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Archived Articles
Fiber Optics Break Ground in Remote Field Production
by Richard A. Cerny
President, Telecast Fiber Systems, Inc.
Prepared for Broadcast Engineering Magazine March 1996
In the recent movie Sabrina, Harrison Ford illustrates his character’s business vision when he states, "I know it the way I knew that fiber optics would replace coaxial cable". This replacement has been steadily occurring in fixed TV broadcast communications, just as it has completely occurred in data communications and voice communications. The transition from coax to fiber is now taking place outdoors in remote production environments, and it is gaining momentum fast.
A few years ago you never heard of fiber optics being used as the primary cabling from camera sites to the production vehicle. Now you see fiber at all the grand sporting events— Olympics, Super Bowls, World Cup, Formula One racing. The same with entertainment events, like Woodstock and the Three Tenors in Concert, and news events, like D-Day coverage and papal visits. To appreciate where fiber systems best fit, it is important to understand fiber’s advantages and its limitations.
There are two main application areas for fiber in outside broadcasting. The first application is to feed the switched video and audio program to an uplink or to the demarcation point of a common carrier. This has been the predominant use for fiber in the field for the last fifteen years. The other, emerging application, is as lightweight "snakes" linking the cameras and microphones to the production switch. This is the area that has required so much technology development.
We already take for granted that the resultant program feed will be shipped cross country through a long distance fiber optic network, such as the one operated by Vyvx or by your telephone company. Instead of stringing long lengths of coax and audio pair to bring your switched program feed to your uplink, or to your carrier’s demarc point a thousand or more feet away, you simply carry a compact reel of fiber cable, and let it pay out behind you.
Fiber optic systems are essentially standard interface adapters that convert from electrical signals on copper, to optical signals on fiber cables, and back again to electrical on copper. They may be attached when desired, and moved from location to location, and equipment to equipment.
Fiber optic systems made their field debut back in 1980, in such televised events as the Democratic National Convention and the Winter Olympics. At that time, and until the 1990’s, broadcasters adapted conventional rack mounted systems by "suitcasing" them in shock mount units. While these systems offered the inherent advantages of fiber optic communications, they carried a lot of baggage with them. Today’s fiber systems make field teleproduction a much more convenient and reliable undertaking.
Those rack mounted systems still are the mainstay of fixed video and audio communications for studio-transmitter links, for campus and intrabuilding networks, and for medium to long haul telecommunications. Typically, such systems employ FM video with multiple audio subcarriers diplexed above the video baseband or, in long haul systems, one or more video channels digitized and multiplexed with their associated audio channels onto one optical fiber.
Like all fiber optic systems, they provide superior signal transmission and eliminate virtually all the problems associated with copper, including ground loops and hum, electromagnetic or RF interference, crosstalk, lightning problems, and high frequency rolloff. The newest generation of fiber systems couples these advantages with the overwhelming benefit of portability and weight reduction. The cable is more durable than ever, more so than coaxial or triaxial cables, and the electronics are housed in miniature enclosures built for field environments.
Since one slender (0.25 inch) cable can replace as many as six coaxial cables plus more than 64 audio pairs, every 1,000 foot section of fiber optic cable, weighing less than 15 pounds, can replace over a ton of copper. The labor savings in set up and strike time and effort is amazing. The savings in troubleshooting is even more impressive.
One of the lingering myths about fiber is that since it is lightweight, and since the "conductors" are made of glass, it must be fragile. On the contrary, today’s fiber elements are stronger than steel, and the cables are designed for battlefield use. Such "tactical" cables have proven to stand up to golf spikes, automobile traffic, and even the environmental horrors of Woodstock ‘94. The military tests are easy—they are quantitative tests conducted with specific sized hammers and mandrels. Broadcasting rigors are not—they involve random knots, slamming doors and forklift traffic. The proper fiber optic cable will survive most of these.
The field fiber cable will have a polyurethane jacket for crush resistance, not PVC. It should be tightly linked to the reinforcing Kevlar yarn, yet be strippable. You can test for integrity by taking a one meter length of cable, and try to stretch it will all your might. You won’t damage the fibers, but you want to see if the outer jacket wrinkles up when tension is released. It shouldn’t. Bend the cable into a small loop or knot, and again the jacket should be relatively smooth, not wrinkled.
What happens when a lawnmower or street sweeper cuts a cable—or, as ESPN experienced, a mountain goat eats a section? The cable can be quickly spliced temporarily to get on the air, or the ends may be reterminated with bayonet type ST connectors, then barreled together. In most cases, however, the one cable will have to become two shorter cables, so plan on buying a few inexpensive fiber barrels to link them together.
