ProAVmag

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, 7/29/2006 7:06 AM Eastern

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.”
    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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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.

Another benefit of using light rather than electrical waves is that the AV traffic is immune to interference, nor does it cause any, so it can be installed near electrical lines or cables such as Cat5. That's good to know in long runs, where odds are higher that sooner or later, the cable will have to be installed near existing infrastructure. “There's a misconception that fiber cable has to be installed in some special way,” Lopinto says. “I've had people tell me that they've put it in separate conduits or isolated it from the AC power.”

  • Electrical wiring – Audio and video can be piggybacked on a building's electrical grid, such as by using a technology called HomePlug. As its name implies, the technology typically is used for home networking applications, but it's practical for some commercial applications, too. “There are customers that use it for apartment buildings, where they send low-rate video, such as for security cameras,” says Nimesh Doshi, the HomePlug product line manager at Conexant, a San Diego-based vendor.

    The latest version of the technology, HomePlug AV, is designed for audio and HD video, with throughput of up to 200 Mb/s, quality-of-service mechanisms, and 128-bit AES encryption. Its predecessor, HomePlug 1.0, supports up to 10 Mb/s. Both technologies have a range of 300 meters, or about 990 feet.

    One caveat that affects distance and throughput is the building's wiring. “If you're going through multiple circuit breakers, or if the building's wiring is from the 1920s, that affects the data rate,” Doshi says.

    • Wireless – Although 802.11 WiFi occasionally is used in pro AV for applications such as connecting a laptop to a projector for a presentation, it's a dicey choice at distances beyond 300 feet. “It depends on how conservative you want to be,” says Jim Petranovich, a product line manager at Conexant. “I'd say 250 feet for 802.11g.”

    One issue is the frequency. Although 802.11a has a data rate twice as fast as the more widely used 802.11b, it operates at a higher frequency. As a rule, signals travel farther at lower frequencies, so if WiFi is the only option, 802.11b or g may be a better choice for stretching the link as far as possible.

    A forthcoming version of WiFi — 802.11n — could be viable for 300-foot-plus links, thanks to an antenna technology that makes the most out of weak signals. “N should be two to three times the range of g,” Petranovich says.

    One caveat: Although 802.11n equipment is commercially available today, it's based on a preliminary version of the standard and could have interoperability problems with the official version. The standard should be finalized later this year.

    Going the distance

    With so many options, the obvious question is, which technology is best for runs of more than 300 feet? Unfortunately there's no simple answer, partly because there are other considerations, such as cost, bandwidth, and ease of installation. Nevertheless, focusing on a technology's ability to handle lengthy links at least helps narrow the field.

    One issue iwhy UTP cable over long distances is skew, which results from the different twist rates of the cable pairs. The result is that color information arrives at the destination out of synch, causing image distortion.

    One issue iwhy UTP cable over long distances is skew, which results from the different twist rates of the cable pairs. The result is that color information arrives at the destination out of synch, causing image distortion.

    “Gefen recommends fiber optics cables for distances over 330 feet (100 meters) due to its more robust nature and ability to carry uncompressed HD data unimpaired at these distances,” says Hagai Gefen, president and CEO of the Woodland Hills, CA-based company. “Fiber optics cables are combined with sender/receiver systems that enable standard and HD data to be transmitted instantaneously up to 1,640 feet (500 meters). We're currently in testing and development using similar technologies to extend HD audio/video up to several kilometers with complete HD clarity. We aim to have this solution available before the end of the year.”

    On the copper side, Cat5 often is a better choice for long video runs. “You can go farther over Cat5 than you can over five coaxes (R, G, B, H, and V) even with today's booster/amplifiers,” says Keith Mortensen, president of Magenta Research, a Milford, CT-based vendor.

    One thing to watch for is the distance and resolution specs on the receiver. “Typically you'll find specifications that say things such as, ‘Up to 1280x1024 up to 500 feet,'” Mortensen says. “When you read the fine print, those two specs aren't necessarily married. It may be 500 feet for 640x480, but 1280x1024 is only 175 feet.”



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.

The labor factor

When bidding a job that involves lengthy spans, one variable to consider is labor costs, particularly in major cities. For example, suppose that you have to pull five pieces of coax and then terminate 10 BNCs. “A worst-case scenario is New York City,” Mortensen says. “The Local 3 electricians union charges roughly $1,000 to pull and terminate a piece of wire. So if you're pulling five coaxes to a plasma screen, you've got a $5,000 cost just in pulling and terminating that cable. Chicago and Boston aren't much better.”

