2017年6月8日
By 1月时特约编辑
专题文章

苹果拥抱HEVC:对编码意味着什么?

从编解码器的角度来看, the key announcement at Apple's Worldwide Developer Conference (WWDC) 2017 was the inclusion of HEVC 成 HTTP直播 (HLS). 流媒体's Dan Rayburn covered the significance of that announcement to the market as a whole in his article, "Why Apple's HEVC Announcement Is A Big Step Forward For The 流媒体 Industry.“你可以从标题上看出，雷伯恩认为这是一件大事，我也同意. 在本文中, I'll focus on exactly what Apple announced regarding HEVC and its implications from a technical perspective. 我还将讨论HLS支持的新字幕格式, 以及苹果推出的一种新的图像格式.

HEVC在High Sierra (macOS)和iOS 11中的支持

我从WWDC第504次会议中收集了以下大部分信息: “HTTP直播技术的进步," by Roger Pantos, the original author of the HLS specification, and Anil Katti, both AV基金会工程师.

以下是我所学到的. 第一个, 苹果将在High Sierra支持HEVC, 下一代MacOS, 和iOS 11, 这两款软件都将在今年秋天免费升级. 图1 标识所有将与iOS 11兼容的设备.

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图1. 兼容iOS 11的iOS设备

如图所示图2, 苹果将通过硬件和软件解码的结合来支持HEVC, and you should expect software decoding to consume more power and shorten battery life. 例如，在戴尔笔记本电脑上进行测试由TechSpot表演 found a 39% drop in battery life when playing HEVC without hardware support as compared to H.具有硬件支持的264. 除非证明不是这样, I would expect similar decreases in battery life with Apple mobile devices and notebooks using software-only HEVC decode. 从积极的方面来说，在HEVC和H.264硬件支持, the TechSpot article found very little difference in battery life when playing the two formats.

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图2. 硬件和. iOS 11和MacOS对HEVC的软件支持

这对哪些设备有影响? 只看iphone, 根据维基百科, iPhone 5s使用的是A7处理器, iPhone 6和6 Plus使用的是A8处理器, 回放将在软件中进行. iPhone 6S机型使用的是A9处理器, 而iPhone 7使用的是A10 Fusion, 其中可能还包括基于HEVC硬件的解码. So, 向iPhone 5s发送HEVC流, iPhone 6, and iPhone 6 Plus will likely significantly reduce their battery life; for newer devices, HEVC应该有最小的影响.

虽然没有列在幻灯片上, 在4点15分的时候, Pantos states that the Apple TV will also receive a software update that enables HEVC playback, 你能在苹果公司确认一下吗 tvo指南.

另一个不同的演讲，第515节，“HLS创作更新," noted that all software and hardware decoders support up to Main 10 Profile, Level 5.0和High Tier. This presentation also discusses many additional changes to the HLS specification and associated tools, 对于任何创建HLS内容的人来说都是值得的. 另一个演示，“我介绍HEIF和HEVC(第503次会议), 由Gavin Thomson和Athar Shah主持, 超出HLS的细节, Apple's HEVC decoders can play HEVC encoded video in either a QuickTime or MP4 wrapper.

其他HEVC实现细节

图3 介绍几个附加的实现细节. 首先，也是最重要的，HEVC必须封装在苹果采用的碎片化MP4格式 when it added support in HLS for the Common Media Application Format (CMAF) at last year's WWDC. 第二个, 苹果将继续使用CBC(密码块链)加密, 谷歌在Widevine中采用的是什么, 但到目前为止，微软还没有在PlayReady中这么做. So, 而你可以重用你的编码fMP4片段的DASH和HLS, 您仍然需要两个加密内容的传递有效载荷—一个用于CBC, 另一个用于PlayReady中使用的计数器模式(CTR).

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图3. 其他实现细节

The rest of the slide deals with notification requirements for HEVC encoded content in the HLS manifest files, 这很简单.

