Using Digital Fiber in AV Applications

Sept. 13, 2005

In today’s tech-heavy facilities, architects are increasingly tasked with designing plans that include more sophisticated uses of audio and video than ever before. Facilities requiring multiple signals and high-definition video are becoming the norm.

Digital fiber-optic products and systems have become easier to work with and less expensive, and offer increased functionality. The benefits, which are many, include:

  • Digital fiber can transmit signals, including video and audio, with greater fidelity than either copper wire or coaxial cable. Baseband quality is consistent over all usable distances.
  • It can support higher data rates, and at greater distances, than coaxial cable. Useable lengths without repeaters from
  • 1 km to 100 km are common.
  • Fiber is immune to virtually all kinds of interference, including lightning, and will not conduct electricity. It can therefore come in direct contact with high-voltage electrical equipment and power lines. It will also not create ground loops. Because the basic fiber is made of glass, it will not corrode and is unaffected by most chemicals.
  • A fiber-optic cable, even one with many fibers, is usually much smaller and lighter in weight than a wire or coaxial cable. It is easier to handle and install, and uses less conduit space. (It can frequently be installed without conduit.)
  • Fiber-optic cable is ideal for secure communications systems because it is difficult to tap. It does not radiate any of the signals being transmitted, and it does not cause interference to adjacent equipment.
  • Fiber is expandable. Additional applications can be used on one fiber infrastructure.

The Basics

The basic Point to Point fiber-optic transmission system consists of three basic elements: the optical transmitter, the fiber-optic cable, and the optical receiver. The intent is to carry a signal as transparently as possible from point A to point B.

The transmitter, which is analogous to a radio or TV transmitter and nothing more than a simple “black box,” converts an electrical analog or digital signal into a corresponding optical signal, or beam of light. Sources of the optical signal include a light-emitting diode (LED), used in multimode systems, and a solid-state laser diode (LD), used in single mode systems. The most popular wavelengths for operation for optical transmitters are 850, 1310, or 1550 nanometers.

The fiber-optic cable consists of one or more glass fibers, which act as waveguides for the optical signal. Fiber-optic cable is similar to electrical cable in its construction but provides special protection for the optical fiber within.

The optical receiver receives the beam of light, or signal, and converts it back into a replica of the original electrical signal. The detector of the optical signal is either a PIN-type photodiode or an avalanche-type photodiode.

There are two types of fiber cable: multimode and single mode. The difference is in how light travels inside the fiber, and as a result it’s bandwidth, and not how many signals it can handle. In essence, multimode fiber can be compared to a dirt road while single mode is like a 10-lane superhighway.

Multimode, which is traditionally used in security applications, has a relatively thick core - 50, 62.5, and 125 microns. Multimode fibers use LEDs in the transmitter box and are limited in their bandwidth and expansion capabilities.

Single mode fibers have a thin core of 5 to 10 microns. The preferred choice for new, permanent installations, they can be used at any distance, at any bandwidth. They use lasers (LDs) in the transmitter box, a technology with greater upfront costs but more cost-effective long-term costs.

Here’s an example of the difference between multimode and single mode: You have a subway station with digital signage. In this case, multimode fiber allows you to go 750 meters from the transmitter to the receiver. The transmitter is in the building next door, so you run the fiber from that building to the application, using all the bandwidth of the fiber. A couple years later, the building owner wants another plasma display at the opposite end of the platform. To add that display, you’d either have to install a single mode fiber or an optical distribution amplifier to regenerate the signal. Both would be costly retrofits. Another problem could arise if the owner wanted to replace the original display with one requiring higher bandwidth.

Bandwidth is not an issue with single mode fiber. In the previous example, starting with a single mode system would have allowed you to add or replace the plasma display without the costs associated with the upgrade from the multimode.

Key Issues to Confront

To maximize the use of a digital fiber-optic system, it’s important to know when to use fiber:

  • When distance is too great for copper.
  • When the environment is hostile.
  • When security is a requirement.
  • When installation costs are critical.
  • When multiple signals need to be communicated.

Fiber can be run next to AC lines, video and audio lines, control lines, lighting control lines, in water, and oil and gas pipes. It can have a relatively narrow bend radius, and it’s rugged.

Fiber-optic transmission can transmit video, audio, and data over much farther distances than traditional coax cable and twisted pair wire. The exact distance that can be supported in any given system is a function of many factors, including the type of cable being used, the frequency of the signal transmission, the bandwidth of the fiber, and the number of splices and connectors used across the entire transmission distance.

It’s important to be aware of factors involved in terminating the fiber cable. Let’s say, for example, that you need to install a length of fiber 500 feet long with ST connectors. With preterminated fiber cable, the connectors would be installed in a controlled factory environment. The opposite of that would be to buy the raw fiber without any connectors on it. The connectors would be terminated, or installed, onsite. This is called field terminating. Because this process is labor intensive, problems can arise when the installer is inexperienced, unskilled, or uses the wrong tools.

Sometimes, two pieces of cable must be joined together, or spliced. Traditionally, a contractor would perform fusion splicing, an expensive process typically costing $30,000 for a modern fusion splicer. Now, there are inexpensive “field splices,” typically costing $30, that allow mechanical splicing to be done between two fiber cables at much less expense. While field splicing will never entirely replace fusion splicing, sometimes it can provide an effective alternative.

