评论:HEVC和AV1-A幕后看视频编解码器竞赛
There’s a codec race underway between HEVC and AV1, and recent events are making it even more interesting. Today, HEVC, the incumbent codec, 包括三星Galaxy手机和苹果的Safari浏览器在内的20多亿设备都支持, Apple TV, Mac, and iOS devices. This makes HEVC playable on close to 50% of smartphones. Despite its broad support on devices, HEVC has not been widely deployed, mainly due to licensing issues. 然而，HEVC Advance最近取得了进展 waiving the streaming fee, which is a big step forward for the codec. Streaming Media conducted a survey 在苹果的支持下，研究了HEVC的部署场景. 研究发现，到2018年底，24%的运营商已经部署了HEVC, 64% will have deployed a HEVC service.
On the other side of the video codec race is AOM (the Alliance for Open Media), with members that include Apple, Google, Mozilla, Facebook, Hulu, Netflix, Microsoft, and Amazon, among others. AOM just released its AV1 specification, 旨在通过提供免版税许可计划来挑战HEVC在OTT应用部署方面的地位. The main use case for AV1 is OTT VOD. Live can also be supported, although it might face some encoder scalability issues, as discussed later.
Harmonic and research institutes including HHI, B<>COM, 和威斯巴登大学对AV1进行了研究. In particular, Harmonic and B<>COM compared the AV1 open source codec with the HEVC HM reference model on different reference test sequences provided by ITU-R and EBU, using 1080p resolution across a broad range of bitrates. The results, 哪些将在2018年NAB展上向公众展示, 证明AV1视频质量与HEVC相当. 测试发现，AV1在PSNR和VMAF指标上优于HEVC, but not on subjective testing. 从整体的编解码效率来看，与HEVC相比没有太大的区别.
Meanwhile, 我们必须以更全面的方式比较AV1和HEVC，其中也包括编码器和解码器的复杂性. Table 1 提供AV1和HEVC之间差异的总结:
Features | HEVC | AV-1 |
Royalty free | No | Claims to be |
Compression efficiency vs HEVC | NA | On par with HEVC |
Open source | No | Yes |
Installed based | 2 billion | Not significant before 2020 |
Live encode | Affordable | 编码器比现在的HEVC实时慢1000倍，一旦优化，预计会慢100倍 |
Decoder cost | TCO =解码器芯片(目前量产)+内存+ HEVC许可证 | 需要考虑额外的解码器+内存成本，并与HEVC解码器TCO进行比较 |
Deployment environment requirement for live | 可以在软件设备、裸机容器或原生云上实现吗 | Cloud is the only technology to enable live UHD |
Broadcast support | Yes | No |
Table 1. 比较HEVC和AV1的优缺点
AV1 was designed with unicast delivery in mind, 这意味着我们可以预见不同类型设备的不同编解码器, i.e. HEVC-capable and-AV1 capable. At the beginning of AV1’s life, we will see AV1 supported only on web browsers in 2018; note all major web browsers are members of AOM. 2020年之后，我们将看到互联设备更大规模地采用AV1.
From an encoder implementation point of view, adding a new codec will not be a problem, as the encoding is done in software. 然而，基于当前的可扩展性数字(100倍vs HEVC)，它将增加成本. Currently, the AV1 encoding requirements are high, and a cloud-based architecture is well adapted, 因为它将使其他节点能够实例化，以便实时编码, especially for high-resolution use cases like UHD. AV1将迫使编码器公司创新并实现基于切片的处理, where each slice will be distributed to a specific node. 注意，这种方法会降低编码效率, 因为预测总是局限于切片边界. On the decode side, to cut the cost, 必须建立一个可编程结构来同时支持HEVC和AV1, as both do not need to be decoded simultaneously.
编解码器竞争中出现的另一个问题是市场细分. Depending on the service and devices, 服务提供商将需要在HEVC(未来几年)或AV1(从2020年开始)中进行编码。. 这将需要大约36个月的过渡期(是移动设备更新周期的两倍)，以及HEVC和AV1的同时播出, meaning dual encoding and CDN storage.
直到在下一代视频编解码器的竞争中出现强有力的领先者, there is room for enhancement of AVC, which has been used for more than a decade. 随着内容感知编码的出现，我们看到了AVC的复兴, 在Netflix和YouTube为视频点播(VOD)做了同样的事情之后，Harmonic率先将这项新技术应用于直播应用. With content-aware encoding solutions, 视频内容和服务提供商可以减少高达50%的OTT传输带宽, decrease CDN costs, 并提供高质量的体验，而不需要对现有的底层交付基础设施和视频播放器进行任何更改. 为了取代HEVC, AV1必须向运营商展示除许可条款之外的实际利益. More specifically, AV1需要在解码端展示一个具有成本效益的解决方案, 特别是如果双解码(HEVC-AV1)必须在解码器中实现. On the encode side, 我们看到HEVC和AV1在编码时间上的巨大差异(今天是1000倍), 视频点播可以接受，但可能成为实时部署的障碍. 随着AV1对HEVC授权商的持续压力，业界需要找到解决方案. A dual codec scenario is not ideal, but could be likely.
[编者注:这是一篇由供应商提交的文章 Harmonic. Streaming Media 接受供应商署名，完全基于他们对我们读者的价值.]
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