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Long-Distance AV Signal Transmission

How far can a signal go? Not surprisingly, the answer depends on many factors, including the technology that's carrying it. But for runs of more than 300 feet, there are even more issues to consider, including the cost of cabling and labor, bandwidth, and ease of installation.

How far can a signal go? Not surprisingly, the answer depends on many factors, including the technology that's carrying it. But for runs of more than 300 feet, there are even more issues to consider, including the cost of cabling and labor, bandwidth, and ease of installation.

Long cable runs show up in plenty of places, such as corporate campuses, shopping malls, and houses of worship that simulcast a service across multiple buildings. Here's an overview of technical issues to consider when designing and bidding long-distance projects.

Technology options

AV pros have several wired and wireless choices for long-distance runs. The main ones are:

  • Co-axial cable – Coax comes in a variety of diameters, with thick, stiff cables having lower signal loss and more resistance to interference than their skinnier counterparts. All varieties are relatively fast and easy to terminate in the field. “In general, for distances up to 300 feet or so, coax is still the most common of the major signal transport technologies,” says Joe Da Silva, director of product marketing at Anaheim, CA-based Extron Electronics. “Low-resolution analog video — composite, S-video, or interlaced component analog — is typically good for several hundred feet on standard RG59 coax. When a line driver or an equalizing distribution amplifier is employed, low-res video signals can be run up to 1,000 feet on RG59 with very good quality.”
    Fiber? Coax? Or Cat5? There's no single cable technology that's ideal for all long-distance applications, so the choice usually boils down to issues such as the cost of material and labor, bandwidth, and ease of installation. For example, in a bandwidth-intensive application, fiber may be more cost-effective because it can handle more
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 data than a grouping of Cat5 or coax.

    Fiber? Coax? Or Cat5? There's no single cable technology that's ideal for all long-distance applications, so the choice usually boils down to issues such as the cost of material and labor, bandwidth, and ease of installation. For example, in a bandwidth-intensive application, fiber may be more cost-effective because it can handle more
    data than a grouping of Cat5 or coax.

    However, the quality of the coax cable itself varies from spool to spool, or even within multiple lines under the same jacket, occasionally creating problems. The culprit is usually the dielectric insulator that's sandwiched between the mesh shield and the center conductor. “The speed of signal transmission is dependent on the dielectric constant, which has to do with physics, chemistry, and density of that foam,” says Ali Haghjoo, CEO of Hall Research Technologies, based in Tustin, CA. “With the more expensive cables, you don't run into this situation, but with the normal VGA extension cables that are usually made in China or Taiwan, we've had batches where a customer calls about focus issues.”

    Hall now tests all cable and has rejected up to 10 percent at times. One way to identify dielectric-related problems in the field is to look for a vertical misalignment of red, green, and blue. “White has all three components, so white doesn't look like white,” Haghjoo says. “It has tinges of colors.”

    What do dielectric problems have to do with distance? For one thing, having the cables tested before installation is a way to avoid extracting a bad cable and then repulling a second one — both of which incur additional labor costs.

  • Cat5/6 cable – This technology is literally the backbone of most Ethernet local area networks (LANs), and the number after the Cat indicates the generation. All Cats consist of twisted pairs, but not all are equally useful for AV applications. For example, in LANs, data is sent and received, so the pairs are twisted at different rates in order to eliminate crosstalk. But with one-way video traffic, that design creates skew, so for video applications, it's important to take steps to minimize skew using skew-free cable or compensation circuitry.

    Because it's widely used in LANs and other applications, Cat cable is cheap: less than 5 cents per foot, depending on type and volume. That low cost makes it tempting to pull liberally, but overzealousness can backfire in certain applications.

    “If it's digital signage, with big characters and pictures, it probably doesn't matter much,” Haghjoo says. “But if it's smaller fonts, it's going to matter. It depends on the resolution or refresh rate. The higher those are, the shorter the pixel times are. It can become an issue, depending on the length of the cable. At 300 feet, you can be sure that you'll have that problem.”

  • Fiber optic cable – There are two main types of fiber optic cable, both of which use hairlike strands of glass. The first is single-mode, also occasionally called mono-mode, where all information is carried on a single beam of light. The second is multimode, which handles multiple beams simultaneously and thus has a larger girth than single-mode cable.

    One major benefit of fiber optics is that the signals are sent as beams of light rather than electrical waves. So unlike copper technologies, the signal doesn't get bogged down by electrical resistance over long links. Because light tends to scatter over distance — a phenomenon known as modal dispersion — multimode fiber is typically used for spans up to 10 miles, and single-mode cable for greater distances. However, although both fiber types require transmitters (called optical emitters), single-mode fiber uses transmitters that are two to four times more expensive — about $1,000 — than ones for multimode applications, according to LuxLink, a Hicksville, NY-based company that specializes in fiber optics.

    Yet single-mode fiber may be a better choice even for links shorter than 10 miles. One reason is bandwidth. “People think that fiber is an infinitely large pipe,” says John Lopinto, president and CEO of Hauppauge, NY-based Communications Specialties. “But multimode has severe bandwidth limitations, particularly as it relates to digitized baseband signals such as RGB video. Single-mode fiber doesn't have that.”



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