2019年7月29日
By 蒂埃里Fautier 谐波公司视频策略副总裁
专题文章

3提高OTT质量质量的创新技术

OTT视频火爆. A recent report from Parks Associates predicted that video streaming services will accelerate globally over the next 5 years, with more than 310 million connected households having at least one OTT service by 2024—equating to some 586 million subscriptions overall.

然而,, a major challenge OTT service providers face is that consumers expect the OTT experience to be the same as broadcast TV. 消费者对视频直播的热情高涨, one would think the quality of experience (QoE) would be exceptional—at least on the same level as broadcast. 但情况并非总是如此. 可伸缩性问题有时会导致最终用户体验到较差的质量同时观看OTT直播服务. This article will explore the challenges and three future-forward technologies that are helping operators deliver the best quality of experience (QoE) to subscribers on every screen.

OTT QoE前景及相关挑战

纯OTT服务通过与互联网流量共享的非托管网络提供, 哪些会导致QoE问题. 当重大体育赛事举行时, 全球数十亿观众正在一系列相互连接的屏幕上观看比赛, 包括电视, 个人电脑, 智能手机, 和平板电脑. 提供高知名度的体育赛事是一项挑战，因为观众希望得到一致的结果, 所有屏幕的高质量体验, 具有与广播相当的延迟. 在交付工作流程中有多个点可以为实时OTT服务创建QoE问题.

图1提供了视频交付链的一般视图, 头端包括压缩和源服务器, 然后是CDN, 接入网, 显示链中可能产生可扩展性问题的所有点.

在信号传递方面, several issues can occur when a high number of subscribers want to suddenly view an event at the same time. 可伸缩性问题可能发生在原始服务器上, CDN, 访问网络, 在送回家的最后阶段. 在原始服务器级别, 问题的发生是因为服务器被设计为支持增值功能，如重新开始电视, 追赶电视和有针对性的广告插入, 当同时有大量用户观看视频时, 它在原始服务器上创建一个工作负载, 最终产生HTTP 404错误.

Scalability issues generally occur at the CDN level when video service providers haven’t properly anticipated how big an event will be and the maximum network load is reached. 在交货的最后一英里(1).e. 在接入网中，会出现影响QoE的各种情况. 通常，这些问题取决于网络的类型(如.e.(固定线路，DSL或光纤或移动)及其相关技术. 在送货到家的最后阶段(1).e., 家里的门), contention can happen when too many devices connect to the same access point or when the connectivity to the client is bad, 从而降低比特率并影响QoE.

提供OTT服务的另一个挑战是延迟. 延迟, 或者存在于现场生产和最终用户显示之间的延迟, 是一个在体育赛事直播中非常明显的常见OTT问题吗, 特别是当广播或社交网络引用在几秒钟内发生时.

早期的流媒体格式, 主要为SVOD开发, 是为了避免在播放器上渲染视频时重新缓冲而设计的吗. 但要让它在任何设备上都能运行, 内存缓冲区必须在工作流中使用, 尤其是在玩家中, 引入端到端延迟. 苹果最初的HTTP直播(HLS)协议, 2009年发布, recommended using 10-second segments and specified that players should not buffer less than three segments. 这就解释了为什么许多OTT服务显示典型的40秒甚至更长时间的延迟. 苹果后来将推荐时间修改为6秒, 这仍然相当于客户端延迟了18秒. 对于SVOD服务来说，20秒或更长时间的延迟不是问题, 但这是体育直播的问题. 在实时事件期间，需要约5秒的延迟, 或者延迟会变得非常明显，让观众感到沮丧.

Video compression technology can help to improve the QoE by reducing the amount of data that is moved from the 起源 server to the end-user, 但这是有代价的. 压缩比特率越低, 就越容易将单播传输到具有卓越QoE的连接设备. 但是如果比特率太低，质量就会受到影响，QoE也会降低. OTT服务提供商可以通过使用最先进的编解码器(i.e.但目前的许可条款阻碍了该技术的部署. 因此，该行业需要找到更智能的方式来分发内容.

Let’s have a look on three future-forward technologies that are helping operators deliver the best QoE to subscribers on every screen: content-aware encoding, CMAF用于低延迟, 边缘缩放.

Content-Aware编码

Content-aware encoding (CAE) is an innovative technology that is currently used by 网飞公司 (named per-tile or per-segment encoding) and other leading OTT service providers around the world to address HEVC licensing issues and deliver the best picture quality using AVC codec. 与CAE, 运营商可以让编码器根据视频复杂度实时调整编码参数. 逐块编码技术的工作原理类似于statmux的VBR, except only one program is encoded and the video quality measurement is more refined since it is based on the Human Visual System.

最大限度地提高视频质量测量的准确性, CAE是使用人工智能进行离线训练的. 对于VOD, CAE可以一次使用，就像直播一样. 这提供了最高的可伸缩性(i.e.(编码速度)，但不是最低压缩比. 另外, 这可以通过几个步骤来完成, where each encoding parameter set is encoded in parallel and the decision is made at the end of each encoding batch.

