As more TV stations begin to broadcast in high definition, an area that must be given careful consideration is interconnection. For the most part, TV stations use coaxial cable for SD transmission, driving signals more than 300 meters at the SD data rate of 270 Mbits/s. Once the shift is made to HD, however, the data rate is raised to 1.5 Gbit/s. At this rate, the maximum transmission distance over coaxial cable drops dramatically to a range of 100 to 150 meters, and then only if high-quality coax is used (e.g. Belden 1694) along with a suitable cable equalizer at the receiving end.
The restrictive transmission characteristics of coax at HD data rates make the use of optical fiber transmission very attractive. Two types of optical fiber exist: single-mode and multi-mode. Single-mode fiber has a very small (9-micron diameter) center core, which is used to carry the optical signals. The narrowness of this light carrying fiber minimizes signal dispersion effects that limit the distance performance of wider core multi-mode fibers. Signal dispersion is literally the tendency of light to go in the wrong direction, and the smaller the core, the less likely this will happen. Thanks to its low dispersion characteristic, at the HD data rate of 1.5 Gbit/s, transmission distances up to 20 kilometers are readily achieved over single-mode fiber.
The worldwide adoption of a single-mode fiber-based solution by the telecom industry has seen many barriers of adopting high-speed optical networks considerably reduced. Widespread component availability at affordable prices now is a reality. A studio built today and equipped with current, off-the-shelf, single-mode fiber will have a network infrastructure that can physically support data rates from zero to in excess of 10 Gbit/s as well as the ability to support multiple signals over the same fiber. Such a level of “future-proofing” would be unthinkable with a coax solution.
The first decision to be made obviously will be whether to use fiber at all. For short distances, coax connection generally will be the choice. Snell & Wilcox manufactures a range of modular distribution products designed for the demanding world of high definition known as “System HD.” These are far from typical as they can drive up to 150 meters of good coax cable.
Most high-definition products are limited to approximately 100 meters. Even this capability will not get you very far in a stadium, TV station or even in a large studio. For longer distances, fiber makes sense and often is the only practical alternative. Other considerations include fiber’s immunity to electromagnetic radiation as well as its small size and light weight, both of which make it a fantastic signal transport medium.
When designing a fiber system, the specifying engineer needs to make some basic decisions. These are all concerned with counting how many signals will be carried and then plotting where they will be sent. A system will consist of some or many fiber transmitters and receivers. While the cost of fiber has decreased dramatically, the specifying engineer will want to conserve transmitters in the basic design.
The single-mode fiber transmitters in the Snell & Wilcox System HD modular enclosure can be supplied with one, two or four optical outputs like traditional coax distribution amplifiers. These multi-output variants are highly cost-effective for one-to-many connections. If the same signal is to be fed to two or more destinations, this is the way to go.
The next decision to be made when designing the fiber system is whether two signals should share the same fiber. Fibers are small, light and very easy to lay. So the first choice is to install enough to cover any requirements. In fact, many people install extra unused “dark” fibers just in case.
There are times, however, when it is just easier for two signals to share a fiber. This is where Wave Division Multiplexing comes in. By ordering transmitters with different wavelengths (colors) of laser light, a combining rear connector panel can join two signals onto one fiber. System HD’s dual transmitter can be equipped in this fashion.
A demulti-plexer at the receiver splits the signal back into two separate wavelengths. This splitter type unit is mounted in front of two ordinary receivers that can receive either laser wavelength, therefore requiring no modification.
One special variation of the two signals on a single fiber idea is duplex operation. In this configuration, two signals are passed in opposite directions down a single fiber. This is useful where, for example, a return path such as a camera and its return viewfinder feed must complement a video feed. System HD provides transceiver modules that can be configured for exactly this kind of operation. As with the previous dual simplex case, two wavelengths of laser are used, one at each end of the fiber.
Though optical transmission networks provide superior signal handling characteristics to coax networks for long-distance/high data rate applications, they cannot work miracles. A poor quality electrical input signal to an optical transmission system will result only in a poor quality optical signal being transmitted. Signals should be re-clocked prior to being placed on the fiber, and the system designer should bear this in mind. To simplify the design and installation, System HD’s optical modules equalize and re-clock any HD electrical input signals before optical transmission and then re-clock the recovered electrical signals before distribution.
Because they are so easy to install, fibers generally can be thought of as robust. Some environments, however, always will be more dangerous. The specifying engineer can consider a solution such as System HD’s “changeover” function. This has been built into the dual receiver module and allows for two identical optical feeds to be input to the module. The unit then can be switched between inputs or can automatically switch if one signal fail. The measured light level can drive this on the fiber, making for a secure redundant system.
Once installed, the user will be concerned with the correct operation of the fiber connections. System HD provides comprehensive monitoring of many factors affecting the continuing operation of the system. These can be read locally or remotely using the Snell & Wilcox RollCall equipment monitoring and control system. On an optical transmitter module, for example, monitoring includes board temperature, input signal quality, the presence of CRC (cyclic redundancy check) errors in the video, the line standard and frame rate being transmitted as well as the laser device health status. It also can estimate the length of coax feeding its HDSDI input.
Similar video diagnostics are available for the recovered optical signals in a receiver module along with a direct indication of the amount of light present at the optical receiver device. This particular feature is extremely useful both in the installation phase and general life of the unit. During installation, it will indicate whether the fiber and all its connections have exceeded the best practice for system design. Then, throughout the life of the unit, it will indicate the general health of the system.
Richard Schiller is senior product manager, modular products, and Spencer Barr is senior design engineer for Snell & Wilcox.
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