The ST type connector is the industry standard single-fiber connector that is commonplace in campus backbone applications. One ST is required for each end of each fiber, similar to a BNC on coax. With, perhaps, four fibers in a cable, if you use these low cost ST’s, you need to be aware of proper labeling and signal directions.
A new type of field connector is beginning to take over. It is the multipin, hermaphroditic military connector. Originally developed for the Army, this connector is more reminiscent of a triax connector, only more expensive. It has two male fiber contacts and two female contacts in the screw-on plug. The cable crosses over such that each fiber has a male contact on one connector end and a female contact on the other end. Thus, the cable cannot be installed in the wrong direction; there is no sexed end to worry about. Plugs can be directly interconnected together forming extension cables, and no barrel adapters or intermediate patch panels are needed.
Fiber connectors become very lossy when dirty, so they must be kept clean. You must always put the dust caps on fiber connector plugs and receptacles when they are not in use. When there is any doubt, clean the fiber contacts.
Reels can be the bane of a fiber optic field installation. With coax, you don’t expect the reels to last very long, because the cable is disposed of soon anyway. So you throw out the reel, too. But fiber cable will last many years, and it’s more expensive, so you have to care for it better. A reel can hold anywhere from 500 to 3,000 feet of tactical cable. If the reel gives out or falls apart, the cable can spill off sideways, and the untangling nightmare begins. Avoid a reel with flanges that are bolted through the hub—the nuts may fall off in transit, and there goes the cable. Also, avoid split reels or reels with external hangers for the inside course of cable. The connectors will find a way to flop, causing possible damage during deployment and take up. Use reels that have compartments in the hub (without axles) to store and protect the inside length of cable.
The cable may be pulled from a stationary reel or, as previously noted, payed out behind the reel as it is carried. Since there is not a multifiber rotary slip joint, the cable is deployed, then connected to the electronics. Otherwise the inner length of cable will twist with respect to stationary electronics. Plugs attach to mating hermaphroditic receptacles mounted on portable shell-type enclosures or on the panel of the production vehicle.
If a production vehicle is dedicated to the fiber system, the electronics can be mounted in standard 19" racks. Fiber pigtails lead from the hermaphroditic receptacle to ST type connectors on the rear of the electronics frame. The frame contains plug-in modules, each being either a transmitter or receiver for various types of signals—video, audio, intercom, etc.
At the camera location, the corresponding modules have to be housed in a more robust enclosure. Typically, this portable "shell" will have a hinged lid and be gasketed to provide environmental protection. The hermaphroditic connector will screw onto the externally mounted receptacle, and all electrical connectors (BNC, XLR, etc.) will be on side panels. Although the enclosure may be weather resistant, unused XLRs can leak, so its a good idea to bag the enclosure in inclement weather.
Speaking of weather, this is not office environment in terms of temperature, either. Both cable and electronics should be able to operate without fans or heaters in temperatures ranging anywhere from -40°C to +70°C. Military specs are even wider. The same unit has to be capable of televising both summer and winter events.
In fly-away applications, occasional rentals, or where flexibility is paramount, both ends of the system will use shell-type enclosures. Then all electronics and cable are transported in a single case suitable for checking as airplane luggage. News from Bosnia is reported over such fly-away fiber systems.
Modules may be reconfigured as event communications require. A common configuration is the 3-camera shoot. Typically in that setup, three remote cameras each send video (NTSC or PAL) plus two audios to the vehicle, genlock and two audio or IFB channels return. Two channels of intercom, camera control data and tally/call switch closures are transmitted in both directions.
In remote broadcasting, intercom is the tail that wags the dog. With 2-wire intercoms (RTS, Clear-Com, and Telex), power must be provided to belt packs from the portable shell, and fiber cables carry lightwave signals, not electrical current. Thus, the distant shell must be powered locally at the camera end. Power can be supplied by AC or batteries. A camera battery, like an Anton/Bauer brick, can power a fiber shell for five to six hours. A deep cycle marine battery will power it an eternity, and a battery is a lot lighter and easier to transport than a ton of copper. Inside the shell, rechargeable NiCad batteries serve as a 30 minute UPS in case external power is lost or has to be changed. (If the UPS takes over, an internal alarm may trigger, so it’s good to know where the mute switch is.) Also inside the shell is a DC-DC converter that steps up your 12 volt input to 24+ volts for power to a short series of intercom and/or IFB belt packs.
