2018年1月15日
By Jan Ozer 特约编辑
专题文章

HLS中的HEVC:流媒体视频开发者的10个关键问题

在2017年6月的全球开发者大会上, Apple announced support for HEVC playback in HTTP 在线直播。 (HLS) delivered to iOS, MacOS, 和tvOS终点. For many, 这一声明引发的问题多于答案, 我们希望在本文中解决这些问题.

顺便说一下背景, 在2017年流媒体西部, I co-produced a preconference session on encoding HEVC for HLS with David Hassoun and Jun Heider from RealEyes Media, 流媒体咨询公司. Much of the materials presented herein was assembled for that session, and you can 下载资料在那个演讲中. 和会前会议一样, I’m going to assume that you know the basics of HLS production and I will focus primarily on the HEVC-specific aspects.

Here are the top 10 questions for developers seeking to add HEVC to their HLS streams.

1. 在HLS中哪些设备支持HEVC播放?

在HLS中支持HEVC播放的设备有三类:

iOS: 所有设备兼容iOS 11. 这包括5s以后的所有iphone, iPad Air和iPad Pro型号, iPad mini 2及以后的机型, 第五代iPad, 和iPod touch第6代设备.

Mac: 所有设备兼容MacOS 11, 或者High Sierra, 包括MacBook(2009年末或更新版本), MacBook Pro(2010年年中或更新版本), MacBook Air(2010年末或更新版本), Mac mini(2010年年中或更新), iMac(2009年末或更新版本), Mac Pro(2010年年中或更新版本).

Apple TV: Apple TV 4K.

在这一点上，也值得讨论一下你 don’t 通过在HLS中支持HEVC获得，至少目前是这样. 具体地说, 尽管Android支持HLS和HEVC, at this point Android can’t play back HEVC video included as part of an HLS presentation, 虽然这随时都可能改变. 对于支持HEVC硬件的Windows 10平台上的Microsoft Edge也是如此, 哪个可以播放HEVC流, 只是不是HLS中的HEVC. 一旦你有编码的HEVC流, 您应该能够转换成DASH以交付到两个平台, 不过这又增加了一个过程.

2. 对电池寿命有什么影响?

为了了解电池寿命，我们比较了H.264和HEVC播放流媒体西部会话积极的结果, 你可以在文章中读到 HLS中的HEVC:它如何影响设备性能?” 精辟的总结陈述, “Overall, while you may have issues with the oldest generation of supported mobile devices and computers, the next generation in all cases show only a slight increase in CPU use for HEVC playback, 而新款iphone至少表现出了相对的平价. Publishers considering deploying HEVC should do so without concerns that the higher-end format will create significant battery life issues for most potential viewers.”

3. 支持HEVC能给我带来什么?

Supporting HEVC will deliver multiple benefits, including economic savings and service improvements.

带宽节省: HEVC应该节省一些带宽, though the benefits will vary from service to service and will depend on multiple factors. In terms of compression performance, HEVC should deliver about the same quality as H.以更低的比特率, 在1080p下降低50%, 尽管在较低的分辨率下，这将大幅下降. 以确定这将为您节省多少实际带宽, you’ll have to start by checking your server logs to see the distribution of streams that you’re currently delivering.

At one end of the spectrum, assume that your typical viewer is watching a 4Mbps 720p H.264 -编码流. Switching to HEVC would deliver little bandwidth savings because after making the switch, 你可能会传送一个1080p的4Mbps的流. While the perceived quality of the stream would increase, the bandwidth would be the same.

另一方面, I recently chatted with an OTT provider in Denmark who reported that 93 percent of streams delivered were the highest quality 1080p H.264流可用，编码为8Mbps. 在这种情况下, bandwidth savings could be close to 50 percent because the service could deliver the same video quality using HEVC at about half the bandwidth.

体验质量 Quality of Experience (QoE) benefits also depend on the streams that you’re currently delivering. 在丹麦OTT提供商的案例中, 由于1080p H的感知质量，QoE变化很小.264 stream encoded at 8Mbps would be nearly identical to the 1080p HEVC stream encoded at 4Mbps.

