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The Gordion Knot

Oct 1, 2002 12:00 PM, Peter H. Putman, C.T.S.


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Cable pulls used to be so easy. To run audio, you'd grab a spool of plenum wire and go. Video? No problem. You could move as many signals around as you wanted to. Even control cables were easy: get a soldering iron, some 9-pin RS-232 plugs and jacks, and some multistrand wire, and you were ready to set them up. Now that you finally have a system that works, the manufacturers are driving you crazy with “new and improved” ways to move signals. You must choose between Cat-5 wire, fiber optics, RF transmission, pulse-width modulation, LEDs, receivers, and transmitters — yikes!

The truth is there are better ways to transport video, audio, and data, particularly in multiroom environments. Discrete analog wiring is a good thing as long as the cable runs aren't too long and don't need to be split and redistributed too many times. But today's typical residential or commercial installation is a bit of a crapshoot. The question is: how futureproof can you make an installation when you don't even know what types of analog and digital signals will need to move down those wires? There are other considerations, too, not the least of which are labor costs and cable prices. It would be nice to see significant reductions in both areas and still be able to ensure some degree of future expansion capability for your clients. I'll start with some tips and directions that can help your wiring systems stay useful over time and under budget.

THE JOYS OF MULTIPLEXING

The most efficient way to transmit any combination of oddball signals is to multiplex them. Multiplexing isn't a new idea. Television broadcast stations have been doing it for years, using a 6 MHz RF “pipe” to carry amplitude-modulated video, frequency-modulated audio subcarriers, and a color burst subcarrier that also contains horizontal and vertical sync pulses. That makes wiring really easy, because just one coaxial cable carries all the information from the transmitter to the receiver (through an antenna or a cable drop). Because the receiver has a great deal of selectivity and filtering, multiple channels of RF can be transported across that one cable. The only technical limitations are the losses in the cable and any signal degradation that occurs in the analog amplifiers, splitters, and distribution equipment.

Because this model works for the broadcast of TV signals (not to mention satellite), why not adopt the broadcast model for distribution of A/V signals and eliminate the need for all those cable pulls? Better still, imagine installing a system of wiring that has sufficient bandwidth to handle all kinds of digital and analog signal sources. Then you wouldn't have to worry about what you were connecting to the signal pipeline you've provided because it would be able to handle just about anything.

Digital over LANs

The broadcast multiplex pipeline system for residential and commercial wiring can be adopted in several ways. One that I have covered in previous issues of S&VC that's getting a lot of attention from manufacturers of projectors and displays is using ordinary Cat-5/6 wire to carry all signals as digital bits of information. This pure packet system has many advantages. Because it's all digital, all that's required is that any receivers of data have some sort of unique address, such as an IP address. With this system, audio, video, and data packets are received only by the equipment they are addressed to, and large volumes of packets containing everything from e-mail to still photos can be mixed in with multichannel audio, video, computer display signals, control information, and even digital TV programs.

The downside to all this is that you will have to set up some sort of local area network (LAN) within the facility or home to communicate with all of these devices. That might not be too daunting a task, but it does require a degree of system intelligence beyond simple wiring. Other potential roadblocks exist, though. Many companies with existing LANs are resistant to having A/V equipment hooked into their computer networks. That means you'll need to set up a separate, parallel LAN for the immediate future until everyone in the IT departments is convinced your projector or plasma monitor won't crash his or her server when it sends out a service e-mail.

Another problem is the availability of equipment with IP addresses. So far only Sony has gone on record as saying it will build IP addressing into every product it makes from now on, including its broadcast VTRs, digital cameras, display devices, and DVD players. If you want to build a pure LAN-equipped A/V facility, you may still need to rely on RS-232 control for screens, audio amplifiers and mixers, drapes, and so on.

LANs are also shared bandwidth networks. A 100 Mbps Ethernet connection means you have 100 Mbps maximum data rate (or headroom) available to all users on the network. It's easy to gobble up data, too. In a residential network, watching a streamed DTV program would chew up 20 Mbps of data. PVRs that stream programs such as Sonicblue's Replay system will also grab a chunk of data, as will high-resolution digital photos, music from CDs, and DVD movies.

That isn't to say you should dismiss LANs for A/V installations — far from it. But the concept is still in its infancy, and it will take time for manufacturers to incorporate the necessary IP addressing into their products. The upside is that you'll need only to pull one type of cable, Cat-5/6, for everything, which will considerably whittle down labor and product costs. As of now, you can get a limited degree of LAN connectivity and enjoy some benefits such as remote diagnostics of equipment, automatic e-mail alerts, and status monitoring. Streaming video and audio and sharing presentations and still images are not quite there yet for IP-connected A/V devices.

Radio frequency

The broadcast multiplex pipeline concept translates almost perfectly to twisted-pair systems that use Cat-5/6 cable as inexpensive balanced transmission lines. Instead of moving digital data, a small transmitter modulates the video, audio, and sync signals as RF carriers and subcarriers. For a five-wire signal system, the red, green, and blue channels are each discrete RF signals with sync and audio traveling as FM signals.

This concept has been around for a few years. It provides a simple way to accomplish point-to-point RF distribution of signals. Assuming the output of the transmitter is matched carefully to the transmission line (again, Cat-5/6 cable), then signal loss due to attenuation is tolerable and allows cable runs of 1,000 feet with base-band video and VGA (640-by-480) signals. There are also RF signal multiplexers for Cat-5/6 wiring, just as you'd have with CATV and MATV systems. If you need to move a signal to more than one display device, all you have to do is add a splitter, which is nothing more than a broadband RF distribution amplifier with pads and terminations to accommodate Cat-5/6 wiring.

