Distribution
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Ask the experts John Lopinto, Communications Specialties, Inc. With the increase in bandwidth requirements for signal distribution systems, which infrastructure provides greater benefits — copper or fibre? Can you explain why? B oth copper and fibre each have their place in today’s broadcast infrastructure. As we know, the useable bandwidth of coax cable is a function of both its physical construction and the length of the cable between the source and load. Modern IC line drivers and equalizers offer some ability to compensate for distortions associated with long lengths of coax to make them useable in more applications. But there are limits and costs associated with these techniques, and when taken together with the physical size and cost of the coax, their economic impact must be carefully considered in the overall system design. Clearly, for relatively short lengths, coax cable offers a straightforward choice for single channel signal distribution. But today’s modern facilities, with changing and varied signal formats, require a more flexible and economic cable infrastructure. Fibre cable has many technical and economic advantages compared to the coax cable needed for today’s high definition video transport. Physically, it is thinner and lighter. Even when each glass “core” is individually jacketed, it is only a fraction of the size and weight of HD coax for a given length. Keep in mind that fibre carries no electrical current nor does it contain any metallic contact. These characteristics allow it to be installed in a variety of facilities that would otherwise be considered off limits for copper cabling, such as ones that are adjacent to AC power lines, fluorescent ballasts and motors. Single 34 | TV-BAY MAGAZINE y iver Del mode fibre can also transport multiple signals, and in a bi-directional manner, on one core. We keep hearing that fibre optic systems are difficult to install and maintain. Can you comment on that? Years ago the reliable termination of a fibre connector onto the end of fibre was a black art. It required patience, skill and a high tolerance for failure. I’m happy to report that all that has changed for the better. Today, several manufacturers have developed pre-terminated crimp type fibre optic connectors in many different styles supporting both single mode and multimode fibre types. If you can terminate a BNC connector onto coax you have the skills to terminate a fibre connector in a matter of minutes. Termination kits are not expensive and consist of a collection of hand tools, a cleaver and stripper. Additionally, there are also low cost, simple to use optical power meters and light sources to facilitate the installation, maintenance and troubleshooting of fibre installations. Once you understand the basic concepts of link budgets and wavelengths, and you have a practical understanding of optical connections, maintaining a fibre installation is really straightforward. One tip: When in doubt, clean the connector tips. It’s amazing how just this one simple maintenance action can eliminate the majority of fibre system problems. Just use the same diagnostic skills you have developed in your experience with copper based systems, such as substitution of components and paths that are suspect. The same techniques also apply to fibre. Over time, you an increase the array of diagnostic equipment in your tool kit to make installation and maintenance more efficient. Which applications are most ideal for fibre optic technologies? For copper? There is no specific guideline to say when copper or fibre should be used. Certainly, when the transmission length is beyond the capability of coax cable, even with equalization, then fibre is the ideal solution. However, in all these considerations of copper versus fibre, the number and complexity of the array of signals to be distributed, along with the distribution topology, needs to be taken into account. Too often designers think of fibre as only a long distance solution to be used when copper cable no longer works effectively. While this is one decision point for fibre deployment, it doesn’t take into account the technical and economic characteristics unique to fibre that make it the right choice for many more applications. Consider the case where multiple HD-SDI signals need to be transported from point A to point B over a distance of 250 meters. The copper solution would be to have an HD grade coaxial cable for each HD-SDI signal. The resulting bundle would be quite thick, heavy and inflexible. Yet, no interfaces would be required at the source and load. However, with one core of single mode fiber we could easily multiplex all of these HD-SDI signals and have a very small (< 3mm) cable that is very low cost, easy to install and lightweight. Yes, the proper optical multiplexing equipment is required, but this total system cost must be compared to the alternative total cost for a copper solution. As system requirements become more complex, you’ll find the tradeoff between copper and fiber will be less about distance and more about cost and performance. What are the main differences between single mode fiber and multimode fiber? How do you know which one is right for your facility? Bandwidth, bandwidth, bandwidth! Decades ago when fibre cable was first manufactured, it was difficult to create fibre cable thin enough to contain the light travelling through it without a lot of dispersion, that is the scattering of the light beams (or “modes”) as the light travelled through the fiber. This dispersion caused a fair amount of attenuation and distortion of the light >>