Fiber-optic transport

As the demand for the transport of high-resolution video at higher bandwidths increases, the use of fiber in facilities will be more and more prevalent.
Author:
Publish date:

I t's been almost 30 years since the first commercial use of fiber-optic cable for the transport of a broadcast television signal. In 1980, broadcasters of the Winter Olympics, in Lake Placid, New York, requested a fiber-optic video transmission system for backup video feeds. Because of its quality and reliability, the fiber-optic feed soon became the primary video feed, making the 1980 Winter Olympics the first use of fiber optics for a live television production in history.

Despite the long history of the use of fiber-optic cable as a means for transporting video and audio signals, it is amazing that resistance still exists. Production studios are often limited in size to support video transmission over traditional copper coax or twisted pair. The control room is designed to be as close as possible to the camera positions. In the short term, this approach reduces some of the system costs, but the strategy can prove to limit the future of a permanent installation. In the past few years, the proliferation of HD-SDI, 3G HD-SDI and DVI formats has exceeded the limits of copper coax and twisted pair.

There are many applications in electronic newsgathering and sports where fiber is unavoidable. Fiber cables for video transport are widely used in these venues. Many broadcasters and systems integrators will use fiber only when absolutely necessary, only to spend more money to upgrade a system later when a new higher-resolution video format arrives.

Benefits

There are many reasons to use fiber. Early adopters of fiber-optic transport are now enjoying increased bandwidths for the new 3Gb/s HD-SDI and DVI dual-link formats. Users are limited to less than 100m over coax for 3Gb/s HD and 10m over copper for DVI dual link. Broadcast facilities are requiring the transport of 1080p 3Gb/s HD, and DVI single- and dual-link are used in just about every control room with a multiviewer or monitoring.

Users who have installed fiber for their SDI and HD-SDI infrastructure are now ready for the transition to 3Gb/s HD. Those who installed fiber for their RGBHV infrastructure can now easily upgrade to DVI.

In addition, fiber offers better signal quality over longer distances and better noise and interference immunity. Single-mode fiber provides virtually infinite bandwidth to future-proof your system. With all its benefits, fiber is affordable. Explosive construction and expansion in Asia has skyrocketed the cost of copper, while the cost of fiber-optic cable and components has steadily decreased.

A misconception is that fiber is expensive to install and maintain. Fiber-optic cable is available preterminated with any connector type. Bulk fiber can be purchased and pulled through conduit. The connectors are then fusion-spliced with preterminated pigtails using inexpensive slicing gear. Field installations and repairs can be achieved using epoxy-less connectors. A technician with basic skills terminating coax with a BNC can terminate an epoxy-less fiber connector.

Fiber-optic cable is available in many configurations for most applications. Cables are available with multiple fibers from one strand to hundreds of strands. Fiber-optic cable is available in configurations that meet all building and installation codes, including plenum-rated fiber for installing fiber cable in a plenum space; riser-rated cable that supports its own weight for vertical runs through walls in tall buildings; and tactical and armor-rated fiber cable, which is available for outdoor and military environments.

There are many applications for fiber-optic communications. Any application that requires high-bandwidth or high bit-rate communications is ideally suited for fiber-optic transport. Television and video applications are perfect examples. Analog television has a relatively high-bandwidth signal of more than 5MHz. Digital television or HDTV has bit rates of more than 3Gb/s. High-resolution DVI computer graphics can have bit rates exceeding 10Gb/s. The following is an examination of fiber's applications.

Broadcast television transmission

Television production and broadcast engineers have always sought out the best technology for media events such as the Olympics. In the 1980s, fiber-optic transport was introduced to the television industry. Today, fiber optics are used in all aspects of production and distribution of video and audio signals.

With the introduction of digital video in the 1990s, fiber-optic transport continued to enjoy growth in the broadcast industry. Digital video was encoded into 144Mb/s to 360Mb/s. These high bit-rate video signals could only travel over copper up to about 300m. A transport distance beyond 300m required fiber.