混合HEVC和H.264在主播放列表

图4 详细介绍了如何结合HEVC和H.由主播放列表文件定义的单个HLS演示. 如你所见, you can mix the two codecs in a single playlist just as you could include video encoded using different H.264年资料. The key requirement is that you designate the codec as discussed above (and as you had to do when mixing H.264配置文件).

Essentially, this means that the player will have to seamlessly switch from playing H.264 encoded video to HEVC encoded video; it will be interesting to see how smoothly this works, 特别是在使用软件解码的设备和计算机上.

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图4. 混合HEVC和H.264在主播放列表

注意，不像HEVC，它必须是fMP4, H.264 HLS content can be formatted as either fragmented MP4 files or MPEG-2 transport streams. 这意味着你不需要将你的流与HEVC混合, 一个很好的便利因素. The last line details that Apple updated their HLS Authoring Guidelines for HEVC (the official document is called the 苹果设备HLS创作规范). 让我们看看更新后的指南是什么样的.

40%? 谁说的40%?

图5 显示新的编码阶梯, and it's interesting that the highest bitrate saving is 29% and never gets to the 40% number that Apple bounced around in multiple sessions. It's also interesting that Apple hasn't yet extended its guidelines beyond 1080p, 尽管iPhone 7和许多其他设备都支持4K播放.

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图5. 来自Apple HLS创作规范的更新编码阶梯

不管. You should be able to achieve 35-50% savings at 1080p resolution, and 30-40% savings at 720p. 除了这些决议，H.264从来都不是一个好的选择，所以比较是没有用的.

字幕怎么样??

苹果还推出了对IMSC1的支持 w3c推荐标准用于为流传输优化的TTML标题, 是CMAF的基本字幕格式. 这也是强制性的可互操作的主格式规范像Netflix这样的内容发布商所承诺的.

根据苹果的文档, IMSC1标题作为XML文本在fMP4片段中携带, 并在主播放列表文件中标识. Note that IMSC1 captures are in addition to the WebVTT captions already supported by HLS, 不是代替.

HEIF是什么鬼东西?

在上述题为“介绍HEIF和HEVC”的演讲中," Apple also debuted an image format called the High Efficiency Image File Format, 或HEIF, which is an image standard developed by the Moving Picture Experts Group (MPEG) for storage and sharing of images and image sequences. 就像你看到的图6, 苹果使用的HEIF格式使用HEVC压缩, 哪个演示者状态提供比JPEG高2倍的压缩.

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图6. 有关新HEIF格式的详细信息

Support f或HEIF image decode looks a lot like HEVC decode; that is, 使用A9芯片和(可能)更高版本的设备的硬件, 还有其他所有人的软件. 这引起了两个问题. 第一个, HEIF解码器需要支付版税, 虽然可能, 苹果已经在自己的更新中解决了这个问题. 第二个, that HEIF images will be a proprietary format that you can't easily share or utilize. It may be that you'll first have to decode the image 成 JPEG with an Apple supplied or other decoder to use it outside the Apple ecosystem.

关于版税

根据多份报告, 苹果目前拥有约2500亿美元现金, 所以它当然能负担得起与HEVC相关的版税. 有趣的是，苹果公司被列为许可方(和许可方) MPEG - LA池，但不是作为持牌人 HEVC预付款池. 我检查了HEVC Advance, 这证实了苹果目前并不是授权方, 不过他们希望苹果发布更新后情况会有所改变. 最新的游泳池，来自 Velos媒体但该公司尚未确定条款.

我从业界听到了什么?

正如你所料，业界对苹果的举动非常积极. Bitmovin 已经支持 HEVC in HLS, and JW Player plans to once the updates are available and demand for the format begins.

The one publisher I spoke with expressed concern about the costs of adding a completely new set of encodes to its CDN footprint, and shared it they probably would wait until iOS devices with hardware decode comprised a high percentage of all supported iOS devices.