Not long ago, most broadcast facilities were designed without the benefit of fiber-optic infrastructures and relied primarily on copper-based coaxial and twisted pair cabling. Now that fiber-optic technologies have become more cost-effective and the bandwidth demands of uncompressed video signals have soared, designing a facility without fiber is unthinkable. In the mobile OB environment, there are similar but even more compelling, needs for a fiber-optic infrastructure. However, the design considerations surrounding OB infrastructures differ significantly from fixed installations.
So many connectors
While fixed installations are designed around generic single-fiber, physical contact (PC) connectors like the ST, SC, FC and LC, OB and ENG operations normally need specialized ruggedized multifiber connectors. In the case of cameras and other hybrid fiber/wire devices, there is often a requirement to also carry power on the same cable/connector assembly.
Multipin connectors can be categorized in several ways, including butt joint (PC) pin and socket versus expanded beam, sexed versus hermaphroditic, fiber-only versus hybrid, etc. Additionally, ruggedized fiber connectors are commonly available in fiber counts of up to 12. With the exception of the SMPTE 358M four-fiber and the SMPTE 304 two-fiber hybrid connectors, multifiber interfaces tend to be proprietary in nature and are frequently sole sourced.
The complexity of design, the relatively small market and high skill required to assemble make the cost of multipin optical connectors an order of magnitude more expensive than single-fiber connectors. Repairing multipin connectors requires a skilled staff with an investment in training and tools. The more fibers/pins per connector, the more expensive it is, and the more difficult it can be to assemble or repair without damage.
On the plus side, ruggedized multipin connectors offer a far greater reliability and ease of setup. Making a single connection in an application that is repeatedly set up and taken down saves significant labor and time and will require less troubleshooting. Swapped or mislabeled fibers can burn a lot of time when it is least affordable. In addition, a multipin connector on the end of a TAC-4 cable is physically more robust than a cheap ST connector on the end of a breakout patchcord.
Although expanded beam connectors are generally more expensive than PC types, they are more reliable and maintainable because the optical mating surfaces never touch and have no cavities to collect debris. These connectors do, however, experience higher baseline loss, typically 1.5dB per mated pair compared with less than 0.5dB in a mated ST pair.
Know your optical loss budget
Fiber equipment manufacturers specify the minimum output power and minimum receiver sensitivity in order for their equipment to operate properly. These two values are absolute power measurements referenced to 1mW and are measured in dBm. The difference between these numbers, called the optical loss budget, is a relative power measurement and is measured in dB.
So, if a SMPTE fiber-equipped camera has an optical output of -8dBm and a receive sensitivity of -18dBm, the optical loss budget will be 10dB. By calculating the total number of connectors times the loss per mated pair from the output of the camera to the input at the controller, as well as the cable's length times loss per kilometer, we can verify the camera is within its operating limits.
Cameras with a 10dB budget will often alarm with losses as low as 6dB, staying well away from the envelope. Leave a healthy margin to allow for the practical realities of dirty connectors and stressed cables, and avoid having a really bad day.
When to multiplex
The two chief technologies to concentrate more signals onto a single fiber strand are time-division multiplexing (TDM) and coarse wavelength division multiplexing (CWDM). Both add complexity and cost to the terminal gear relative to transporting, for example, one HD-SDI per fiber. In contrast to fixed installations where transmission is normally a short hop within a facility, fiber is cheap and the connectors are inexpensive, in OB the cost of the connectors, the cable and the labor to deploy a high fiber count cable becomes expensive.
In the case where a 12-fiber tactical battlefield-rated cable is required to link an announce booth position 6000ft from the production vehicle, one solution might involve four 1500ft TAC-12 cables, not counting associated patches and jumpers, terminated with either STs or military-style connector. If 5 percent of the connectors need cleaning or repair, statistically, there may only be eight working fibers out of 12 in a concatenated path before the troubleshooting begins, and that's when the overtime clock is typically running. (See Figure 1.) The largest single expense in deploying a high fiber count cable is the overtime spent correcting air gap problems.
In addition, connectorized cable assemblies are the items that take the most beating in the field and will frequently need repair and/or replacement. They account for a considerable percent of the total life cycle cost of the installation. Therefore, minimizing cabling costs is crucial, and the obvious solution is to use a smaller, less expensive two-fiber cable instead of the TAC-12.
By applying TDM and CWDM, fiber counts for large booth operations can easily be reduced to two fibers, typically one in each direction. (See Figure 2) In fact, CWDM technology, as used in powerful coax-to-fiber and fiber-to-fiber repeating wavelength managers, can enable a single fiber to carry the equivalent of 16 individual fibers. (See Figure 3.) This capability begins to exploit the promise of fiber having “infinite bandwidth.”
Smaller cables allow longer lengths on reels and fewer connectors in-line, each with fewer pins to maintain, resulting in fewer problems. From a practical point, a commonly available TAC-4 cable with four fibers can support the largest remote positions with 100-percent redundancy. In a pinch, even “trusty” SMPTE 311 hybrid cable can carry all of it well beyond its intended use as a single camera cable.
Here are some things to keep in mind when executing fiber in the field:
- Plan aheadKnow where all connections should be before deployment starts, and pre-label the cables accordingly. The crew should be able to make connections with confidence.
- Verify functionality before cables leave the compoundHaving both ends of the cable in one place makes verification easy. Putting 1000ft between the ends creates a project.
- Inspect deployed cables for stressTight bends and knots may not break the glass but may attenuate light signals to an unusable level. The use of tie wraps to dress the cables should not be allowed.
- Plan your link loss budgetCount the connections and know the lengths. Allow for dirt and lossy connectors, and then add 3dB for safe measure.
- Train the crewThe crew should understand the link loss budget. The key to a successful deployment of any gear is familiarity of how the equipment operates and what the indicators mean. If the equipment is to be used in a way that has not been tried before, experiment well before a critical event.
- Equip the crew properlyHaving inspection scopes, optical power meters and adapters will take some of the “black magic” out of the technology. If they can see it and measure it, they can understand it.
- Where possible, one fiber, one directionThis simplifies troubleshooting and minimizes sensitivity to back reflections.
Properly executed fiber in the field is a “just plug it in and it works” proposition. This is the promise of fiber technology and what OB operators should reasonably expect from their fiber infrastructure.
Eugene E. Baker is VP and CTO of Telecast Fiber Systems.
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