But this system has a limit on the number of video and audio signals you can move around, a ceiling determined by the operating frequency of the transmitter and receiver and rolloff in the Cat-5/6 cable itself. For example, the allowable cable lengths drop with an increase in resolution, horizontal scanning frequency, and refresh rate. Moving SXGA graphics or HDTV 1,080i signals becomes problematic at lengths of more than 400 feet. UXGA images will cut that length to about 200 feet, according to the specifications I have seen from manufacturers of twisted-pair transmitters and receivers.

For that reason, I view twisted-pair interconnects as temporary technology. The Cat-5/6 wire pulls needed to make up a twisted-pair system are perfectly suited for conversion to a full-blown LAN once the IP-addressed equipment and peripherals are available. All you need to do in the future is pull out and toss the transmitters and receivers, replacing them with hubs, servers, and other interconnections.

Optical fiber

A third path to future connectivity entails doing away with coaxial or twisted-pair wire altogether and using fiber-optic cables. Yes, I've heard that fiber is expensive, difficult to terminate, fragile, and so on. Maybe that was true once, but the situation today is much different. Fiber-optic cable has one big advantage over Cat-5/6 cable: bandwidth. Single-mode cable drops about 0.25 dB of signal for every kilometer at 1,550 nanometers (nm). That works out to about 0.04 dB per mile of fiber, a figure that leaves conventional wiring gasping for air in its wake. Even multimode fiber can achieve about 5 dB/km performance at 850 nm. Because the bandwidth of fiber-optic cable goes well into the terrahertz range, multiplexing of video, audio, and control signals is a pretty easy task.

The original implementations of fiber used analog signal transmission, similar to the concept of a preamplifier and an amplifier. Variations in a sine wave were amplified (not always faithfully) and reproduced at the receiving end of the fiber link. But analog signals could easily be degraded by imperfections, light distortion, and refraction in fiber. The answer (like everything else these days) is to go digital, as there are fewer problems switching 1s and 0s. Because fiber is a two-way system (like a LAN), the transmitter and receiver can move any kind of digital data and even provide for forward error correction. Lasers and LEDs are both used for fiber transmitters, though LEDs are typically reserved for multimode fiber and lasers for single mode.

The cost of fiber-optic cable has dropped dramatically. With terminations, fiber can be had for about 15 cents per foot. The terminations have also become easier to use, eliminating the need for polishing and adopting a crimp-style connector that is about as complex to install as a BNC plug. For more secure connections, the polish-and-epoxy system is still used, albeit in a much more expedient way. Like other systems, repeaters can be installed to boost fiber signals over really long runs; however, it's not possible to use a fiber distribution system to split fiber into multiple bundles. Rather, a signal distribution system working at the actual signal frequency is required, unless the fiber system is carrying pure digital packets around some sort of LAN.

Another advantage to using fiber is its immunity to crosstalk, EMI, RFI, lightning, and other AC-coupled interference that can be problematic with any copper-based system. No voltage flows through fiber, so it can be run alongside high-voltage or utility pipes and won't create hazards if it breaks (unless it runs through a photographic darkroom).

SO MANY CHOICES

There is no clear-cut advantage between these three approaches. Twisted-pair wire is a sensible solution for many applications, but it is bandwidth limited and not practical in really long runs. Cat-5/6 in a LAN configuration using pure digital data is an efficient solution, but the network needs to be set up to run Gigabit Ethernet or Fibre Channel to allow streaming of bandwidth-hogging video and audio. If more products were available to support IP addressing, then the Cat-5/6 LAN approach would make good sense. It's pretty simple to set up a small A/V network and add hubs and routers as needed. All that really needs future upgrading from that point on is the equipment, as the wiring should be good for 100 to 200 Mbps applications.

Fiber-optic cable offers immense bandwidth, long transmission distances, and competitive prices when compared with Cat-5/6 cable. Some spec sheets show single-mode fiber at 75 cents per foot, as opposed to Cat-5 bulk cable at 12 cents per foot. But you need to do signal distribution and splitting post-fiber to make it practical.

If I had to make a choice, I might pull both for a commercial installation. The fiber links would be for serving up high-bandwidth video and computer graphics from a central location or for point to point. For lower-bandwidth applications, including networked displays, I'd haul in some Cat-5/6. With that setup, I'd eliminate a lot of bundles of control, audio, and analog video wiring, not to mention a bunch of distribution amplifiers. The system would be futureproof for an all-digital signal distribution system. The cable is certainly inexpensive enough, and plenty of interfaces are available for both systems.

Extron, FSR, Altinex, and Inline support twisted-pair connections, and some of those companies are taking baby steps into pure LAN and IP interfaces. Sony, Epson, Sharp, InFocus/Proxima, and other display manufacturers have joined them in that market and are incorporating IP addresses into projectors and monitors. Check out all the offerings from those companies and pay careful attention to bandwidth specifications before choosing the wire for your future installs.

On the fiber side, Communications Specialties has several RGB, video, and audio interfaces for fiber connectivity. (Thanks to John Lopinto of CSI for providing background materials about fiber.) I've covered twisted-pair and LAN connectivity in previous issues of S&VC, but if you want more information on fiber, see “A FOC Primer” in the July 2002 issue. There are a lot of sources of information online, but you can also check out CSI's Web site (www.commspecial.com) or contact the company through e-mail (info@commspecial.com) for some great educational booklets.



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