2017年4月18日
By 1月时特约编辑
专题文章

Bitmovin Pushes AV1 Forward, Joins Alliance for Open Media

Just when it seemed that the Alliance for Open Media's AV1编解码器 would be on the backburner through the end of 2017, Bitmovin 宣布将AV1添加到其VoD和实时编码服务中，并在NAB上演示AV1实时编码和播放. 在一个博客关于公告, Bitmovin详细介绍了AV1迄今为止的进展, and reveals how far it needs to go to become usable and how quickly it can get there. Long story short, like objects in your rearview mirror, AV1 may be closer than it appears.

AV1播放器

Bitmovin于2013年在奥地利成立，最初是为了推广基于dash的HTML5播放器云编码和分析. Earlier this year, Bitmovin announced a managed on-premise encoding service based on Docker and Kubernetes.

开放媒体联盟(AOM) 2015年推出 to consolidate the open source codec development efforts of several its founders, 包括谷歌(VP10), Mozilla (Daala), 思科(雷神), 微软, 和英特尔, with Amazon and Netflix also joining as founding members. 2016年，AMD, ARM和NVIDIA 加入小组， AOM网站目前显示 23个成员, not including Bitmovin, which joined in April 2017.

的 AOM的主要焦点是开发“媒体编解码器”, 媒体格式, 以及相关技术，以满足市场对视频压缩和网络传输的开放标准的需求." In short, a royalty-free codec to use instead of the exceedingly royalty-bearing HEVC. 质量目标是“比VP9/HEVC高50%，在编码和播放复杂性上有合理的提高”,2016年4月我们写了冻结比特流的目标日期是“2016年底到2017年3月之间的某个时间”." Since then, sources had advised that the target date might slip to the end of 2017. 在其他细节中, Bitmovin在其博客文章中指出, "的 AV1 bitstream freeze should be in Q3 2017."

Given AOM's membership, things can move quickly from there. 这是, 谷歌, 微软, and Mozilla can quickly add decode to their respective browsers, 而Netflix, 谷歌 and Amazon can start encoding trials for content distribution. While hardware support for AV1 will obviously take longer, 联盟中的硬件合作伙伴在开发过程中一直与编解码器开发人员一起工作(详见下文), which should accelerate the availability of AV1 encode/decode in CPUs, gpu, soc, 以及其他硬件.

Bitmovin宣布了什么

4月18日, Bitmovin宣布，其云编码和本地管理服务“现在支持AV1视频点播和直播编码”." 的 company also announced that it would "showcase the first ever AV1 livestream, 提供1080p播放在1.4月24日至27日，我们在拉斯维加斯NAB展(SU9007CM)的展位上提供了5Mbps的广播质量." Let's deal with the second announcement, then circle back and address the first.

Bitmovin将在NAB开幕式上展示AV1直播工作流，该工作流由实时编码程序开放广播软件(OBS)生成的12Mbps 1080p 30fps流。. 这是通过RTMP发送到谷歌云，在那里它被Bitmovin云编码器编码为“广播质量”1.5 mbps AV1. 关于质量, 这篇博文说, “使用传统的编解码器(如H264)，您需要大约4到15Mbps才能提供相同的质量.编码后，视频将通过AOM播放器和FFmpeg流式传输到桌面进行播放.

图1 illustrates the difference between a single notebook encoding the stream, and encoding performance in the Bitmovin cloud. 这是, Bitmovin将在NAB上展示的40秒《百家乐app下载》预告片将在一台配备4核i7-4800 MQ CPU的笔记本电脑上进行编码，耗时近9个小时. In contrast, the Bitmovin cloud can produce the same output in 34.5秒.

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图1. Bitmovin cloud compared to a 4-core notebook.

