Deploying fiber for OB
Dec 1, 2009 12:00 PM, By Eugene E. Baker
The design considerations for OB infrastructures differ significantly from fixed installations.
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
Multipin expanded beam connectors make repeat matings easy and reliable. Flush, noncontacting lenses simplify cleaning compared with pin-and-socket connectors, which tend to trap dirt.
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.
Figure 1. Individual fiber failures in high fiber count cables make troubleshooting a time-consuming process. Fiber failures accumulate when multiple faulty cables are concatenated in series.
Select figure to enlarge.
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
Figure 2. In an OB booth application, two fibers can carry all the bidirectional camera signals, as well as associated HD-SDI video, microphone and return audios, intercom/IFB, Ethernet, and monitor video returns.
Select figure to enlarge.
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.
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