Broadcast Engineering - February 2009 - (Page 38)

Technology in TransiTion New products & reviews The right connections wire, cable and fiber hold your facility together; make sure they’re properly installed. By John Luff lumbers have pipes, and broadcasters have cables, but the concept is the same. Cables transport electrons while plumbing transports water or other fluids and gases. Voltage might be equated to pressure and amperage to volume. Frequency is hard to equate because plumbing is a DC medium, but it can be compared to velocity. This article will explore how these concepts can be instructive in understanding what’s at the core of your facility. What’s under those cables? It’s important to know what kind of plumbing you’re connecting. Until relatively recently, you could look at the outer jacket of a coax cable and be pretty sure what kind of signal was in it. That’s no longer true. In most broadcast plants, there are many types of signals on the same coax medium, including NTSC, SDI (270Mb/s NRZI coding), HD-SDI (1.485Gb/s), SMPTE 310 (19.39Mb/s), AES audio (3.07Mb/s) and ASI (270Mb/s NRZ coding — phase sensitive!), not to mention RF signals that vary from slightly above DC to just below light. To ease the confusion, 1V is the standard for most of the pressure in our plumbing metaphor. But the velocity (frequency) is clearly all over the map! Cables vary from 3Mb/s to 1.5Gb/s, a ratio of about 483:1. Broadcasters cannot expect the plumbing to work equally well in a wide range of uses. Broadcasters use many types of coax because it’s convenient to snake more pipes into a given space. Coax diameters for precision use vary from about 4.1mm to 10.4mm, with performance trade-offs that are well understood. At short distances a coax pipe can carry p any signals, but of course with different performance. For example, SMPTE 292 HD video might be expected to work in miniature coax for only 9.3m, though high-quality fat pipes would be good for about 5X the distance. Think about the municipal water system again, with 762mm main lines and 25.4mm pipes into your house, which works for similar reasons. A 762mm main to run to your bathroom ing cable properly are important. We don’t often think about the mechanics of cable, but as frequencies increase, from AES to 3Gb/s SMPTE 424 HD interconnects, cable performs completely different. Anything that changes the return loss characteristics of the cable assembly will directly affect signal integrity. Sharp bends, any mechanical deformation of the cable and Fiber-optic distribution cables can be used to support the data backbone for broadcast and video applications. makes little sense, as does 12.7mm copper under your street, but both work in the right application. With cable density increasing in television plants, it is appropriate to look at all types of cable, including fiber, before picking the medium you should use for any application. Proper installation is half the battle Like plumbing, installing cable correctly will make a world of difference in performance. Using true 75V connectors, minimizing patching and other signal interruptions, and install- mishandling will limit the performance and change what comes out the other end of the pipe. Coax is called precision cabling because it truly is just that. Many of us have learned by experience that proper handling of microwave interconnects on heliax and waveguides is critical at gigahertz frequencies, but often that level of caution and consideration is not given to how we deal with video signals in the digital domain. Cable manufacturers advise that their product must work at twice the bit rate to adequately handle the content. With SMPTE 424, that means a 38 broadcastengineeringworld.com | February 2009 http://www.broadcastengineeringworld.com

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Broadcast Engineering - February 2009 Contents Viewing the Slowdown in 3-D Examine Workflows Tapeless Technology Digital Audio 24p and 25p Judder Video Routing: A Look at What's Next Managing AFD The Right Connections Solid State Logic's AWS 900+ SE and More... Advertisers Index

Broadcast Engineering - February 2009

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