02.16.2005 03:50 PM
BBC OB Unit 12 wired with eye towards HD teleproduction

Wiring Unit 12 from the start to accommodate the bandwidth requirements of HD was a critical design imperative.
Unit 12, the latest outside broadcast vehicle in the BBC fleet has been designed and wired to meet the unique demands of HD teleproduction when the U.K. public broadcaster converts the truck to serve growing HD field production requirements in the future.

The OB vehicle, commissioned in 2004 following the successful completion of Units 10 and 11, was integrated by UK-based independent systems integrator Megahertz Broadcast Systems.

The new vehicle will provide linking and studio facilities for sporting events around the world. It will be fully upgraded to HD as programming demand increases. Following a similar design to that of Units 10 and 11, both of which were integrated by Megahertz, Unit 12 is based around a large, double-expanding trailer that was custom coach built by A Smith’s of Great Bentley.

The vehicle is equipped with Sony cameras, a Sony SD DVS-9000 production switcher and wiring for an additional Sony MVS-8000 HD switcher. A significant part of the audio system will be AES using a Calrec Sigma and Pro-Bel freeway routing matrix connected via a MADI interface.

As with the BBC’s Units 10 and 11, Unit 12 will have the capacity for up to eight VTRs, eight EVS LSMs and 20 cameras. It will also incorporate two 128-square SD SDI routing matrices with capacity for another 64 square HD to be installed later.

To make the vehicle HD-ready, great care had to be taken with the cabling, which needed to be of the highest quality. According to Megahertz Technical Director Steve Burgess, equipping a truck for high-definition broadcasting meant specifying cables, connectors and patchbay panels that are all capable of handling a higher bandwidth signal.

From a systems integration point of view, this is more complex than a traditional installation infrastructure, which would be designed to cope with a maximum data rate of 270Mb/s for standard definition digital video signals. In the case of HDTV, the installation must accommodate 1.5Gb/s with a third harmonic at 2.25GHz, he said.

According to SMPTE 292E, the required bandwidth for the interconnections is 2.4GHz. Furthermore, insertion loss at HDTV frequencies is significantly greater and care has to be taken to keep this below 20dB. “Normally we would aim for headroom of 6dB allowing a loss of only 14dB,” he said. In an analog environment, character impedance of the interconnections was virtually ignored, he explained. Integrators often mixed 75ohm and 50ohm components. With the advent of SDV at 270Mb/s, care has to be taken to ensure all components are truly 75ohm, he said.

For more information, visit www.megahertz.co.uk.

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