It’s too early to say whether DASH will be a marketplace success.
Video industry groups around the world are trying to get their hands around DASH—Dynamic Adaptive Streaming over HTTP—to understand what it is, and decide whether to use it. So here’s a short tutorial.
The problem that DASH addresses is the varying channel quality encountered by streaming video programming delivered over the Internet. Sometimes there is congestion, sometimes not.
But with video, you want a continuous stream of video, you don’t want stuttering. So the DASH solution is to break up the program into short segments, and send each segment at a data rate adequate to deliver a continuous, non-stuttering stream. If that means delivering some segments at less than high definition quality, so be it.
DASH was invented by the MPEG committee (formally, the Moving Pictures Experts Group), which is a committee administered jointly by the International Standards Organization (ISO) and the International Electrotechnical Commission (IEC). Be careful when you see the term “MPEG.” It might refer to the MPEG committee, it might refer to a video compression scheme (MPEG-2), it might refer to a protocol for delivering video metadata (MPEG Transport), and now it might refer to MPEG-DASH.
A DASH server might, for example, store three versions of a movie: a 5 Mbps version, a 2 Mbps version and a 0.5 Mbps version. Each might be broken into segments that are, say, 5 seconds long. So every 5 seconds the client, which is in your receiving device, would assess the quality of the transmission path and tell the server which version to send. The switching between versions is done seamlessly. At any time, you might be getting the 5 Mbps version while across town the network is congested and viewers are watching the 0.5 Mbps version. Or maybe your neighbor’s Wi-Fi is receiving interference from another neighbor’s Wi-Fi, and she is watching the 0.5 Mbps version.
The concept behind DASH is not original. Apple’s HTTP Live Streaming, Microsoft’s Smooth Streaming and Adobe’s HTTP dynamic streaming all came earlier. But they are all proprietary methods, and different receiving devices may support any of the three protocols.
Program suppliers may need to support all three. The MPEG committee aims to make DASH the single publicly-documented world standard.
One important point: DASH is not for broadcast delivery, it’s for point-to-point delivery. A cable operator would not use DASH for linear programming carried on a dedicated frequency band on the cable, since that transmission path has a constant quality level. Rather, a cable operator (or a broadcaster or indeed any other program supplier) would use DASH for over the top (OTT) delivery via the Internet, which is known in the cable industry as TV Everywhere.
Cable operators already use a cruder form of program segmentation, in order to insert commercials commercials using SCTE standard messages and protocols. So it was important to convince the MPEG committee to make DASH segments compatible with ad insertion segments.
The DASH specifications are contained in a family of international standards known as ISO/IEC 23009. Part 1, the core specification, was published in 2012, but a second edition is expected later this year or early next. Lke other standards developed by the MPEG committee, the DASH specifications offers a toolkit of functions and capabilities, and some further guidelines are needed to assure interoperability. And so, companies formed a DASH Industry Forum to promote interoperability and to promote market adoption.
DASH-IF will sponsor demonstrations and interoperability tests, develop conformance specifications, manage the use of a logo and brand name, and generally promote the use of DASH as the worldwide standard for streaming video over transmission channels of varying quality.
At this point, DASH-IF has published an “Interoperability Points” document that specifies the use of certain profiles of H.264/AVC for video coding and DTS, E-AC-3 and HE-AACv2 for audio coding. Work is underway on specifying use of HEVC video coding.
The DASH-IF web site lists nearly 60 members. Microsoft and Adobe are members, which I guess means that they are willing to abandon their proprietary schemes. But I did not see Apple listed as a member, nor any cable MSO or U.S. broadcaster other than Cox.
For now, it’s too early to say whether DASH will be a marketplace success. In order for that to happen, most importantly Apple would need to support it in iOS devices like the iPad. That hasn’t occurred yet.
So maybe DASH is the next big thing.
Surely the cable industry’s TV Everywhere needs a big thing to create some momentum, since it seems to be floundering. But for DASH, the jury is still out.