2017年4月18日
By 1月时特约编辑
专题文章

Bitmovin推进AV1，加入开放媒体联盟

就在开放媒体联盟 AV1编解码器在2017年底之前都将被搁置， Bitmovin announced that it is adding AV1 to its VoD and live encoding service and demonstrating live AV1 encoding and playback at NAB. 在一个博客关于公告, Bitmovin详细介绍了AV1迄今为止的进展, 并揭示了它需要走多远才能变得可用，以及它可以多快到达那里. 长话短说，就像你后视镜里的物体一样，AV1可能比它看起来更近.

AV1播放器

Bitmovin was founded in Austria in 2013, initially to market a DASH-based HTML5 player, then adding 云编码和分析. 今年早些时候，Bitmovin宣布了一项管理基于Docker和Kubernetes的本地编码服务.

开放媒体联盟(AOM) 2015年推出巩固几个创始人的开源编解码器开发成果, 包括谷歌(VP10), Mozilla (Daala), 思科(雷神), 微软, 和英特尔, 亚马逊和Netflix也加入了创始成员行列. 2016年，AMD, ARM和NVIDIA 加入小组， AOM网站目前显示 23个成员其中不包括Bitmovin，后者于2017年4月加入.

的 AOM的主要焦点是开发“媒体编解码器”, 媒体格式, and related technologies to address marketplace demand for an open standard for video compression and delivery over the web.简而言之，这是一种无需版税的编解码器，而不是需要支付高额版税的HEVC. 的 quality target was "50 percent over VP9/HEVC with reasonable increases in encoding and playback complexity,2016年4月我们写了 that the target date for freezing the bitstream was "sometime between the end of the 2016 and March 2017.从那以后，有消息称，目标日期可能会推迟到2017年底. 在其他细节中, Bitmovin在其博客文章中指出, AV1比特流冻结应该在2017年第三季度."

考虑到AOM的成员资格，事情可以从那里迅速发展. 这是, 谷歌, 微软, 和Mozilla可以快速添加解码到各自的浏览器, 而Netflix, 谷歌和亚马逊可以开始为内容分发进行编码试验. 而对AV1的硬件支持显然需要更长的时间, the hardware partners in the Alliance have worked alongside the codec developers (more below) during the development process, 这应该加速AV1编码/解码在cpu的可用性, gpu, soc, 以及其他硬件.

Bitmovin宣布了什么

4月18日, Bitmovin announced that its 云编码 and managed on-premise offering "now supports AV1 encoding for VoD and Live.该公司还宣布将“展示有史以来第一个AV1直播”, 提供1080p播放在1.5Mbps in broadcast quality at our booth at the NAB Show in Las Vegas (SU9007CM) from April 24-27.“让我们先处理第二个问题，然后再回过头来处理第一个问题.

的 live AV1 workflow Bitmovin will present at NAB starts with a 12Mbps 1080p 30fps stream generated by live encoding program Open Broadcaster Software (OBS). This is sent to the 谷歌 Cloud via RTMP where it's encoded by the Bitmovin cloud encoder to "broadcast quality" 1.5 mbps AV1. 关于质量, 这篇博文说, "you would need around 4 to 15Mbps with traditional codecs like H264 to deliver the same quality." Once encoded, the video will be streamed to a desktop for playback via the AOM player and FFmpeg.

图1 说明对流进行编码的单个笔记本之间的区别, 以及在Bitmovin云中的编码性能. 这是, the 40 second 钢铁之泪 teaser Bitmovin will demonstrate at NAB would take close to nine hours to encode on a notebook powered by 4-core i7-4800 MQ CPU. 相比之下，Bitmovin云可以在34小时内产生相同的输出.5秒.

bitmovinpushes1

图1. Bitmovin云与4核笔记本相比.

的 obvious question is how many cores in the cloud are necessary to achieve this performance. 在NAB，该公司预计它将需要多达200个核心. 然而, Bitmovin CTO Christopher Mueller commented that "it's really early stage and that we mainly want to show the flexibility of our encoding stack...I am very confident that we can bring down the hardware requirements to encode a single 1080p AV1 stream to 8 to 32 cores sooner than later.这是有道理的, 考虑到大多数繁重的优化工作直到位流冻结后才开始.