显而易见的问题是，要实现这种性能，云中需要多少核心. At NAB, the company expects that it will require up to 200 cores. 然而, Bitmovin首席技术官Christopher Mueller评论说:“这真的是早期阶段，我们主要是想展示我们编码堆栈的灵活性...我非常有信心，我们可以降低硬件要求，将单个1080p AV1流编码到8到32核.这是有道理的, given that most heavy optimization work won't start until after the bitstream freezes.

然而, 至少在短期内是这样, the real gate to deployment isn't the encoding side; it's the playback side. 这是, 任何想要流媒体播放AV1视频的公司都必须在谷歌之前提供播放器或插件, 微软, and Mozilla enable browser-based playback. 对于大多数视频服务, 那是不可能的, particularly if browser-based playback is coming, perhaps as soon as the fourth quarter of 2017. Note that Bitmovin does offer an Android SDK with AV1 decode, so if Android playback provides sufficient economic justification to start deploying AV1, 你可以在年底之前开始工作.

鉴于这些现实, let's consider Bitmovin's claim that its cloud and managed platform now supports AV1 encode. 虽然毫无疑问是真的, 短期内播放选项的缺乏可能只与一小部分百家乐软件app最新版下载有关.

Beyond describing the detailed inner workings of their NAB presentation, Bitmovin的博客文章还提供了有关AV1开发过程的有趣细节, 以及AV1质量与HEVC的比较, VP, 和H.264. 让我们快速看一下两者.

AV1发展

这篇博文解释了这一点, AV1编解码器源于谷歌的VP9/VP10编解码器的代码库，另外还有77种实验性编码工具已经添加并正在考虑中. 在这77种实验编码工具中, only 8 are currently enabled by default (adapt_scan, ref_mv, filter_7bit, reference_buffer, delte_q, tile_groups, rect_tx, cdef), but the performance of the codec is already appealing."

换句话说, AV1碱基主要是VP10, 通过添加不同的实验性编码技术来提高质量和/或性能. 在77个可用的编码中，Bitmovin只启用了目前默认启用的8个编码. While this gives us a preliminary look at performance, 在比特流被冻结并提供编码建议之前，我们无法评估最终性能. 同样的道理, 以前基于旧代码和启用实验的AV1的任何外观几乎肯定不能代表AV1的最终质量.

博客文章继续描述了“如何将实验添加到AV1编解码器的高级过程:”

Coding tools are added as experiments into the AV1 codebase. 的y are controlled at build-time by flags (e.g., –enable-experimental –enable-).
硬件团队(AOMedia内部的一组硬件成员)审查实验，以确保它可以在硬件中实现.
Each experiment needs to pass an IP review to ensure no IPs are violated.
Once reviews are passed the experiment can be enabled by default."

如上所述, the hardware members in the group were apprised of codec developments in real time, so they won't be starting from scratch when the bitstream is frozen, and all experiments should be hardware-friendly. This will accelerate the availability of hardware-based encoding/decoding.

第二个, the group is clearly hoping to ensure that they don't step on anyone's IP, so that free and open source will remain free and open source. 没有人能以任何程度的保证预测AV1是否会因侵权而被起诉, but these types of procedures are assuring.

AV1质量

的博客 then describes comparisons between H.264, HEVC, VP9, and AV1, though it's difficult to generalize their results. 例如, one set of comparisons involved PSNR and SSIM analysis; but these were performed on the animated movie Sintel (图2). Since codecs often perform very differently on animated and real-world footage, 从这些结果中无法预测AV1与真实世界视频的比较性能.

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图2. AV1 proved best in PSNR comparisons on the Sintel video.

的 blog also includes comparison frames from 钢铁之泪 (图3)，但测试案例是如此有限(1080p@500Kbps@24 fps)，很难得出任何广泛的结论.

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图3. Ditto here in this limited comparison with HEVC.

我向穆勒提出了这些担忧, 他承诺Bitmovin将“扩展我们的实验，并在NAB之后发布更多包含运动的典型测试集的结果”, 动画和电影." While I expect AV1 to perform well in these comparisons, we won't know until the results are released.