Andreas Hilmer /
08.01.2009
Originally featured on BroadcastEngineering.com
Fiber-based transport
Fiber cables can transmit data over long distances and at extremely fast speeds.

Broadcast productions and event installations are getting ever bigger. Throughout the last few years, those productions have gotten more complex than anybody could have ever imagined. Many different signals now need to be integrated into the infrastructure, which makes cabling a serious challenge. This is especially so when it comes to combining different signals and distributing them synchronously and in real time to various destinations. Furthermore, the areas where these installations are located have significantly expanded, making traditional cabling with copper time-consuming. Working with copper also means high transportation costs due the increased weight.

It's no surprise, then, that fiber-based transmission has become increasingly popular. The advantages over other technologies are apparent; the increased bandwidth and the greater distances that can be bridged with optical fiber offer huge benefits in costs, effort and weight. But there are more advantages to fiber-based signal transport, especially in installations in noisy electrical environments. These especially profit through the use of fiber because of its immunity against electromagnetic radiation.

Optical fiber cabling also has the advantage that it is able to combine different signal types. Broadcast and event installations particularly benefit from this, because audio and video signals can be distributed and routed together on a single fiber link without a problem, whether it is intercom, video, IT network or control data for cameras. By multiplexing digital signals, the amount of data that can be transmitted through a single cable run of optical fiber is increased by a large factor. In light of these benefits, equipment for fiber-based, real-time networks for signal routing and distribution have been introduced to the market.

How fiber works

At the beginning of a fiber, the transmitter converts any electrical signal, whether it is analog or digital, into an optical signal. To transmit analog signals, the electronic inputs of the transmitter are used to modulate the light source, while for transmission of digital signals, the digital pulses are converted to light pulses. A converter on the receiver side of the fiber changes the light waves back into electrical signals. Different signals can be transmitted simultaneously using different wavelengths of the light in the optical fiber.

The core of a fiber cable transmits the light information. The core has a diameter of 125µm and is clad with a material that keeps the light within the core using a reflective material on the inside. Most optical fiber uses light with wavelengths in the ultraviolet spectrum, which makes it invisible to the human eye.

More signals per fiber

Multiplexing offers a way to increase capacity. The different signals are converted into light data streams but with different wavelengths. Each signal is consolidated into a single stream with the help of a prism. On the receiver side, a similar prism splits the multiplexed stream into separate signals with different wavelengths. Depending on the quality of the fiber and the prisms, different amounts of multiplexed signals are possible. While wavelength-division multiplex (WDM) offers a bandwidth of up to three channels, coarse wavelength-division multiplex (CWDM) allows up to 18 channels, and dense wavelength-division multiplex (DWDM) provides for up to 180 channels on a single fiber. Because CWDM allows the use of cheaper fiber cables and transceivers, it is currently the most economical solution available. It provides a good balance between channel count and costs for fiber and multiplex technology.

To get the most benefit from the high capacities of fiber, different signal types can be combined onto one fiber link. One approach is to use a combination of electrical time-division multiplexing (TDM) and optical CWDM. Each mainframe — or “node,” to use a more network-related term — contains a processing card, which handles 16 4.25Gb/s high-speed ports. The idea here is to provide each carrier frame within this network with a bandwidth of 4.25Gb/s. To optimize the bandwidth usage of the carrier frame, the carrier is divided into subframes with 6.4Mb/s of bandwidth, which corresponds to the smallest signal to transport: AES3 audio. These subframes can be filled with any type of data, such as HD-SDI or SD-SDI video, MADI or AES audio, intercom, or control data. Each native signal is sliced into 6.4Mb/s segments. These slices are distributed to one or multiple destinations in real time. At the destination, the system recreates the native signal and, with additional software, can provide signal processing and conversion features at the outputs. Such a system can provide within a single node a signal router for 32 × 32 720p/1080i signals; 160 × 160 SD-SDI signals; 27,000 × 27,000 AES signals; or any combination of these.

Single mode vs. multimode

Being that there are different multiplex solutions, there also are different types of fiber. In total, fiber offers three different basic types, each with its own variants: multimode step index, multimode graded index and single mode. The fiber that allows the longest distances or the fastest connections is single mode. It can easily bridge up to 2km or provide speeds of up to more than 10Gb/s per wavelength. To achieve such performance, the lasers for this kind of operation are very expensive, which makes single-mode links far less cost-effective for shorter distances or where such high-speed performance is not needed. Multimode step index fiber, on the other hand, is mostly used in common consumer electronic fiber links such as in CD players or home entertainment technology. This link offers a transmission with high attenuation and low bandwidth. The most common fiber link that incorporates a good compromise of high performance and lower cost is the multimode graded index fiber.

Transmitting data on fiber

The power transmitted for any form of data is measured in decibels. As with audio volume, the decibel is a logarithmic and relative function. Similar to the transmission of audio, the receiver has to be fed with a correct signal level. Too high of a signal level could drive the receiver into distortion, with insufficient signal means, so not all the information arrives at the destination. Receivers, connectors and fiber cables all have an influence on the optical budget that gets transmitted through the fiber link. Depending on the type of application, different fiber solutions are used. For applications such as security cameras or simple speakers, a simplex connection is sufficient. For any link that uses bidirectional transmission, duplex fiber links provide the optimum cabling. Broadcasters can also use two simplex links for this application, but the costs are much higher compared to a duplex link.

Cables are available for any sort of application. They range from very light and thin to ruggedized types that are suitable for outdoor installation and are also available for underwater or buried operations. Furthermore, the number of fibers in one single cable may vary from manufacturer to manufacturer or even from version to version. As much as there are different fibers in one cable, the connectors also vary considerably, from heavy-duty connectors that can be installed underwater to light connectors that are mostly installed in benign environments and don't need to be changed often.

One thing is important for any fiber: Unlike copper cabling, even the slightest bit of dirt or dust can prevent the optical cable from functioning properly. Because of this, any installation that makes use of fiber must be set up with caution to ensure the system operates reliably.

Outlook

We are just at the beginning of a new era of fiber-based broadcast production infrastructure. What is already in use and common in wide area networks is now being considered for smaller applications like events, broadcast solutions and campus installations. These innovations are just the beginning of a new generation of broadcast technology that will open up completely new approaches to the way installations are planned, set up and operated.


Andreas Hilmer is director of marketing and communications for Riedel Communications.



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