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TECHNOLOGY Take it to the bridge If the broadcast industry is to fully embrace IT networking for live production it needs a highly reliable way of delivering low latency, synchronized audio and video. AVB is one such option. by Will Strauss S ince the late 1980s, studios, facilities, OB trucks and many other professional broadcast set-ups have relied on digital video standards such as SDI (Serial Digital Interface) to move audio and video around. Using SDI it has been possible to happily construct a reliable, error-free, plug-and-play, real-time operation capable of successfully getting a show to air. But while it still works well, it is not perfect. SDI only allows for one- way, single-purpose and point-to- point infrastructures that are usually expensive, require highly skilled engineers and have limitations when it comes to higher resolutions, frame rates and other advances. As a result, there are moves afoot that might eventually see SDI and other professional studio interfaces, signals and equipment replaced by generic IT technology and packetized networks using Ethernet. Sounds great so far. The broadcast facility of the future will essentially be a data centre, with all the efficiencies that this brings. But while sending full, finished TV programmes via Ethernet is child’s play, streaming synchronous uncompressed HD video as a live source is much trickier. You need 46 | TV-BAY MAGAZINE: ISSUE 91 JULY 2014 enough bandwidth and not too much latency, yes, but also a range of devices that obey a standard network protocol so that everything goes smoothly. What are the benefits? Audio Video Bridging (AVB) is potentially the answer. A ‘trend’ for many years (talked about at least as far back as 2009) it has become fairly commonplace in audio, but only now is the technology being rolled out with video in mind. Using AVB has various benefits. For example, audio and video streams can be synchronized within a microsecond of each other, with low delay, and with minimal data loss. It also allows for: simplified remote production; fully bi-directional/duplex connections; multiple different signals to be multiplexed and transported on one cable; reduced cabling requirements. What is it? AVB is a manufacturer agnostic open standard based on the WAN and LAN standard IEEE 802.1 (and its sister standards). It is essentially a networking language for the connection of real-time media devices that is designed for bridging, carrying, routing, synchronizing and switching audio and video signals over standard Ethernet connections. Essentially, AVB makes a standard Ethernet network suitable for live television production. The basis of AVB is a compliant switch – aka the bridge – that connects different types of equipment. There is a whole industry association, the AVnu Alliance, dedicated to the development of AVB. Its job is to test and certify devices. Axon Digital Design, Calrec, Dolby, Harman, Avid, Barco, Riedel, Shure and Sennheiser are amongst the companies involved in this. This ability to have one connection that can carry multiple streams and work as both an input and output is what you might call “game changing” (if you worked in marketing). It’s not faultless though. AVB is what is referred to as a Layer-2 protocol. This makes it ideal for extremely low latency and tight synchronization but does mean its use is currently restricted to within a Local Area Network eg a facility or OB truck. If you want to send broadcast signals down an Ethernet cable from, as an example, a sporting or music venue to a facility then a layer-3 protocol is required. An example is the BBC R&D developed Stagebox (but that is a subject for another article). While AVB does in theory have a Layer 3 derivative its availability is considered to be some distance away.