Megahertz Broadcast Systems prepares BBC for a high definition future

UK-based independent systems integrator Megahertz Broadcast Systems has completed the management, wiring and installation of the BBC’s new Unit 12 outside broadcast vehicle – the first new vehicle of the BBC’s extensive OB fleet to be made ready for high definition broadcasting.
The vehicle, which was commissioned in 2004 following the success of its predecessors (Unit 10 and Unit 11), will provide expert linking and studio facilities at sporting events around the world. It will be fully upgraded to HD as programming demand increases and will then sit alongside outside broadcasts existing unit 2.
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 coachbuilt by A Smith’s of Great Bentley. The vehicle is equipped with Sony cameras, a SD DVS-9000 vision mixer and wiring for an additional Sony MVS-8000 HD crate. A significant part of the audio system will be AES using a Calrec Sigma and ProBel 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 when the BBC makes the commitment to broadcast in HD.
In order to make the vehicle HD-ready, great care had to be taken with the cabling, which needed to be of the highest quality throughout. Megahertz’s technical director, Steve Burgess, says that while many broadcasters are not currently broadcasting in HD, having an HD OB truck means that they can produce and record programming at a higher bandwidth and store that so it is there and ready to use when they need it.
He said: “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 tradition installation infrastructure, which would be designed to cope with the maximum data rate of 270Mbps for standard definition digital video signals. In the case of HDTV we are dealing with a data rate of 1.5Gbps, with a third harmonic at 2.25GHz. SMPTE 292E states a required bandwidth for the interconnections of 2.4GHz. Furthermore, insertion loss at HDTV frequencies are 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.”
Burgess added that in the days of analogue, character impedance of the interconnections was virtually ignored, with integrators happily mixing 75 and 50ohm components. With the advent of SDV at 270Mbps, care has to be taken to ensure all components are truly 75ohm.
“In the case of HDTV, we have to be even more careful that we are seeing a purely resistive 75ohms with the minimum of reactance, in order to avoid reflections on the cables,” he said.
(GB)