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, any company attempting to stream AV1 video would have to supply a player or plug-in until 谷歌, 微软, 和Mozilla支持基于浏览器的播放. 对于大多数视频服务, 那是不可能的, 特别是如果基于浏览器的播放即将到来, 也许最快会在2017年第四季度. 请注意，Bitmovin确实提供了带有AV1解码的Android SDK, 因此，如果Android播放提供了足够的经济理由，开始部署AV1, 你可以在年底之前开始工作.

鉴于这些现实, 让我们考虑一下Bitmovin声称其云和托管平台现在支持AV1编码. 虽然毫无疑问是真的, the short-term lack of playback options probably makes this relevant for only a very small group of streaming producers.

除了详细描述NAB演示的内部工作之外, the Bitmovin 博客 also provides interesting details regarding the AV1 development process, 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, "the AV1编解码器 has its roots in the codebase of 谷歌's VP9/VP10 codec with an additional 77 experimental coding tools that have been added and are under consideration. 在这77种实验编码工具中, 目前默认情况下只启用了8个(adapt_scan), ref_mv, filter_7bit, reference_buffer, delte_q, tile_groups, rect_tx, cdef), 但是编解码器的性能已经很吸引人了."

换句话说, AV1碱基主要是VP10, with different experimental encoding techniques being added to improve quality and/or performance. Of the 77 available, Bitmovin enabled only the eight currently enabled by default in the codec. 虽然这让我们初步了解了性能, we won't be able to assess final performance until the bitstream is frozen and encoding recommendations are provided. 同样的道理, any previous looks at AV1 based upon older code and enabled experiments are almost certainly not representative of AV1's final quality.

的博客 goes on to describe "the high-level process on how experiments can be added to the AV1编解码器:"

编码工具作为实验添加到AV1代码库中. 它们在构建时由标志(如.g., –enable-experimental –enable-).
的 hardware team (group of hardware members inside of AOMedia) reviews the experiments to ensure it can be implemented in hardware.
每个实验都需要通过知识产权审查，以确保没有知识产权被侵犯.
一旦审查通过，实验就可以默认启用."

如上所述, 小组中的硬件成员被实时告知编解码器的发展情况, 所以当比特流被冻结时，他们不会从头开始, 所有的实验都应该是硬件友好的. 这将加速基于硬件的编码/解码的可用性.

第二个, 该组织显然希望确保他们不会侵犯任何人的IP, 这样自由和开源就会一直保持自由和开源. No one can predict with any level of assurance whether AV1 will or won't be sued for infringement, 但这些类型的程序是可靠的.

AV1质量

这篇博文随后描述了H.264, HEVC, VP9和AV1，尽管很难概括它们的结果. 例如, one set of comparisons involved PSNR and SSIM analysis; but these were performed on the animated movie Sintel (图2). 因为编解码器通常在动画和现实世界的镜头上表现得非常不同, it's impossible to predict AV1's comparative performance with real world video from these results.

bitmovinpushes2

图2. AV1在Sintel视频的PSNR比较中证明是最好的.

该博客还包括来自钢铁之泪 (图3) but the test case was so limited (1080p@500Kbps@24 fps) that it's again tough to draw any broad-based conclusions.

bitmovinpushes3

图3. 在与HEVC的有限比较中也是如此.

我向穆勒提出了这些担忧, who promised that Bitmovin will "extend our experiments and publish more results after the NAB with a typical test set containing sports, 动画和电影.“虽然我预计AV1在这些比较中表现良好, 结果出来后我们才能知道.

这是什么意思

整体, Bitmovin's announcement is a very impressive technology demonstration by a relatively small company that revives hopes that AV1 will be meaningful in the relative short term. 然而, you should resist the urge to proclaim AV1 the quality king until more comprehensive comparisons are available.