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.
“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.”