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10 Tips for Better AV Systems

We polled manufacturers, integrators, and end users to help identify lingering pain points and asked them for quick solutions and insights. Depending on what you're working on, you should be able to find a handful of these tips worth clipping out and stuffing in your back pocket for future reference.

Audio Over Cat Cable

It's not widely known that you can send balanced-line audio over category cables, though doing so is actually fairly simple. In standard shielded twisted pairs, most of the noise rejection comes from the fact that the pair is twisted. Rejection of noise by the twisted pair is called Common Mode Rejection Ratio (CMRR). And because the pairs in category cable are extremely precise, they have extremely good CMRR.

Category cables have a foil shield for RF protection, but foil shields don't start to be effective until 10 MHz or so. Braid shields go down to 1 kHz; below 1 kHz, there is no shield that has any effect on noise or interference. At that point you're entirely dependent on the twisted pairs–and CMRR–to get rid of noise.

In addition, the best category cables have bonded pairs, in which the two wires are affixed together along their longitudinal axis. In this configuration, no matter how you bend the cables, the two wires stay right next to each other, maintaining consistent concentricity to maintain good CMRR. Thus, the limit of noise reduction on that pair is the balance (CMRR) of the source and destination devices.

The only time that a category pair won't help is if you need a ground wire to deliver something besides just shielding, such as phantom power in microphones, or in some intercom systems that use the shield as the return path for the audio. In that case, unshielded twisted pairs (UTP) won't work. But if all you have is audio, then you only need two connections (pins 2 and 3 in an XLR) for the signal. Put in a pair of Cat-5e or -6, or -6a wires, and listen to the music.

Powered Over Ethernet

Distributing power over an Ethernet cable (PoE) can help simplify the installation of devices that must be installed someplace where there isn't an AC outlet–for example, in a plenum, over the span of numerous digital signs, or outdoors. But the more devices you need to drive using PoE, the more planning you should do. If you have multiple AV systems you're running power to, consider a PoE switch. It may be more cost-effective than individual PoE injectors. Just remember that almost all PoE switches incorporate a fan, so if you use a PoE switch, take into account the audible noise it might give off.

There are also a variety of multiport injectors–devices that initiate power down multiple UTP Ethernet cables–including 6- and 12-port versions. Keep in mind, however, that multiport injectors offer less flexibility in installation design because they can be used only where all the devices connect back to the same wiring closet or switch.

Even devices that weren't designed for PoE can be powered via Ethernet using an adapter, or tap, which converts PoE to DC. There are two basic types of taps depending on your needs. A passive tap takes the voltage from the UTP cable and sends it to the equipment without adjustment, so if 48 volts of direct current (VDC) are injected into the line, 48 VDC will be delivered to the device. A regulated tap takes the voltage on the UTP cable and converts it to another voltage, including 12 VDC, 6 VDC, and 5 VDC. In most cases you'll want to use a regulated tap.


AMX PoE injector

Bonus! Calculating the Usable Bandwidth of a Fiber-Optic System

When designing an AV system that operates over fiber-optic cables, you have to take into account the fiber's usable bandwidth so you know whether it can support both your AV needs and everything else that might run over the line. In general, multimode fiber suffers from greater signal loss and less bandwidth than single-mode fiber, so calculations are key.

Most manufacturers of multimode fiber-optic cabling do not specify the dispersion characteristics of their products–i.e., how the light traveling down the cable "spreads out" and eventually dissipates. They will, however, provide a useful figure of merit known as the bandwidth-length product, or just bandwidth, in units of MHz-kilometer (km). For example, 500 MHz-km translates to a 500-MHz signal that can be transmitted 1 km. In this case, the product of the required bandwidth (BW) and transmission distance cannot exceed 500 (BW x length (L) = 500).A lower bandwidth signal can be sent a longer distance.

Single-mode fiber typically comes with a dispersion specification provided by the manufacturer. The dispersion is in picoseconds (ps) per kilometer (km) per nanometer (nm) of light source spectral width, or ps/km/nm. This loosely translates as the wider the spectral bandwidth of the laser light source, the more dispersion.

The analysis of dispersion for a single-mode fiber can also be complex but affects bandwidth calculations. An approximate calculation can be made using the following formula: BW = 0.187/(disp x SW x L). "Disp" equals the dispersion of the fiber at the operating wavelength; "SW" equals the spectral width of the light source in nanometers; and "L" equals the length of the fiber cable in kilometers.

So for example, a single-mode fiber with a dispersion of 4 ps/km/nm, spectral width of 3 nm, and transmission length of 20 km, would have a bandwidth of 779,166,667 Hz or about 800 MHz.

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