The transition to HDTV has created a need to transport signals with a bit rate as high as 3Gb/s. HDTV or HD-SDI in its native or uncompressed form is 2.97Gb/s, which can only reach about 100m over coax. Once again, fiber is the only choice to reach distances beyond 100m.

Systems can be designed using many of the technologies described above. We can mix analog and digital signal transport. We can combine signals using time-division and optical multiplexing.

A broadcast television station may typically reside in a downtown metropolitan area, while the television transmitter and satellite up and down links may be on a distant mountaintop outside the city. This situation is a perfect application for fiber transport. The system may require both analog video and DVB/ASI digital video, because the station might be in the midst of its conversion from analog to digital broadcast. It will also require signals in both directions to support downlink satellite video.

Another application is where many channels of video and audio are combined together over one fiber for backhaul feeds, cable television, common carrier or telco. The system uses time-division multiplexing (TDM) to combine groups of eight channels of video with audio into single wavelengths. The optical multiplexing (CWDM) technology is used to combine the wavelengths with groups of eight videos onto one fiber. The combined technique of TDM and CWDM provides a fiber transport capacity of more than 144 video channels on one fiber.

Fiber-optic video networking

Solutions are available to distribute video over large networks. The current trend is the transport of video, audio and data over an IP or Ethernet network. These systems require compression and a high-bandwidth infrastructure to move many signals at one time. IP systems can have issues with compression artifacts and latency due to limited network bandwidth, but they are simple in design and configuration.

Systems are available that offer the same simplified design and configuration, but in an uncompressed, real-time, high-quality video transport. These new systems transport and distribute uncompressed video, audio and data over fiber. This new technology drastically simplifies the architecture of a fiber-optic network, reduces equipment costs, simplifies design, and maintains uncompressed, digital, broadcast-quality video from end to end.

Fiber-optic transport technology has numerous applications in sports, ENG/SNG, field and studio production, broadcast, cable, satellite, and the studio to transmitter link (STL).

Figure 1. The fiber receivers can be integrated with the multiplexer into a fiber hub to save rack space.  Click to enlarge

In a traditional system, there are multiple fiber-optic transmitters at each video source. The fiber-optic signals are then typically routed to a central location or node. At the central location, the fiber-optic signals are converted back to analog video, audio and data. Then up to eight channels of video, audio and data are combined once again using TDM into a single fiber-optic signal. Two TDMs are required to transport 16 channels of video over one or two fibers. The equipment at the central node typically occupies 8RU to 10RU. The 16 channels of TDM video are then decoded at the receiving end using three or more rack units of equipment.

A new fiber transport system reduces the equipment required at the central node from 8RU to 10RU to 1RU or 2RU. (See Figure 1.) As in the traditional system, a fiber head is used at the source location to encode the video, audio and data onto one fiber. At the central location or node, the 16 fiber-optic receivers and two TDMs are combined into one device called the fiber hub. The fiber hub supports up to 16 fiber-optic inputs from up to 16 video fiber head units. The fiber hub unit has one high-speed, fiber-optic output for the main fiber.

The equipment complexity and size is also reduced at the destination point from 3RU to 4RU down to 1RU to 2RU. Signal quality is maintained by digitizing once as opposed to digitizing twice. The video, audio and data signals are digitized once at the fiber head and then are decoded back to an analog signal at the fiber receiver hub.

The technology is ideal for a head to hub or star configuration. Another configuration is a self-healing ring. The system can be designed using a combination of the head to hub and self-healing ring topologies. (See Figure 2.)

Optical repeaters and distribution amplifiers

There are applications in fiber-optic communications where a signal requires regeneration and replication. The function required is similar to that of a distribution amplifier or digital signal reclocker. A passive splitter can be used to split an optical signal, but each signal is significantly weaker after the split. A device called an optical repeater or distribution amplifier can be used to repeat or regenerate a weak optical signal. This is helpful on long fiber-optic runs where a fiber signal is reaching its limit, because the repeater can be used to regenerate the signal for further distribution.

