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MPEG DASH almost ready to roll

Broad adoption of the MPEG DASH adaptive streaming protocols is likely following publication of the first implementation guidelines for public review before final release on March 31. This move by the DASH Industry Forum will take DASH beyond the realm of early adopters into mainstream OTT and hybrid services. The only major significant impediment remains Apple’s failure to endorse the standard, although DASH now has strong backing from other big hitters, including notably Microsoft and Adobe, architects of the other two main existing streaming standards — Smooth Streaming and HTTP Dynamic Streaming.

The standard, now called DASH-264, has tied up loose ends that threatened to produce variants that would be interoperable by catering for too many options. It is now more specific about audio and video codecs, file types and the precise syntax for transmitting over HTTP, and can be adopted with much higher confidence that it will be interoperable.

The guidelines, published as Version 0.9, specify support for both live and on-demand services, and crucially give detail of how different encoding profiles are linked to interoperability points needed for conformance testing. For example, DASH-264 supports presentation of up to 720p HD based on H.264/AVC Progressive High Profile, but also recognizes that some clients may only be capable of the lower H.264/AVC 11 Main Profile. Therefore, as specified in the guidelines, content authors can indicate a subset of DASH264 12 by providing a profile identifier referring to a standard definition presentation. This is an interoperability point now clearly defined.

The other main area of clarification concerns end to end content security around the DRM and encryption. DASH unifies encryption around the Common Encryption (‘cenc’) protection scheme, to enable interoperability and avoid having to support multiple encryption methods, which would increase device processing costs. But all along DASH has avoided specifying an end to end end-to-end DRM so that vendors and service providers can choose their own. So DASH 264 provides a framework for multiple DRMs to protect a content file by adding instructions or relevant proprietary information in predetermined locations to a file that is encrypted with Common Encryption. This then specifies encryption parameters that can be applied by a scrambling system and key mapping methods based on a common key identification scheme.

The point of this single key identification scheme is that it enables the same encrypted version of a file to be used by different DRMs. The new guidelines reduce the encryption parameters and associated metadata to specific use cases for VOD and live content, simplifying the overall system and again improving prospects for interoperability.