Back to basics: MPEG Dash


David Springall TV-Bay Magazine
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by David Springall, CTO & Founder, Yospace

Just when you thought that you had got your head around the different flavours of MPEG, another one comes along. What on earth is MPEG-DASH, and why do we need to care about it?

The DASH part stands for dynamic adaptive streaming over HTTP. MPEG-DASH is an attempt to create a standardised approach to delivering content to devices – not conventional television sets – over some form of the internet, which might include mobile connectivity.

Let’s forget the MPEG standard for a moment, and think about the problem. As we are all too aware, in some places the internet goes faster than others. Content owners offering their product online want to deliver a reasonably consistent quality of experience. So the search was out for a way of reconciling these two conflicting requirements.

The solution lies in adaptive bitrate streaming. In essence, the receiving device works out how much bandwidth is available to it and calls for content that will sit within that capacity. The sender – usually a content delivery network (CDN) on behalf of the service provider – actually stores multiple resolutions of the file, and so can send as much detail as the receiving device, and the data path between them, can accept.

The streaming bitrate is adapted dynamically to match the combination of circumstances at each moment. In the metadata alongside the content as it is delivered is a media presentation description file (MPD), which you may hear referred to as a “manifest”.

The reasoning behind this is clearly a good idea, and the technology exists, so people set about developing practical implementations. In 2009, when the iPhone 3GS was launched, its operating system included a new streaming protocol called HTTP Live Streaming (HLS). Microsoft shortly afterwards introduced its equivalent, SmoothStreaming. Adobe has its own format, too: HDS or HTTP dynamic streaming, used for Flash.

While these are proprietary standards they are widely adopted. Indeed, HLS has become something of a de facto standard, being used for Android devices as well as iOS, and also serving Blackberry 10, Roku, Boxee, Samsung connected televisions, and even on Xbox (which not many people are aware of). Perhaps more predictably, Microsoft does not support it natively in Windows 8, nor is it supported in Flash, although we have SDK solutions to handle HLS on both of these platforms (as do other specialists in the field).

Which brings us back to MPEG-DASH and the attempts to standardise online delivery. It is concerned with the MPD – the manifest – and does not address codecs nor much of the client/server interaction. These details are defined by profiles within MPEG-DASH, which enable you to create the complete end-to-end specification. The standard allows you to develop as many profiles as you need.

So it would be perfectly practical to develop an MPEG-DASH SmoothStreaming profile, and this is happening in something called DASH264. Adobe is behind this profile too, so it will subsume HDS. And it would be equally practical to develop an MPEG-DASH HLS profile, which again is the basis of another initiative, the MPEG-TS profile (apologies for all the acronyms).

But the important point is that the DASH264 profile and MPEG-TS profile are as incompatible with each other as Microsoft SmoothStreaming and Apple HLS are. Will either or both of those organisations give way and change years of research for an MPEG standard? I think we can guess the answer to that.

Why does it matter that there are multiple incompatible streaming systems? Each takes hardware to prepare, so content owners have to invest to support additional formats. And the processed files have to be stored, both at the content owner’s facility and at the edge servers of the CDN. Dealing with a single format represents a significant cost saving in bandwidth and server space.

But for now, at least, I cannot see that MPEG-DASH will deliver that single solution even if the “freight train” of continued device support for HLS support is derailed. Instead, we may migrate from three competing and incompatible protocols to two or three competing and incompatible profiles within a rather broad standard. You may, therefore, feel it unsafe to rely on the new MPEG initiative to bring about the concept of a unified streaming solution in the near future.


Tags: iss076 | Yospace | mpeg-dash | streaming | http | mpeg | cdn | bitrate | David Springall
Contributing Author David Springall

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