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.
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.”