另一方面, 如果你目前正在向手机用户提供中级流媒体, QoE的好处可能相当可观, 如表1所示. 简单地说，为了完成这个表，我在H.264和HEVC两个测试剪辑， 钢铁之泪 and Sintel. 然后我计算了每个片段的VMAF分数, with the Delta column on the right showing the improvement from using HEVC instead of H.264.

顺便说一下背景, VMAF分数为6分或以上表示仅显着差异(JND)。, 这意味着75%的观众会注意到差异. By using higher-resolution streams for HEVC at 365-2000Kbps, the QoE benefits are quite substantial. 另一方面, 如前所述, 当两个编解码器都以1080p显示时, QoE的好处是微乎其微的. The bottom line is that while many vendors will tout that HEVC delivers bandwidth savings, 体验质量改进, or both, 里程数会因制作人而异, and you’ll need to check your own logs to gauge the benefits of adding HEVC to the mix.

高动态范围(HDR): 尽管在本文中我不会花很多时间在HDR上, 值得注意的是，最新版本的HLS确实整合了HDR. 这简化了HDR视频到所有支持的HLS端点的传输.

4. HEVC支持的成本是什么?

HEVC支持成本有多个类别.

编码和存储成本: 显然，您必须将视频编码为HEVC格式. 如果你在内部编码, you’ll have to calculate the cost of buying and maintaining additional encoding platforms, if needed. 如果在云端编码, 成本将根据编码阶梯的阶数而变化, 以及分辨率和数据速率. At high volumes, you should be able to achieve encoding costs of well under $20/hour for all rungs. 你必须继续用H编码.264格式的其他目标，所以这些成本将在H.264. 对于原始服务器上的存储也是如此.

PPV和订阅服务的版税: If you’re distributing subscription or PPV video, you may already be paying royalties for H.264在MPEG LA H下的使用.264专利池. 对于HEVC，有三个池，MPEG LA, HEVC Advance和Velos Media. 这显示在 Figure 1, which is adapted from a presentation given by Divideon’s Jonatan Samuelsson at Streaming Tech Sweden in November 2017.

Figure 1. HEVC知识产权所有者和专利池

在三个池子里, MPEG LA的许可条款不包括内容版税, HEVC Advance收费0美元.2018-2019年每位订阅者每月015美元. Velos Media尚未公布任何拟议的版税条款. 2017年8月28日，该网站的Q&A stated, “因为这与内容有关, we will take our time to fully understand the dynamics of the ecosystem and ensure that our model best supports the advancement and adoption of HEVC technology.“因此，内容版税可能会摆在桌面上.

对于左下角没有加入池子的公司, it’s impossible to say whether they plan to charge content-related royalties or not. If you’re looking for a reason that streaming producers haven’t jumped aboard the HEVC/HLS train, 很可能是与内容相关的版税的不确定性.

球员发展: 如果所有的播放都是在iOS/MacOS浏览器中实现的, 玩家的发展应该最小化, 因为本地HLS播放器都应该自动处理HEVC. 如果你是为了交付而部署应用，可能会涉及到一些开发成本.

5. 我应了解哪些管制文件?

您应该熟悉两种文档来源. 首先是苹果设备HLS创作规范它包含了与HEVC使用有关的大多数规范. 第二种是苹果在go2sm上提供的HLS示例.com/hlsexamples that fill in most of the details missing from the Authoring Specification. 例如, 创作规范声明, 为了向后兼容，一些视频内容应该用H编码.264.“苹果的例子恰恰说明了HEVC和H.264 bitstreams Apple included in its HLS presentation, as we’ll share with you below.

6. 我知道如何用H编码.264. 我还需要知道什么来生产HEVC?

如果你理解H.264编码，你不需要知道更多的产生HEVC. HEVC很像H.264和MPEG-2之前, 你知道的关于数据速率的大部分知识, 关键帧间隔, 比特率控制, 其他常见配置选项的工作原理非常相似. Like H.264, HEVC has different profiles, two of which are available for HLS—Main and Main 10. As the name suggests, Main10 encodes in either 8-bit or 10-bit bit depths, while Main is 8-bit only. HLS can handle either, though you’ll need to produce in Main 10 format for HDR output. Note that the Authoring Specification has detailed rules for 比特率控制 for live and VOD streams you should learn if you’re new to HLS encoding.