The same device can be used to replicate an optical signal. One optical signal can be replicated up to 16 times with one device. Unlike the passive split where the optical output is diminished, the output optical signals are regenerated to full optical power.

The device can also be used as a mode converter or wavelength re-mapper. The device can be configured with a single-mode input and multimode outputs. This gives the ability to convert from multimode to single-mode or from one wavelength to another wavelength. The device can convert an optical signal to CWDM wavelengths.

Fiber-optic routing switchers

Most broadcast and audiovisual systems today have a video and audio routing switcher. The switcher gives the user the ability to control the source and destination of a given video and audio signal. As more and more video and communications migrate from copper to fiber, there will be a need for an optical routing switcher. The optical routing switcher is a new concept for the video market, but it has been used for many years in the telecommunications industry to route and control telephone traffic.

Optical switching starts to make more sense as the complexity increases with dozens of different video and encoding formats. If all or most of our video information is in the optical domain, why not switch in the optical domain?

In broadcast or video applications, there may be analog video, component video, SDI and HD-SDI. To switch all these signals, we would need a different switcher for each type or format of video. If we transport signals in the optical domain, we will have to convert back to electrical to switch, and then back to fiber after the output. If we switch optically, only one switch is required because an optical switch can switch virtually any format signal in the optical domain. There are two basic types of optical switching: photonic fiber-optic and electro-optical.

Photonic fiber-optic switching is 100 percent optical switching using 3-D MEMMS technology, which uses electronically controlled mirrors to route optical signals. This type of switch has an optical input, an optical crosspoint and an optical output. The abbreviation for this technology is OOO. An OOO switch provides only point-to-point switching. One input cannot be multicast to many outputs, because the mirrors cannot point to more than one output at a time. The use of mirrors does permit multiple wavelengths in both directions.

Switches are available in sizes from 8 × 8 to 256 × 256. Pure optical switching is available for multimode and single-mode applications and supports both analog and digital optical signals. Photonics switching has virtually infinite bandwidth.

An electro-optical switch uses a hybrid approach. The input is optical, the crosspoint is electrical, and the output is optical. The abbreviation for this technology is OEO. An OEO switch supports point to multipoint or multicast switching. Any input can be switched to every output, if necessary. Because the optical signal is converted to electrical, only one wavelength can be switched at a time. Also, an electrical crosspoint only operates in one direction; therefore, only one wavelength in one direction is supported.

Electro-optical switchers can be configured for 4.25Gb/s and 10Gb/s bandwidth, which will future-proof system designs to 10Gb/s. Broadcasters that purchased an optical switch a few years ago for SDI or HD-SDI can easily upgrade to 3Gb/s HD-SDI today.

An optical switch supports a wide range of formats, from 19.4Mb/s ATSC through 3Gb/s HDTV, as well as NTSC, PAL, SECAM, SMPTE 259M Serial Digital (SDI) Video and many more. Optical switcher technology can be used in the field to support applications requiring reliable, high-quality video distribution, such as mobile production trucks, sports venues and professional video facilities. Optical layer protection and fault-tolerant switching can be configured for mission-critical, nonstop applications.

Optical switching is extremely cost-effective for any application requiring 32 or more switched optical ports. It eliminates the need for expensive video transceivers to convert signals between electrical and optical formats. Switching the signals in optical format can save thousands of dollars per port in fiber-optic transport equipment costs.

The future

Systems are currently in development for the transport of high-resolution video at bit rates exceeding 40Gb/s. Digital cinema and the proliferation of HD television will demand fiber-optic transport systems with high-bandwidth capabilities.

Fiber transport to the home for video, telephone and Internet traffic is slowly becoming a reality in many North American communities. This will fuel the demand for high-speed content delivery and distribution throughout the globe.

Jim Jachetta is senior vice president of engineering and product development at MultiDyne Video and Fiber Optic Systems.