When it comes to the world of IT infrastructure, “category cabling” (Cat5, Cat6, etc.) is undoubtedly king. It is cost effective and easy to terminate, and its ubiquity only leads to more ubiquity in the marketplace. That’s why manufacturers have found ways to use twisted pair cabling for the distribution of audio and video, whether that is over the network or for “point-to-point” distribution from a transmitter to a receiver.
However, there is another network cabling technology, fiber, that is also available, and it provides more bandwidth over longer distances than twisted pair. It is less prone to electromagnetic interference and the cable’s use of light vs electrical signals makes the technology more resistant to on-cable signal “sniffers” (making it a popular choice for government installations in particular). For those who can afford the cabling costs, fiber is an excellent choice for audio and video distribution. However, to use it effectively in AV system designs, it is important to understand the various choices available for distributing over fiber. The first and most important of these is the selection of the type of fiber cabling you want to use.
There are two types of optical fiber: single-mode and multi-mode. While people sometimes confuse single vs. multimode with the number of fibers required, the actual difference is in reference to the size of the glass center of the cable. To understand the differences, you first must understand how optical fiber works. Optical fiber is a very thin strand of pure glass, which acts as a wave-guide for light rays over long distances. Single-mode and multi-mode fibers differ in the way they construct these glass pathways to distribute data. Both cables guide light down the center of the fiber called the “core”. However, the fundamental physical difference between single mode and multimode fiber is the core size, which has a direct impact on the most fundamental features of fiber: distance and speed.
The size of the glass core of a fiber cable has a direct relationship with how light travels down the core. When light travels, it travels in several directions (called “modes”). The core on multimode fiber is wide enough that the light distributes in multiple “modes” at once, meaning the light “bounces” along the sides of the cable, causing more of the light to bleed out into the cladding around the core. As the different modes of light “bounce” along the sides of the core, the “bouncing” light (the transverse modes) slow down, causing “modal dispersion.” This “modal dispersion” is similar to how indirect sound reflecting off walls or the floor is delayed (or echos) from the source sound, something we’ve spoken about before . The result of the light bleeding out and the modal distortion is that the signal degrades as the light travels along the cable. Multimode fiber can distribute data far (up to 300 meters depending on the signal type), but it could travel much farther without the modal dispersion.
This is the main point behind single mode fiber. Single mode fiber has a very small core (only 9 microns vs 50-60 micros for multimode) that allows only one “mode” of light to travel down the core. This means less light bleeding into the cladding and no modal dispersion. Thanks to this narrower cable and a thicker cladding, there is nowhere for the light to go except straight down the line. The result is a cable that can carry data, including video, up 10 kilometers from the source.
However, because single-mode fiber cores are so small, the technology required to make it work must be small as well. This makes the cabling and other elements (light emitters, detectors, etc.) in a single-mode fiber system more expensive. Because the signal length of multimode fiber is still sufficient to transverse most buildings, the more cost-effective multimode fiber is more commonly used in commercial building installations, for example.
So how is fiber used in audio and video distribution? First, you must decide if you are wanting to use fiber for point-to-point distribution from a transmitter to a receiver (or transmitter to matrix AV switcher to receiver), or if you are wanting to distribute over a fiber IT network. If you are distributing audio or video over an IT network, fiber only matters to an AV designer if you are connecting fiber at the endpoint (the encoder or decoder). Otherwise, it is a larger network discussion (of which fiber would be only one component). If you are connecting twisted pair at the endpoint, the backbone being fiber does not affect the design (other than bandwidth considerations, etc.). Of course, some encoders and decoders, such as the AMX SVSI N2000 4K Series, have both fiber and twisted pair connections, so they will work with whatever network design is available.
It is important to note that even with the higher bandwidth of fiber installations, networked AV systems are still encoded, which means there is some amount of compression involved as well as latency as the signal is encoded, transmitted and decoded. The latency is imperceptible (less than 10 milliseconds for AMX SVSI N1000 Series) and the video quality is excellent and suitable for most applications. However, very sensitive situations (such as government installations) requiring fully uncompressed non-chroma sub-sampled 4:4:4 video will want to go with point-to-point solutions. AMX Enova DGX 100 Series Digital Media Switchers have a propagation delay of 4.8 microseconds (not milliseconds but microseconds) through the transmitter. That’s completely uncompressed pixel-for-pixel video over a distance of up to 10 km with a delay of only 4.8 one-millionth of a second.
If you do choose point-to-point distribution over fiber, then that is when single mode vs. multimode comes into play for the AV system designer. In an AV distribution design, there are different fiber options for single and multimode fiber. The AMX Enova DGX line offer both single mode and multimode connections in 4-port hot-swappable input and output cards. System designers can then select the appropriate combination of fiber cards (along with the accompanying transmitter or receiver) to meet their needs—as well as twisted pair cards, local AV connection cards etc.
Typically speaking, system designers will use multimode fiber to connect different AV systems or devices in a single building. They will then use single-mode fiber to connect multiple buildings in a campus together, leveraging the extremely long range of the single mode fiber. This creates a wheel-and-spoke pattern that allows an entire campus of multiple buildings to be interconnected.
One final point to consider when designing a fiber-based AV system is simplex vs duplex, which addresses the issue that fiber products are commonly designed to send signals in one direction. In the world of point-to-point distributed AV over fiber, simplex systems only have one cable that sends video from the transmitter to the receiver. Duplex systems, on the other hand, have two cables for bi-directional communications. This allows duplex systems to offer Ethernet connections at the endpoints and bi-directional control and monitoring of connected source and display devices.
Simplex is often used in government markets where they have video going from lower classification levels to higher classification levels. In AMX Enova DGX fiber solutions, a single fiber can pass audio and video—including HDCP content—from the source to the endpoint. Simplex solutions do not include a second cable for two-way communication. Because the data is going only one direction in a simplex solution, that comprehensively eliminates the possibility of data back-feeding from a higher classification level to a lower classification level.
No matter which solution you choose, fiber provides an excellent, secure way of providing high-bandwidth distribution of audio and video across long distances. Have any tips for using fiber in audio and video installations? Let us know in the comments.