A major way to minimize cost in any fiber-optic system is to minimize the total aggregate length of fiber cable, because the cost of the cable and the installation labor will probably cost more than the electronics. One of the benefits of fiber is that a large number of signals - baseband signals such as video, audio, and data - can be put onto one fiber instead of using separate fibers for each. This may mean that the electronic boxes, or transmitters, will cost more, but the costs of the fiber cable and installation will decrease.

The Link Budget

The link budget, or loss budget, is the No. 1 issue in fiber-optic system design. It’s a concern in AV systems even though distances are relatively short. Exceeding the link budget is usually the reason why the system doesn’t work when first installed.

The link budget, which is measured in dB (loss of light), is the maximum allowed loss of light from the transmitter to the receiver before the integrity of the signal is lost. In an analog system, the more the optical signal decreases, the poorer the baseband signal quality. (Think of a fading AM radio signal.) In digital systems, however, if there’s enough signal, the operation will work at 100 percent, with no interference. If there’s not enough signal, it won’t work at all.

Elements that contribute to loss include the fiber cable, the connectors on each end of the fiber, the splices in the fiber cable, the patch cables/panels that may be used, and dirt and dust.

Fiber Splices

  • End of one fiber is melted or “fused” to  end of another fiber
  • Fusion splicers are used to weld the ends together
  • Requires low skill level but expensive machine to do it right
  • Budget .2 dB per splice

Fiber Cable Losses

Single mode fiber

  • .25 dB/km at 1550 nm
  • .35 dB/km at 1310 nm Multimode fiber
  • 4 dB/km at 850 nm
  • 1.5 dB/km at 1310 nm

Read the spec sheet for the fiber or the side of the reel

Dirt and Dust

  • This can kill a link
  • Wipe the end of each connector with alcohol and lint-free wipe before installing connector
  • If link loss increases over time wipe the connectors
  • Budget 1 dB per 6 connectors used in link

Connector Losses

  • Budget .25 dB per connector
  • Can range from .1 dB to 1 dB
  • Field terminations about .5 dB
  • Single mode fiber termination more critical

Patch Cables and Panels

  • Premade can be .2 dB
  • Actual loss will depend on how well it mates with panel jack
  • Budget .5 dB per patch cord used


The boxes above show sample attenuations, or losses in the cable. Each fiber cable manufacturer lists the attenuations with their products.

Using the previous numbers to calculate a sample link budget, say a run is 500 meters long of multimode fiber and split into two lengths with a patch panel at each end. Add the following:

  • 500 meters total of MM fiber at 850 nm = 2 dB
  • 4 terminating connectors (4 x .25 dB) = 1 dB
  • 2 patch cords (2 x .5 dB) = 1 dB
  • 8 total connectors for dirt/dust = 2 dB


To determine if 6 dB is too much loss for the application, go to the transmitter and receiver data sheets that the manufacturer supplied. As long as the number calculated is less than the loss budget maximum, then the application should work.


Topologies are maps that show the signal distribution. Traditionally, topologies used in fiber optics have been Point to Point, shown here. (figure 1)

This topology shows the transmitter (TX), the receiver (RX), and a line illustrating the piece of fiber between the two.

In an application putting a video signal into the transmitter, the signal would travel through the fiber and come out the receiver. For example, a stationary camera on a closed-circuit television (CCTV) would send a signal back to a security monitor.

Today, there are more options for fiber transmission. In a Point to Multipoint - Star topology (figure 2), a transmitter would send a signal to an active optical distribution amplifier, which would regenerate the signal, sending three new signals to three transmitters. An example would be plasma displays at an airport. The source of the signal would be in a building at another location and would send the signal to multiple points.

In a Point to Multipoint - Tree & Branch topology (figure 3), the signal would go to an optical DA, and then one output of the optical DA would go to another optical DA. The advantage of such an arrangement is that the link budget starts over at each optical DA, so the system can increase in complexity without losing signal integrity. Such a topology may be used in a shopping mall where displays are widely dispersed.

Many more topologies exist, and they will grow as products become available that increase capabilities while also increasing quality. As end-users become more adept at what’s possible, they’ll demand even more choices out of their AV systems, and the professionals who design them.

John Lopinto is president and CEO of Communications Specialties Inc. (CSI), a New York-based manufacturer of fiber-optic transmission systems and computer-video interface products. He has previously held managerial positions at CBS Television Network, HBO, and Time Warner Inc. He holds a bachelor of electrical engineering degree and is a member of the ICIA Academy’s Adjunct Faculty.

Founded in 1983, Communications Specialties, Inc. is recognized worldwide for its development of innovative products in the areas of fiber-optic transmission and computer-video technology. Among its many products are the Pure Digital Fiberlink® line of point-to-point fiber-optic transmission systems for video, audio, and data and the award-winning Scan Do® line of scan converters and Deuce® video scalers. The company is headquartered on Long Island, NY, and has a wholly-owned subsidiary, Communications Specialties Pte Ltd., in Singapore.

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