Optical media networks
Live action, at its best, needs to convey all the immediacy and vibrancy of the event. Delivering an in-your-face game experience to savvy sports fans, who tune in expecting an increasingly high-quality experience from their large flat-screen televisions, is a primary goal. Yet so is efficiently and cost-effectively transmitting video and data from the field. Today's fiber solutions can deliver on these goals with high-quality, cost-efficient transport and new flexibility.
When compared to coax, optical fiber has many inherent beneficial properties: low loss ratios, virtually unlimited bandwidth and reduced sensitivity to electrical interference. This article will examine some of these advances when applied to a typical live sports broadcast.
The game plan
In our example, a broadcaster needs to originate a national game. The feed requires three field-located HD cameras, one stationary camera and two portable cameras. Two commentators are situated in a press box above the field. Each location needs to be linked and routed to a production facility and fed to a nationwide audience.
While the challenge may sound simple enough, a range of factors must be considered to obtain an optimal solution — one that delivers the highest quality, lowest latency and most reliability at a cost-effective price.
There are three basic options available:
- Option 1: Local on-site production using an OB truck. In this solution, fiber is used between the cameras and the truck.
- Option 2: Remote production via transmission over telco circuits. While cost can be a factor, this is a good option for secure, high-quality transmission and the choice for our example. (See Figure 1.)
- Option 3: Remote production via transport over a single dark fiber — if stadium infrastructure supports it. Unfortunately, the likelihood of an installed dark fiber connection is low.
Ultimately, the chosen solution design often depends on factors outside the designer's control, such as stadium capabilities, production equipment selected and then the broadcaster's infrastructure.
This scenario assumes that single-mode fiber is present at all camera locations and connects to a common meeting point such as a telco closet, or that fiber can be temporarily run to camera locations from a central meeting point.
The connection point
The first challenge is to connect cameras to the production site or truck. The camera/production connections must support bidirectional HD-SDI video with audio embedding, intercom and remote camera control through standard RS communication and GPI. This ensures that camera operators can view what they are shooting, and producers have complete control, command and communication with talent in the stadium box and on the field. A separate Ethernet connection for comprehensive data services is also desirable, along with the ability to monitor connections and switch audio signals as required. Finally, the cabling infrastructure can be greatly simplified and setup time reduced if all needed signals can be carried on one cable.
These requirements can be met by connecting each camera to a lightweight and weather-resistant portable housing near the camera position. A single optical fiber connection is then run back to the demarcation point. (See Figure 2.)
The multiple signals inside the remote box can be optically multiplexed onto a single fiber and brought back to a common location, such as a telco room, where all the remote sites are terminated. The optical signals are converted back to their native electrical formats prior to transporting over the telco network.
Using native format transport, the three-camera shoot will need:
- Nine HD-SDI circuits: Two HD-SDI signals from each camera location and one return signal to each camera location;
- Three Ethernet circuits: One to each camera location;
- Three or more data circuits: Multiple to each camera location.
Maximum multiplexing with DWDM
Once electrical signals are converted to optical, there are options for transport back to the OB van or production site. Our goal here is to get multiple signals onto a single optical fiber. With dense wavelength division multiplexing (DWDM,) up to 80 separate wavelengths can be multiplexed and transported on a single fiber. However, this application doesn't require that level of integration.
For ease of set up, environmental stability and cost, coarse wavelength division multiplexing (CWDM) is used to combine multiple optical signals onto a single fiber. With CWDM, multiple wavelengths, or frequencies, are wavelength-division multiplexed on one fiber, a process similar to RF multiplexing on coaxial cable. Multiplexing takes place inside the remote box, so only one connection runs from the box back to the production truck; all bidirectional optical signals are carried on one fiber-optic cable.
Calculating the optical link budget
Calculating the distance an optical signal will travel requires the determination of several key factors: optical output power and optical frequency of the transmitter, the optical receive sensitivity or optical range of the receiver, the length of fiber plus connection points, splice points, and passive optical devices such as optical multiplexers and splitters. Typical fiber-optic video transmitters provide an output power of 0dBm. Optical video receivers may have a receive sensitivity of -30dBm. In this example, the optical link budget is 30dB.
The common wavelengths used in optical video transport equipment are 1310nm and 1550nm. For design purposes, it is also common to use the following signal loss over a piece of SMF-28 fiber, which is the most common type of fiber deployed for these applications:
- With a 1310nm wavelength, the loss over fiber is 0.35dB/kilometer and typically takes fiber splice and patch panel losses into account.
- With a 1550nm wavelength, the loss over fiber is 0.25dB/kilometer and typically takes fiber splice and patch panel losses into account.
Given the 30dB link budget, a 1.5Gb/s HD-SDI signal can travel:
- 85km @ 1310nm
- 120km @ 1550nm
Using 60mi for this example:
- Convert 60mi to kilometers (1.61km/mile): 60 × 1.61 = 96.6km
- Total loss @ 1310nm: 0.35dB/km × 97km = 33.95dB
- Total loss @ 1550nm: 0.25dB/km × 97km = 24.25dB
- (miles × 1.61km) × 0.35 = fiber loss @ 1310 nm
- (miles × 1.61km) × 0.25 = fiber loss @ 1550 nm
The calculations show that an optical transmitter with a 1550nm wavelength will be needed to traverse the required distance while remaining within receiver's acceptable range of sensitivity, -30dBm.
Fiber best practices
There are some important best practices to remember when working with fiber. While many are obvious, like minimum cable bending radius, other seemingly minor ones can bring an installation to an immediate halt.
For instance, the remote box selected here provides a weather-resistant, dust-proof optical connector interface, but patch panels typically have LC, FC, SC or ST fittings. Mating the two different cables requires a hybrid fiber-optic jumper featuring two different connectors. The need for jumpers is often overlooked in the design phase, so be sure to have several spares in your toolbox.
When using fiber jumpers, it is important to clean the fiber mating surfaces. I recommend a device such as the CLETOP fiber cleaner. It is also good practice to clean new fibers when unpackaged. Always cap the ends when not in use and never look into a piece of fiber, as you never know where the other end is or if light is being transmitted through it. An optical power meter should be part of any toolbox to read optical power levels. Be sure your optical power meter has multiple connector adaptors for SC, LC, FC and ST fittings.
Fiber is often considered costly to install and maintain. Yet, with so many different configurations and connector types now available, the cost has never been lower. Anyone who has purchased copper cabling in the past two years knows of the skyrocketing price. Yet the cost of fiber-optic cable and components continues to steadily decrease.
Glass does not deteriorate, and as technology advances, glass will remain totally video standards agnostic. A well-designed fiber-optic installation will reduce system cost and complexity, improve reliability, and remain easy to maintain. Optical components also have long life spans, predicted at 20 to 30 years or more. The key to a winning strategy and execution include carefully considering long- and short-term goals, existing infrastructure, and smart options that take into account the current state-of-the-art technology and equipment. When you do, fiber optics will likely figure into your system.
Bill Bachmann is senior systems engineer at Nevion.
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