Another factor is ease of installation. For example, fiber optic cable is immune to interference and doesn't cause any of its own, so it can be installed in places that other cable types can't tolerate. In those cases, fiber might be a less expensive alternative if using it means you can install it more quickly or use less of it because you don't have to run it in circuitous ways just to avoid interference. Fiber also can be surprisingly flexible, which also helps ease installation headaches and expenses. “The bend radius on a single strand of fiber is less than that of Cat6 and coax,” says Communications Specialties' Lopinto. “It's about 1 inch.”

Like other technologies, fiber can be spliced in the field, but the choice of technique affects labor costs. One option is to use a fusion splicer, which heats the fibers to fuse them together. Fusion splicers cost around $16,000, depending on features, so it's a tool that few AV pros have on hand. “Typically you bring in somebody who specializes in that as a contractor,” Lopinto says.

A less expensive, more common option is to connectorize the two ends and then plug them together — a process that takes about 5 minutes. “That gives more than satisfactory results,” Lopinto says.

A third, relatively new option is a 2-inch-long tube that accepts the two raw ends and then locks them together mechanically via a twist on each end. The connector costs about $15, and the process takes about 5 minutes using common hand tools to prepare the raw ends.

Whatever technique you choose, make sure that the fiber cables being spliced are the same type. “The only mistake that we hear about is that people tend to splice single-mode fiber to multimode fiber,” Lopinto says. “That's a real no-no.”

Cat5 can also be spliced. “We've had people do a dozen splices without issue with video over Cat5,” says Magenta's Mortensen.

For troubleshooting long runs, it helps to be able to identify where the problem lies. All wired technologies have a selection of tools to determine what's often called distance to fault, or the point where the break or flaw exists. Some of those tools aren't cheap. For example, fiber can be checked with an optical time domain reflectometer, a $5,000 tool that can identify the break within a fraction of a meter.

Giving a boost

For long runs, copper technologies require a booster or repeater to amplify the signal, which becomes attenuated over distance. One inherent downside is that the booster also amplifies any noise picked up along the signal path. Another issue is that simply boosting the signal across all frequencies can cause problems.

“The very highest frequencies in high-resolution video — 100 MHz — has to be amplified more than 80 MHz, which has to be amplified more than 60 MHz, and so forth,” Mortensen says. “Some products amplify everything, and what happens is that the low-frequency information becomes much too hot.”



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.

That problem prompted the industry to create filters that use multiple poles to keep some frequencies from getting too hot. “That's been done on coax for years,” Mortensen says. “But in a perfect scenario, it would be an infinite number of poles to make a perfectly corrected flat curve. That doesn't exist. In the Cat5 realm, we've done something more sophisticated: a seven-pole, complex-state, variable filter. It does a much, much finer correction over cable losses than what's available on the market in Cat5 or coax.”

The signal's strength at the beginning of its journey also determines how far it can travel. That's particularly important with wireless technologies.

“Most microphone transmitters only have 10 to 50 milliwatts (mW) RF output power,” says Joe Ciaudelli, a consultant on the professional products industry team at Sennheiser, based in Lyme, CT. “This is a good level for stage use to prevent overload at the receivers if several transmitters operate in close proximity to each other. However, for long distances, a high-power transmitter rated at 250 mW is recommended.”

The transmitter and receiver antennas also are major factors in the distance that can be reliably spanned. “At the receiver end, an active directional antenna, such as the Sennheiser A12AD can greatly increase the range of the system,” Ciaudelli says. “This antenna has 4 dB of passive gain, plus a 10 dB amplifier, boosting the received signal a total of 14 dB.”

For most AV applications, fiber doesn't require a booster because the signal doesn't peter out as easily as it does over copper. In fact, that's one of the reasons why fiber is widely used for undersea phone lines that span the Atlantic and Pacific oceans.

“Fiber is typically good to several miles,” says Extron's Da Silva. “Fiber repeaters are common, but not inexpensive, allowing signals to be extended hundreds, if not thousands, of miles.”

Depending on the application, it may be possible to mix technologies in order to overcome issues such as cost or distance limitations. “Fiber probably is going to be an increasing market,” says Magenta's Mortensen. “We've seen that starting to come in dynamic signage applications, where people want to go campuswide and run a fiber backbone from multiple buildings and then convert to a copper technology.” 

Tim Kridel is a freelance writer and analyst who covers telecom and technology. He's based in Kansas City and can be reached at tkridel@kc.rr.com.



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