on the cusp of a common way to scale online video delivery to television-sized audiences. Rather than spelling out the technical details of fMP4, I'll point readers to a technically leaning fMP4白皮书 which I authored for Transitions, Inc. 在我的 工作流的博客. 这篇论文, jointly sponsored by Adobe and 微软, contains sections on technical aspects of fMP4 as well as each company's approach to fragmented delivery over HTTP.
As Chris Knowlton pointed out in a recent post on 微软的IIS博客, 微软 was the first to embrace the use of fragmented MP4 files, back in late 2008. 当时, the use of fragmented MP4 elementary streams was a new and non-standardized concept, but 微软 rapidly implemented it in Smooth Streaming, which used AAC for audio and AVC/H.264用于视频压缩.
差不多在同一时间, Apple was also beginning to push the concept of a modified MPEG-2 Transport Stream that it since dubbed Apple HTTP 在线直播 (HLS) as a way to deliver multiplexed (muxed) transport audio/video streams to its iPhone and iPod touch devices.
微软's idea of using fragments of MP4 files for Smooth Streaming was brilliant in its simplicity, for a variety of reasons touched on in the white paper. 最基本的好处, 虽然, might just be the way it solved the asset management nightmare looming with HLS: Rather than having to manage the location of thousands of tiny segments for each streaming video, HLS的情况也是如此, the fMP4 approach allows fragments of a large audio or video file to be identified based on timecode.
All the audio and video could reside in just a handful of files—one each for each discrete video or audio bitrate—limiting the need to manage the thousands of files in competing approaches.
Adobe rapidly moved to adopt an HTTP delivery approach, 太, in its Adobe HTTP Dynamic Streaming (HDS). Adobe chose a different file format called F4F that is, as Adobe's Kevin Towes points out in a recent 博客, based on the fragmented MP4 file format. While Adobe used the same AAC and H.264 codecs for its fragmented MP4 approach, its use of the F4F file format meant that it could deliver Flash video content both via HTTP and RTMP.
Because Adobe and 微软 chose to use the same fragmented MP4 concept, but to package them in different formats, 随之而来的是一些混乱. 我们在StreamingMedia.com found ourselves asking the question as to whether there would be a common ground between these competing formats. Turns out that was also on the minds of Adobe and 微软, but it 太k the emergence of 紫外线 to bring the issue front and center.
紫外线
Prior to 紫外线, 微软 had proposed a format known as 唏 (Protected Interoperable File Format), as a way toward a 通用文件格式 for fragmented MP4. The idea garnered interest, but it was not until 紫外线 settled on a 通用文件格式—now referred to as CFF—that the idea of an interoperable format gained traction. 不管怎么说, the CFF specification that 紫外线 settled on is a published version of 唏, and 微软 has helped keep pace with the CFF by making the 唏 1.3 version directly compatible with CFF.
Once the 紫外线 CFF was published and agreed upon by the studios and DECE members, the next step was to look at a common encryption scheme. 紫外线 settles on five digital rights management (DRM) schemes—one each from Adobe, Google, 马林微软和微软 开放移动联盟.
For all the work by Adobe and 微软, and the additional work by DECE on 紫外线, there had not been a collaborative effort to standardizing an fMP4 approach. 改变了, 虽然, once the 紫外线 通用文件格式 and five-pronged DRM schemes were approved by DECE members.
这让我们想到了MPEG破折号. The primary purpose of DASH is a proposal to standardize HTTP-based content delivery via XML-based manifests (the Media Presentation Description, 或MPD, 用DASH命名法). 这样做的时候, DASH seeks to find common ground between the various approaches that Adobe, 微软, 和其他为fMP4创建的, as well as incorporating a way to generate XML-bases standardized MPDs for the M2TS segment files that Apple uses for HLS.
Alongside these media description manifests, 然而, MPEG felt it wise to adopt two other proposals that complement—but are not part of—the DASH standards: one is a common set of encryptions and the other a 通用文件格式 (CFF).
MPEG wanted to base a standardized CFF on MPEG-4第12部分, the ISO Base Media File Format that has as its original basis the QuickTime file format. 而不是重新发明轮子, MPEG chose to consider existing 通用文件格式s, 选定了紫外CFF. The adopted MPEG CFF now means that 唏, 紫外线, and MPEG-4 use a 通用文件格式, a confluence of fMP4 akin to the DVD论坛's 1995 interoperability specification, which allowed competing device manufacturers to create interoperable DVD players.
除了, MPEG reviewed 紫外线's five-pronged DRM approach, which will be known as the MPEG Common Encryption (ISO/IEC 23001-7 CENC) scheme if pending ratification is approved. The major benefit from a DRM standpoint is that CENC can be combined with the MPEG-4第12部分 ISO Base Media File Format, enabling fragmented MP4 to take advantage of DRM and elementary stream encryption.
This accelerated move to a 通用文件格式 between fragmented MP4 solutions doesn't quite have universal traction, but it sets the stage—along with MPEG破折号's profiles for both fMP4 and M2TS decoding—to allow broadcasters to choose from just a few proprietary and standards-based HTTP delivery options.
We've yet to hear from Apple—which contributed to the MPEG破折号 working group—as to whether it will take advantage of the M2TS profiles in MPEG破折号 to replace the .m3u8 manifests currently used in HLS, but it's become clear Adobe is willing to consider a balancing act between its traditional Flash video approach and a standards-based fMP4 approach.
In much the same way that Adobe has embraced a symbiotic relationship, 说到交互性, 在Flash和HTML5之间, it also appears Adobe sees the two-pronged approach for fMP4 as beneficial. And that's a positive step toward embracing a 通用文件格式 in the near future.
“任何加速发展的技术, a large group of people are motivated to consider different ideas and concepts that improve the technology,Towes在他的信中写道 博客 about the jointly sponsored white paper. "When you are smack in the middle, there are a lot of options available to [broadcasters]. . . . We try to help broadcasters realize new revenue streams, regardless of platform being delivering to."
Flash Player可用的地方, we'll use RTMP or HDS to deliver world-class quality of experience, 加密和权限管理," said Towes in an interview for the white paper. "With other platforms, such as iOS-based devices, we'll do an equally good job. Adobe also sees the benefit of standards-based approaches like the Common File Format, HTML5和MPEG破折号.”
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