HLS现在支持碎片化MP4，使其与DASH兼容

Apple today announced the inclusion of byte-range addressing for fragmented MP4 files, or fMP4, allowing content to be played in HLS without the need to multiplex it into the traditional MPEG-2 Transport Stream. Compatible with newer versions of its operating systems (macOS, tvOS, and iOS 6.0 or higher) this new fMP4 feature makes HLS compatible with the industry standard MPEG-DASH for browser-based HTML 5 playback.

After a 关于fMP4和HLS的会话在今天的全球开发者大会上，苹果公司发布了规格草案. 业界的反应是积极的.

DASH和HLS:回顾

Just under a year ago, the Bitmovin team announced the release of Bitdash 3.0, which enabled Apple HLS delivery inside Bitmovin’s HTML5 and Flash video player.

使用Bitdash 3.0推出了Apple HTTP 在线直播 (HLS)原生支持, which meant HLS was no longer tied to Apple devices but could be used on any web browser.

Still, for all that integration between Flash and HLS in a Flash-powered player, there was one weak spot in any integration of MP4-based playback coupled with HLS playback: the Achilles' heel known as MPEG-2 Transport Stream (M2TS).

M2TS—a transport stream technology harking back to an era where ATM meant asynchronous transfer mode, not automated teller machine—used a packetization technique that carved a video and one to two audio channels into small segments that could be transported over intermittent delivery methods such as ATM or satellite uplinks.

Packets had a program ID (PID) and enough 信息 to allow re-assembly of the video and audio channels at the far end, 即使包裹没有按顺序到达.

That technology served broadcasters well when they had only one resolution and a few alternate audio channels (think SAP), 但一直不适合扩展到无数的分辨率, 比特率, and multi-lingual requirements of a global internet television audience for OTT.

主要原因是多路复用, the act of pairing a single resolution with a discrete audio stream. Add to this the complexity of Adaptive Bitrate (ABR) with its 5 to 7 resolutions and 10 to 20 audio tracks for various languages, and the complexity creating that many permutations of video per second easily expands logarithmically to a big hairy mess.

要解决这个问题, 使用标准的MP4视频文件和AAC或MP3音频文件, the MPEG committees set about creating an ABR solution that used what the industry calls "late binding," where a manifest file (an intra-video playlist of sorts) eliminates the need to pre-segment content.

使用字节范围寻址, 这些MP4和音频文件将保持完整, and the segmenting would be virtualized using a pointer that delivers up a small fragment of one file at a particular bitrate or resolution, followed by a small fragment of another one of the MP4 and audio files if network bandwidth between the end viewer and the hosted content (most often on a CDN) increased or decreased enough to warrant sending the next segment at an equivalent bitrate. 这种来自MPEG的ABR解决方案被称为MPEG- dash.

后期绑定对于这些独立文件非常有效, and the term fragmented MP4 has been used for years now (starting around the time I wrote a joint whitepaper for Adobe and Microsoft on the benefits of fragmented MP4).

在那份白皮书中, 我提到过M2TS, 在HLS中使用, didn't scale well for an internet-sized audience (this was before the term OTT, 或者过度交付, 成为受欢迎的). 不管, there's always been a performance hit when the short segments of a video file needed to be multiplexed into an MPEG-2 Transport Stream.

今天的新闻:为什么分散的MP4支持很重要

Solving for this is why today's announcement that Apple will update HLS to include the use of fragmented MP4 files is such a big deal.

Not only will it allow late binding and eliminate the need for multiplexing, it also has the potential to marry the industry's standards-based DASH players with the de facto HLS delivery standard.

Now before we all do the happy dance, here's a word of caution from the team at Bitmovin:

"Apple still forces content providers to make use of HLS streaming on iOS for assets longer than 10 minutes," 莱因哈德·格兰德写道Bitmovin的解决方案架构师.

Still, he notes, fragmented MP4 inclusion can be a huge advance from an encoding perspective:

“到目前为止, it has been necessary to encode content in different formats to maximize browser coverage and reach most of today’s end-user devices. This increases the storage footprint of your content by 2x and reduces also CDN efficiency as content cannot be effectively reused across devices."

A 2x increase in storage footprint is putting it nicely as most content owners face a 5x increase due to the possible permutations, 因此，碎片化的MP4将节省大量的存储成本.

除了存储节省, 还可以节省运输成本, with fewer files needing to transit the CDNs internal network to be stored at closer caching points.

除了, 工作流的节省也同样巨大, as the just-in-time approach offered by late binding means that files can be output from a non-linear editing system (NLE) as MP4 without further encoding or transcoding required.

This frees the NLE up to do more paid editing work, which means additional indirect cost savings.

换句话说, 使用碎片化MP4的HLS是一件大事, 不仅仅是技术上, 同时也是为了你的底线.