表1 demonstrates the consistent savings that can be achieved with CAE compared with CBR for HD content.

表1. 在CAE和VBR之间节省比特率

除了提供大量带宽节省之外，CAE还增强了QoE. 由于视频以更先进的方式压缩，可以接收额外的高清配置文件. 当比特率较低时，缓冲效果较少.

CMAF

The MPEG Common Media Application Format (MPEG-CMAF) was primarily developed to resolve interoperability issues within the streaming industry, 因为它减少了所需的不同媒体文件格式的数量. MPEG-CMAF是基于fMP4 (ISOBMFF)的媒体容器标准。. Since it can be used by both MPEG-DASH and HLS delivery formats with a common encryption scheme CENC (Common Encyption), MPEG-CMAF极大地简化了OTT的大规模分布.

CMAF工具箱还提供了一些有趣的特性, 包括低延迟块(LLC)选项. 这个工具包含在最初的MPEG-CMAF规范中，以支持以小块(例如.g., 200 ms). 这意味着解码过程可以在一个完整的片段被编码之前开始, 打包, 和接收. 支持高性能和3秒或更短的端到端延迟, MPEG-CMAF与LLC是OTT服务提供商祈祷的答案, 让他们与广播公司在提供体育直播方面处于公平竞争的地位.

为了充分发挥CMAF有限责任公司的优势, OTT服务提供商需要在交付工作流程的所有步骤中支持这项技术, 包括包装机, 起源, CDN, 和球员. CMAF LLC拥有整个行业的支持，包括CDN和播放器供应商. 然而, it is worth noting that players that support CMAF but not LLC would still be able to decode the video after reception of the full media segment. 在这种情况下，延迟将增加几秒钟. 我们在各种玩家和网络条件下测量了DASH CMAF LLC, 我们有信心，一旦生产服务部署完毕，大约5秒的广播延迟将会实现.

边缘扩展

随着OTT服务提供商寻找提高QoE的其他方法, 在网络边缘扩展已经成为另一种途径. The concept of scaling at the edge is simple—service providers deploy edge nodes inside the ISP 访问网络. 对于实时事件，主要是体育赛事，接近订阅者的规模将提供最大的QoE. 然而, the downside is that if the service provider dimensions and builds for these isolated peaks in traffic then the deployed nodes will be underused much of the time.

向这些节点添加存储将启用非线性(i.e.(VOD，追赶电视)服务更接近终端用户. 网飞公司, 例如, provides a large library of non-linear content that is positioned within the ISP networks via its OpenConnect program, 为网飞公司订阅者提供出色的QoE. 虽然这将减少网络影响并改善QoE, 这些服务通常不像现场活动那样对时间敏感, 社交媒体的影响会因为泄露最终分数而降低观看体验吗.

展望未来:下一步是什么?

考虑到今天消费者在视频流媒体服务方面的商业产品数量, OTT service providers don’t have a choice—the QoE they deliver must be exceptional on all devices and should be as consistent as possible over time. 通过部署最新的技术创新, 包括内容感知编码, MPEG-CMAF有限责任公司, 边缘缓存, 服务提供商可以通过低延迟匹配广播提供优质的QoE.

CAE, 特别是, has shown to deliver up to a 50% reduction in bandwidth for OTT live services compared with more traditional encoding. 因为它是100% H.264兼容，OTT服务提供商不需要对今天的生态系统做任何改变就可以使用它. 无论是在设备、虚拟机、云基础设施还是SaaS上，这项技术都是有效的. 随着OTT视频消费的持续增长，CAE技术的发展势头有望形成.

因为CMAF LLC是标准化的，并且足够成熟，可以部署, 对整个工作流程进行微调是下一个合乎逻辑的步骤. 尽管每个组件都可以单独进行优化, optimizing the end-to-end system to deliver the best possible QoE will give additional momentum to the ecosystem.

在CDN方面，最广泛看到的方法是“混合CDN”.” This is where deployed edge nodes for live scaling or caching of non-linear content are combined with a public CDN for off-net reach. The benefits of such an approach include that the edge nodes don’t have to be dimensioned to meet the full demand of the service but will allow for better use of the deployed edge nodes for the average traffic pattern. 除了, 这种方法允许预先确定的“突发”使用水平, 公共CDN处理剩余的流量.

对于特定的现场活动, 可以对流量进行优先排序，以使用已部署的边缘节点, 公共CDN将处理其他服务. The addition of a second or even third CDN provider to this hybrid model will enable a true multi-CDN system that makes optimal use of deployed infrastructure and the geographical coverage that the CDN providers offer. In such a scenario the traffic control system must be able to determine the optimal delivery path as well as load balance incoming traffic requests while still maintaining the integrity of the system and supporting the surge of concurrent user requests as a premium event begins.

[编者注:这是一篇来自谐波. 流媒体 接受供应商署名，完全基于他们对我们读者的价值.]