2015年MPEG-DASH现状

“DRM互操作性是视频流生态系统中必须具备的，因为视频流生态系统中有如此广泛的设备, 有些设备只使用特定的DRM产品, 导致了一个动态但分散的多数字版权管理市场,Laurent Piron说, 解决方案架构师 Nagra. 将互操作性扩展到前端是内容保护信息交换格式(CPIXF)的主要目标。. It allows DRM license service providers to perform easier pre-integration with other components of the streaming backend such as encoders and CDNs, 因此，在提高上市时间和灵活性的同时，显著节省了成本.”

谈到围绕这个倡议的DRM生态系统的融合, Piron补充道:“在一个拥有众多DRM和编码器供应商的市场中，他们都致力于发展流媒体市场, DASH IF中的cppixf工作组, 在NAGRA的领导下, 现在已经得到了互联网和电视领域所有主要参与者的参与, except Apple, 哪个还没有加入DASH IF.

“It shows that the industry is committed to simplify product integration and create a dynamic open streaming ecosystem. It the end, 这对各方来说都是一个双赢的提议, 包括在所有设备上获得最佳体验和内容的消费者.”

With CPIXF, the working group has actually crafted a new XML-based data exchange format that all entities involved in the protected content preparation workflow (CMS, 编码器/代码转换器, 包装机/加密机, DRM client, 和DRM服务)可以用来安全有效地交换内容密钥和DRM信号. 该规范涵盖了现场直播, on-demand, catch-up, 以及电子销售用例, 以及DRM系统和加密器之间的安全推拉数据交换模式. 在简化内容准备工作流程方面，这是一个巨大的进步, 而且在2015年实现的可能性很大, 因为对多重drm的需求正在飙升. After that, 我们只需要看到浏览器提供程序解决相同问题的另一部分, 需要授权各种内容解密模块安装在各种浏览器上, as the perspective of using one DRM per ecosystem is not very attractive when MPEG-DASH is attempting to simplify workflows.

几个组织，包括 Microsoft, Intertrust, ETSI, and Fraunhofer, 是否对CDM接口API提出了需要标准化的建议, 但讨论仍然需要在产生DASH的同一行业融合中进行.

“CDM和cpfx都在解决同样的问题，但在交付链的完全相反的一端,” Piron says. “Browser vendors need to support multiple DRMs to maximize their market reach and thus will also experience similar business benefits as the backend vendors have found out. So it’s expected that the standardization occurring on the backend side of the chain will provide momentum to accelerate standardization on the client side. 这就是MPEG-DASH, 作为一个开放的行业标准, 为整个行业提供了一个强大的工具来推动如此重要的互操作性工作.”

The two other main evolutions to monitor at DASH IF are the introduction of the EBU TT-D subtitling format as the reference format for the future. 目前，由于不同的实现配置文件，定时文本空间被削弱了, 要涵盖所有的工作流程几乎是不可能的, 从内容制作到多屏发行, 具有独特的格式. EBU is currently completing the TT-D specifications set to cover all use cases and to provide mappings from legacy formats to TT-D. 但是像DVB这样的组织已经在他们的规范中引入了新的格式, DASH IF在这个问题上与DVB保持一致是有意义的. ”Timed-Text, W3C目前正在对包括EBU TT-D和其他变体在内的TTML进行分析,Sodagar说. DASH可以提供任何这些html和其他封闭标题格式. But convergence and easy translation from one closed caption format to another would help achieving higher interoperability and greater reach faster.”

这与超高清的情况大致相同. DASH超高清配置文件需要使其规格与广播方法保持一致, in order to allow economies of scale throughout the industry and to see UltraHD contents easily shift from one screen type to another. “On UHD, several organizations are working to define their UHD format and DASH-IF is collaborating with them on DASH UHD profiles,Sodagar说. “The hope is that industry converges on a few UHD formats and that consequently we define DASH UHD Interoperability Points for those formats.”

DASH v3核心实验塑造未来

为了给DASH带来更多的智能和性能, MPEG已经推出了几个名为“核心实验”的探索性轨道.“这些旨在验证提案并奠定DASH v3规范的基础.

