The design and planning of a fiber optic based A/V installation for professional A/V or Broadcast shouldn’t be a problem for the experienced system integrator or project engineer. But you need to keep in mind that working with fiber is different than working with copper distribution. Fiber has unique and beneficial characteristics that can only be exploited to your advantage if they are taken into account at the very beginning of the planning stages. Here are 10 essentials you need to consider when designing a fiber optic system for professional audio/video.
1) Start with a Clean Sheet of Paper
One of the most common mistakes made when designing with fiber in an A/V or Broadcast system is to first design the system using traditional copper products and design techniques and then substitute and shoe-horn into it fiber optic elements. By doing so, you restrict your use of fiber to the technology limits of copper. Fiber optic technology and products are capable of being implemented in ways that copper cannot such as bidirectional transmission and multiplexing multiple, and dissimilar baseband signals on one fiber core.
2) Think A/V not Data
When designing with fiber you may have to work with the facility’s IT staff or their consultants. Remember one thing: when it comes to understanding how baseband A/V signals are communicated and the implication it has on any fiber infrastructure, IT folks are from Venus and A/V folks are from Mars. Even though modern baseband A/V signals over fiber are transmitted as 1’s and 0’s it is not data in the same sense that IT folks understand data as IP based packets using various compression schemes. The biggest difference is that baseband A/V, including SDI, are transmitted on a one-way basis with no error detection or correction protocols. This results in a need for much higher bandwidth and lower bit-error-rate for the end-to-end infrastructure.
3) Determine the Type of Fiber to Use
There are two types of fiber optic cabling called multimode (MM) and single mode (SM) and they are not compatible with each other. If you are given fiber to work with and have no choice it is imperative to know whether it is SM or MM and if it is MM, what type of MM such as 62.5u or 50u. Knowing what type of fiber is being used will be needed in specifying the electrical to optical converters for the signal types being communicated and other ancillary equipment in the system. It will also be used in calculating budget losses and estimating the maximum usable cable lengths. If given the choice, it is recommended that SM fiber always be used for its lower cost, superior bandwidth and low loss.
4) Determine the Lengths of Fiber Cable Segments
It is important to know the lengths of the individual fiber segments, especially if MM fiber is being used on a particular segment. When transmitting uncompressed baseband video signals, such as HD-SDI, the bandwidth required will severely limit the usable distance of MM fiber due to dispersion. As a practical matter, this is usually not a concern with SM fiber in all but the most exceptional cases such as inter-city communications.
If you are forced to work with fiber segments of different types, such as SM fiber between buildings and MM fiber within a building, don’t worry. There are active and passive equipment that can be deployed to interface between different fiber types successfully. And some products work with both fiber types right out of the box eliminating the concern all together.
5) Minimize the Total Amount of Fiber Used
It’s no secret that the cost of installing any cable infrastructure can be the largest cost component of a new system. Many times designers focus on the cost of the fiber optic converter boxes at each end of the fiber segments. But, with SM fiber you have the bandwidth to aggregate signals of all different types to reduce the number of cores to a minimum. Also, unlike copper distribution, multi-point distribution can be done downstream and does not have to be done at the source. For example, if you have a source on the first floor of a building that has to be distributed to multiple points on the 40th, then run just one fiber between the first and 40th floors and then use a passive or active optical splitter on the 40th floor to the destinations on that floor.
6) Understand this Term: “Link Budget”
Fiber is not a loss-free medium. There is attenuation of the light as a function of the type of fiber used, operating wavelength and length of the fiber. “Link Budget”, sometimes called the loss budget, is the maximum attenuation of light between the optical transmitter and receiver that can be tolerated before the receiver ceases to work properly.
The manufacturers of the “black boxes” that are placed at each end of the fiber to convert the electrical A/V signals into and from light specify, or at least they should specify, the link budget for their transmitters and receivers. Additionally, the manufacturer might also specify the individual transmitter launch power and receiver minimum sensitivity. This is particularly important for SMPTE compliant SDI products where interoperability with other manufacturer’s product may be necessary in your system design.
Knowing the optical loss in the fiber you are using and the link budget between the transmitter and receiver you can determine if you will operate with enough optical power margin so as not exceed the link budget.
7) Determine the Optical Connector to Use
There is a wide range of common and specialty fiber optic connectors that are used. Different manufacturers of “black boxes” will offer some, but not all, of the many different fiber connector types. In general, we have found LC connectors used in high-density rack installations and ST connectors used in desktop, throw-down and just about anywhere else. In outdoor and temporary field applications, often time specialty ruggedized connectors like those available from Neutrik and Lemo may be used. The fiber cable itself is usually sold in bulk with connector terminations usually done on site at the time of installation. This makes cable pulls easier. However, almost any distributor of fiber cable will offer pre-cut and terminated lengths on request.
The important thing to keep in mind is that you cannot assume that the fiber connector you have, or will get, on the fiber cable will match the connector that is standard on the product from the manufacturer. Some manufacturers offer more than one type of connector on their products. So be sure to specify, and verify, the connector type for both fiber cable runs and the equipment that will be at each end.
8) Think Bidirectionally
The bandwidth and nature of optical transmission allows full duplex bidirectional transmission over a single core of fiber. In fact, using multiple wavelengths in each direction, a great deal of information and A/V signals can be communicated. Audio, video, Ethernet and data can all coexist bidirectionally on one fiber core. This saves a tremendous amount of money, not only on the cable itself, but on related construction costs as the size of conduits and cable trays become minimal.
Normally, we do not transmit A/V signals bidirectionally over copper, especially when coax is used. But with fiber this can easily be done and signal types can be intermixed and aggregated.
9) Budget and Buy an Optical Power Meter
Your project is going to be expensive. And chances are good that the people who will be tasked to install and make it work will be new to fiber or have little experience with it. Perhaps this is your first fiber optic project too. Here’s a tip to save you time when it comes time to make it all happen: buy a fiber optic power meter. They are not expensive and they can be obtained for just a few hundred dollars, but they are essential to any installation and can save you their cost many times over on just the first job.
Small, compact and easy to use, the “power meter” measures the optical power or “brightness” of the light at the end of a fiber cable segment relative to the power when it is “launched” at the transmitting end of the fiber. The wavelength of the light on the fiber is invisible so the only way to measure its strength, or know if it is there at all, is to measure it electronically with a power meter. Segment by segment you can measure the integrity of your system and also get a first-order determination if your transmitting boxes are working as they should.
10) If You Don’t Understand, Just Ask!
Fiber optics isn’t complicated, it’s just not always clear. Different manufacturers specify their products in inconsistent or incomplete ways that make finding the information you need frustrating. This, together with the “folklore” that surrounds fiber optic communications, can make the system design task somewhat frustrating. But don’t be afraid to ask for help from those in the industry that have many years of practical experience with communicating professional A/V signals over fiber in a wide range of applications. We are always happy to help.
If you have any questions, feel free to contact us at firstname.lastname@example.org or 1-631-273-0404. Alternatively, visit our website, http://www.commspecial.com for the latest product and educational information.