28 Apr 2015

Will Ethernet replace SDI?

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SDI, Ethernet and Video over IP SDI, Ethernet and Video over IP RedShark News

Index

Ethernet is more flexible than SDI. What's to stop it taking over completely in TV studios?

One of the defining technological characteristics of the late twentieth century was convergence, the transition of a wide range of techniques, previously used on a wide range of hardware, toward a single hardware platform. The 1980s and 1990s, for instance, saw this happen to audio editing, which mainly used quarter-inch tape decks, as well as printing and publishing (which used photographic techniques), engineering and technical drawing, banking and many other fields, all of which, of course, ended up being done on computers. We sometimes flatter ourselves that video production and editing, particularly at the levels required for feature film production, was a comparatively late adopter of the transition to desktop workstations, possibly because the data sets are very hard work. However, generally speaking, none of this is new.

What is fairly new is the ability of computer networks to handle high-bandwidth video. It's necessary at this point to define what high-bandwidth really means. The phrase 'video over IP' has been used for a while, in the context of a heavily compressed adjunct to voice chat. Companies like Evertz and Elemental, who are big players in the emerging field of Ethernet as a full-quality studio interconnect, seem to prefer terms like 'software defined video network'. Either way, the approach is the same: to take video frames from (usually) an SDI source and convert them for transport over a high speed (typically ten-gigabit) Ethernet network.

SDI and Ethernet examined

In one sense, this is relatively trivial; all that's happening is the repackaging of a SDI payload into a series of Ethernet frames. It's a stream of digital information in one format being converted into another format. The detail, of course, is somewhat more complex. SDI is a relatively straightforward point-to-point interconnect with no routing capability that accepts data packaged in a fairly narrowly-defined set of formats. Ethernet (or at least Ethernet plus the Internet Protocol standard that is used to route data worldwide) is a general-purpose computer networking protocol with advanced routing capability which can accept anything that can be represented as digital information.

This flexibility is part of the reason that software-defined video networking over Ethernet and IP is attractive, particularly in the context of large studio installations. SDI video routers capable of connecting a variety of inputs and outputs together under software control do exist, but the level of flexibility and capability is nothing like that provided by IP networks. A single ten-gigabit Ethernet link can carry several streams of 1.5 gigabit SDI video (or even a combination of HD and SD formats at various frame rates) in a way that SDI simply doesn't know how to achieve. From a cost benefit perspective, installation of Ethernet hardware (cabling, routers, etc) might be viewed as a more straightforward piece of information technology infrastructure than the task-specific technologies it replaces. What's more, IP networking provides a back-channel; video can flow from camera to studio, while talk-back, cueing information, device control and return video can go back the other way.

Needless to say, the situation is not entirely straightforward. Currently, the very high performance Ethernet hardware required to do this well is expensive, sometimes about as expensive as doing the same job in SDI, under some circumstances. The hardware needs to be a lot more intelligent than a simple SDI router, given the complexities of a fully-switchable routed network. The upsides of it, however, can be considerable. It might be possible, for instance, to connect any production control room to any of several studios using a combination of SDI routing and other technologies. But it's a lot easier to do with Ethernet, so that a control room doesn't have to sit idle while, say, a set is being built in its studio.



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Phil Rhodes

Phil Rhodes is a Cinematographer, Technologist, Writer and above all Communicator. Never afraid to speak his mind, and always worth listening to, he's a frequent contributor to RedShark.

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