Video is transmitted by FM or digital techniques, since both offer high quality with extreme stability. Avoid using AM based video (sometimes called intensity modulated, or IM), since the optical attenuation of the cable dramatically impacts video level. Even with AGC, AM modulated video levels will jump all over when the cable is severely bent or driven over. Similarly, AM systems may have to be recalibrated every time a different cable length is used. With FM and digital, video levels remain steady regardless of cable length or condition.
In specifying video performance, you assure yourself of the highest quality signal transmission when you demand that the system meets or exceeds the "EIA/TIA-250-C, Short Haul" recommended standard. According to this standard, video signal-to-noise ratio should be greater than or equal to 67 dB, but some systems can easily achieve better than 70 dB. The better your originating signals, the better they will end up over the long distance network.
Video quality will fare much better if you avoid audio subcarriers diplexed above the video. So will audio quality when you avoid sync buzz from character generators bleeding through.
Ideally, audio is transmitted digitally, and you can get any number of audio channels multiplexed onto a single fiber. You can add audio channels in pairs by adding plug-in modules, or you can add larger increments of, say, eight or sixteen at a time in stand alone multiplexers. With digital transmission, audio will be flat from DC to 25 kHz, and signal to noise will hover around 100 dB. With subcarrier techniques, the best you will see is about 70 dB, and it is fussy. In many production applications, this is simply not good enough. Take, for instance, a remote effects mic in golf. When audio gain is cranked up to hear the club hit the ball, background noise becomes unacceptable. The viewing public is less tolerant of audio noise than it is of video noise.
Golf and auto racing are excellent examples of fiber optics in outside broadcasting. Applications include interconnecting the vehicle to remote cameras, RF towers and announce booths. Distances are long, and copper cables just don’t reach.
ESPN and NBC (through Mobile Production Services, a division of NEP) both use fiber as a staple for golf broadcasts, as do television networks around the world, including Seoul Broadcasting System. One reason, obviously, is speed and ease of set up, but other factors enter into the equation. Fiber outlasts copper. Triaxial cable punctures easily, and water mixed with fertilizers is terribly corrosive to copper shields. Fiber cable is relatively immune to punctures, since golf spikes just slip off the cable.
A typical booth link uses a four fiber tactical cable, fitted with a hermaphroditic connector, to bring three or four monitor video feeds to the announce booth, and a video back to the truck. It also brings over twenty microphone signals back to the truck, two separate powered IFB lines to the booth, two powered 2-wire intercom lines both ways, and data and switch closure utility lines in both directions. For maximum flexibility, cable sections may be (only) 1,500 feet in length, but more than ten such sections can be linked together to span five kilometers without any repeaters or amplifiers. All this over a cable that weighs less than 15 lbs per 1,000 feet. The equivalent copper cables, if possible, would weigh over 1,000 lbs per 1,000 feet.
In most cases, fiber optic cable is deployed at each event, and reeled up after the event is over. As the technology matures, however, we will see more facilities being pre-fibered, similar to the old way of preinstalling triax into stadiums. At that point, you drive up, plug in, and not have to pull in any cable whatsoever. The beauty of fiber, of course, is that it is capable of transmitting any and all signals, whereas triax is for cameras only, wire pairs are for audio only, etc.
Many non-sports venues are pre-fibered for video and audio, as well. The White House recently made this advance, and smaller facilities, such as the Texas State House has done the same. With fiber in place, nine Texas stations use fiber optic systems to walk up with portable units, and plug in without having to string unsightly cables through the beautifully restored edifice. Thank KVUE and Austin’s Preservation Board for having the foresight to develop and fund the advance in outside broadcasting.
As broadcast technology advances, fiber systems are expandable to accommodate the changes. Survey the industry, and you will find stand-alone systems or plug in modules that transmit digital audio (AES/EBU), digital video (CCIR 601), DMX-512 lighting control, Ethernet LAN extenders, and digital matrix intercoms.
What will the future bring? New adapters will be introduced to interface yet more types of signals—using the same fiber cables. One module will be a dial up telephone interface that permits telephone calls from the remote end of the link without having to rent expensive telco circuits to the camera or productions site. Another is a fiber optic adapter that completely replaces triaxial cable on existing cameras, sending all component program video, return video and genlock, audio, data, intercom and camera control data up to ten kilometers on one optical fiber (four cameras per fiber cable). This will have a monumental impact on metropolitan production networks.
Clearly, fiber optic systems will have to coexist with conventional copper cables for years to come, so it is a technology of interface adapters. Widespread use of fiber-only cameras may be a few years in the future, because you still have to use copper to access older facilities. But make no mistake, fiber has advanced well beyond the stage of a mere solution to an occasional transmission problem. It is now a key part of outside broadcasting equipment—one that is quickly replacing ordinary copper as the primary production link.
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