Acknowledging that there are dozens of use cases where it doesn’t make sense to let the player alone make the decisions on the quality variant it is requesting, MPEG推出了服务器和网络辅助DASH (SAND)计划. SAND引入了DASH感知网络元素(DANE)的概念。, 哪一个能够理解DASH内容正在传递, 以及在DASH客户端之间进行一系列新的信息交换, 媒体来源, the DANE, 以及第三方DASH分发监控服务器. 这些参数包括通过交付网络元素传递的SAND参数，以协助DASH客户端操作, DASH metrics collected by the client on the DASH service and Parameters for Enhancing delivery by DANE provided by 媒体来源. The bidirectional communication channel of WebSocket is tipped as an efficient real-time transport vector for this information. This instrumented approach is supposed to prevent clients from cannibalizing the bandwidth and triggering an infinite cycle in bitrate switches, 同时允许网络运营商更好地控制通过其网络传输的视频.

But the mission of MPEG does not inevitably cover the definition of all standards needed to get SAND implemented across all the chain. “The goal of SAND is to define parameters (basically and most importantly the semantics) that help improving DASH-based applications and services along the delivery/distribution path/ network,” says Dr. Christian Timmerer，首席信息官 Bitmovin. “类似于DASH指标(在bitdash中实现), it’s important to define the semantics of the parameter which shall be interpreted as requirements from a DASH perspective that can help standardizing whatever is needed within the delivery/distribution path/network whoever is working on standards in that domain (e.g. Ietf, w3c, 3gpp...). Thus, I don’t think we should provide too much in terms of specifications as the main purpose is to inform others what we think is needed within their domain to help DASH to evolve.”

Another study track has been launched to see how DASH would have to be adapted to push-based transport protocols HTTP/2 and WebSocket. 它被称为基于全双工http协议(FDH)的DASH. 它旨在为大规模和低延迟流提供新的选择, based on the ability of the DASH delivery node to push the new segments to DASH players as soon as they propagate through the network, 这样就避免了客户端驱动的请求所产生的等待时间. 新的机制将与HTTP/1互操作.1、使用现有的DASH内容，支持cdn缓存. 如果所有这些目标都达到了, the FDH initiative certainly conveys the hope for OTT video delivery to get closer to the scalability and the low latency of the broadcast distribution, 这仍然是一个挑战.

还有其他正在进行的实验. The goal of the URI Signing for DASH (CE-USD) is to create a solution to prevent a client from skipping pre-roll and mid-roll advertisements, 根据令牌授予对内容的临时访问权限, 或者防止深度链接. 为了这些目的，正在审查IETF用于CDN互连的URI签名机制. The SAP-Independent Segment Signaling (SISSI) experiment tackles problems including low-latency live streaming and the Fast Tune In use case, 客户端访问SAP频率高的表示, then switches to a representation with less IDR frames when the playback session is initiated (in both unicast and hybrid cases).

同时认识到SISSI的利益, Timmerer还将问题置于上下文中:“低延迟是实时场景的一个问题, but it’s about solving a problem which starts at wrong assumptions based on OTT delivery chain being attached to the content production chain too late,” he says. “通常，输入到OTT传输链的内容与消费者在电视上看到的内容相同, 这已经引入了明显的延迟, 远远超过OTT本身.”

空间关系描述(SRD)实验.com/dashspatial) is designed to explore how we can combine several spatially related videos in a tiled or zoomed combination, 同时提供了对适应集的普通消费的向后兼容性. This feature is unique to DASH (until it’s copied by HLS) and provides an immersive approach to tiled streaming in the upcoming perspective of the facilitated UltraHD video production.

这些新功能即将问世, MPEG-DASH将自己定位在创新的十字路口. 对于许多行业参与者来说，问题不再是确定DASH革命是否已经发生, 而是拥抱其生动的发展节奏，驯服其复杂性，这也是其成功的关键因素. 关于DASH的最后一句话来自Dr. Leonardo Chiariglione，该组织联合创始人兼主席运动影像专家组 (MPEG): “I do believe that the multimedia industry has now the full protocol stack in place for offering services and MPEG is the capability to embed innovation at all layers of the stack.”

[本文发表于2015年。 流媒体行业资料手册 作为…的国家 MPEG-DASH.]

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