The State of the Art in Fiber

Fiber has been around the broadcast and media industry for a long while: SMPTE 311M, which defines fiber camera cables, was published 20 years ago in 1998. Engineering attitudes have tended to settle around the fundamental pros – high bandwidth, low attenuation so long cables – and cons – difficult to splice connectors and fiber may require optical/electrical converters. It is time to update those attitudes.

Fiber is now a mature solution and standard working practices have been developed to support the use of fiber to transport today’s media signals. Much more important, there are real drivers forcing the industry away from SDI over coax and towards fiber.

While it is possible to move 1.5GHz HD signals down coax, transmission lengths are limited. With growing interest in 4k Ultra HD, fiber becomes the obvious solution.

The second key driver, of course, is the move away from point-to-point connectivity using SDI. Anyone planning a major new installation today is likely to be designing it around IP connectivity and network switching.

Broadcast facilities will soon look more like a data center. It makes sense, therefore, to adopt some of the successful techniques used in data centers, and some of the proven products and technology that is already available.

There are two common broadcast uses for fiber, and they need to be considered separately. The first application for fiber is in fixed installations. The second is for temporary installations. Outside broadcast trucks and touring shows need to balance convenience with robustness.

Fixed Installations

When designing a new fixed installation, the engineer needs to consider bandwidths of up to 40Gb/s per cable. Standards have been set for 100Gb/s and 400Gb/s ethernet, and it makes sense to plan for these future high data rates, if only by installing appropriate cables. This is made easier with the IEEE 802.3ba-2010 standard for physical layer architecture, which harmonizes cable types and lengths across data formats.

A key part of any broadcast or media fiber design is whether to use single-mode or multi-mode cables. Single-mode fiber operates at wavelengths between 1310 and 1625nm, with a 9µm center core, and is almost limitless in bandwidth. Multi-mode fiber operates at 850 to 1300nm with a center core of 50µm.

Why plan for high bitrate capacity like 400Gb/s? The answer is because even if individual signals never get close to this data rate, trunk traffic between switches certainly will. In a broadcast center, a studio might have its own switch or switches, which in turn will transmit over trunk cables to the data center. Those trunk cables could be carrying 20 or more 4K cameras and 80 microphones.

Connectors

Connectors are called MPO (multiple fiber push-on/pull-off), but are also referred to as MTP, the brand name of a high-quality, low-loss connector from US Conec.

The essence of fiber optic connectivity is that the data travels through the cable as pulsed light which is internally reflected within the light guide at the core of the fiber. Connectors need to ensure as much of that light is transferred to or from the device: insertion loss is as relevant in fiber as it is in copper.

Connectors rely on a physical contact between the end of the cable and the receptor in the device. The end surfaces of the cable at the connector are polished to a slightly convex finish, with the apex centered on the fiber to achieve maximum light transfer.

With the micro-dimensions involved here, terminating connectors to fibers requires precision equipment and a controlled, clean atmosphere. For this reason, cable assemblies are best fabricated in clean rooms rather than on site. Specialist cable suppliers have appropriate facilities to do this.

Some cables will be made to length in the factory and shipped complete. Where reasonably accurate measurements can be quoted, and access to pull the cables in is easy, this is the best solution.

Virtually any type of connectivity application is supported in a fiber version.

Virtually any type of connectivity application is supported in a fiber version.

The alternative is to buy bulk fiber and connectors terminated to pigtails. These are spliced on site, normally by specialists who have been trained in the use of the equipment.

Patch Bays

Most installations rely on some centralized distribution point, often a simple patch bay. Fiber termination panels and patch panels are now widely available.

One persistent concern about fiber is that it is not sufficiently robust to be used in patching. Fortunately, cable manufacturers have developed fibers that are extremely resilient for this application. More than a decade ago Argosy led the way by offering a cable called BendBright which contains G657 bend-tolerant fiber. The cable can be bent tightly around a pencil while still transmitting a perfect signal.

Engineers who have been brought up with system designs that depend upon patch panels for occasional routing decisions should be reassured that they can do precisely the same in the fiber era.

Patch cords can also be used to connect multi-fiber termination panels to devices. One of the challenges here is that there is no real standardization on the form factor of the connector. Facilities should keep a stock of converter patch leads for various popular connectors. These adapters are readily available and inexpensive.

Outside Broadcast

Building an outside broadcast system, or the infrastructure for a touring show, follows the same design principles as a fixed installation. A key difference is the mechanical stress the fiber cable endures with repeated installations and then removals.

Camera vendors have largely standardised on SMPTE-hybrid cables. These connectors provide dual fibers for signals along with power to the camera over copper conductors. These cables have excellent armouring.

For other applications, tactical assemblies are available which are specifically designed for the rigors of the road. These add three critical features.

First, and most obvious, the outer jacket protecting the fibers is much sturdier than that used for fixed installations. The armour may also include Kevlar to mechanically buffer the internal fibers. It is never a good idea to leave cables where they can be stood on or driven over, but adding mechanical strength provides a degree of reassurance when out on the road. These features combine to increase protection to the fiber cables that must survive a rigorous life on the road.

Second, outside and mobile fiber cables come with protective end caps. Always use them. Dragging an uncovered fiber connector through the mud of a rugby league ground, or even across the floor after a rock gig, is likely to reduce its future ability to transmit light. The sealing caps are vital and a road crew should always be sure they are correctly fitted before rigging or derigging. Technical crews should also be supplied with the appropriate fiber optic connector cleaning and inspection tools and be trained to properly use them.

Finally, a good tactical assembly for the road should include a means of pulling the cable without yanking on the connector. A captive pulling sock will allow even the longest cables to be dragged safely.

Properly handled, fiber cables are resilient even for the tough environments of outside broadcast. If there is damage, it is likely to be at or near the connector because that is where most of the strain occurs. While fiber cables can be repaired in the field with splicing kits, relying on high-quality tactical assemblies that minimise the risk of damage is a better solution.

In summary, fiber already is the connectivity standard in the IT industry. With the broadcast industry’s move to IP connectivity, it makes sense to adopt a fiber solution to support tomorrow’s high-bandwidth broadcast and media signals.

Josh Simons is director, Argosy Cable.

Josh Simons is director, Argosy Cable.

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