Plumbers 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 0.16in to 0.41in, with performance trade-offs that are well understood. At short distances a coax pipe can carry any signals, but of course with different performance. For example, SMPTE 292 HD video might be expected to work in miniature coax for only 100ft, though high-quality fat pipes would be good for about 5X the distance.
Think about the municipal water system again, with 30in main lines and 1in pipes into your house, which works for similar reasons. A 30in main to run to your bathroom makes little sense, as does 1/2in 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 installing 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 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 bandwidth of about 6GHz, which is in the middle of the range for broadcast microwave RF applications.
Stepping on a cable might seem like a difficult thing to avoid, but if we wouldn't do it with heliax, it makes no sense to risk precision video cabling. Cables have specifications for pull strength, crush resistance and bend radius that must be carefully followed. It is convenient to pull long runs, but care must be taken to ensure that the installation process maintains the integrity of the original product.
One of the most overlooked pitfalls is tying cable in place. Clean installations do not require overly tight cable bundles. Regular and excessively tight cable ties can cause a periodic deformation of the cable, severely compromising the return loss and leading to much shorter working distances before signal degradation.
Unfortunately, you might not know the cabling is poorly installed until years later, when the characteristics of the hardware on either end of the pipe changes. This makes a marginally performing cable visible as errors begin to show up in the content that weren't there at the time of installation.
Now, signals can be interconnected over fiber almost as cheaply as over coax when longer runs require equalizing amplifiers in the middle of a pathway. An HD news studio I worked on in 2008 needed amplifiers between weather graphics computers on set and the transmission rack room, where the production switcher electronics were located. Today, installing fiber is not difficult, and an increasing number of manufacturers offer fiber connections. Fiber itself is actually cheaper than copper cabling, though the signal electronics often make fiber less attractive due to the cost of optical to electrical conversion.
A few years ago, many people thought that SMPTE 292 (1.485Gb/s) signals on copper were impractical. Today, many think the same of SMPTE 424 (3Gb/s) signals. I expect new technologies for signal equalizers to be shown publicly in 2009 that will move the barrier a bit further out. In the long term, we certainly have arrived at the point where fiber is much more technically practical and affordable. It is easy to install, and in some ways less susceptible to degradation over time than copper cabling in plants with typical broadcast interconnection logistics and distances.
John Luff is a broadcast technology consultant.
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