As if we can reduce the many technical issues that AV designers, users, installers, and integrators regularly face to a list of 10! Most of you probably tackle that many hurdles the first day working on a new AV system. But technologies evolve, acronyms breed and spread, and sometimes it's nice just to remind ourselves that if there's a question about a new or existing AV solution, an AV professional has probably thought up a tip, gadget, list, rule-of-thumb, you name it, to help handle the situation.
We polled manufacturers, integrators, and end users to help identify lingering pain points and asked them for quick solutions and insights. Some deal with new technology, some with existing systems, and they all run the gamut from audio to video to signal management.
Will they solve all your AV problems? Of course not. But depending on what you're working on today, you should be able to find a handful of these tips worth clipping out and stuffing in your back pocket for future reference. Go ahead. Get your scissors. We'll wait.
Dan Daley is a freelance AV writer based in Nashville, Tenn. Special thanks for their help with these tips to AV pros at Alcorn McBride, AMX, AVI-SPL, Belden, Crestron, Extron, and Key Digital, as well as Jeff Boccaccio of DPL Labs and Mary Meeker of MEM Systems.
 QSC's PowerLight 3 Series Class-D amplifiers | |
Zap! Class-D Amps and StaticNew Class-D switching amplifiers have high impedance outputs that are very efficient, allowing the circuits to be very small. But they are also high impedance circuits and are thus susceptible to static buildup and resulting circuit damage. However, it's easy to protect against static charge damage. Simply connect three metal oxide varistors (MOV) to each amplifier output. One MOV connects across the pair of speaker wires, while the others connect each wire to the ground. These should all be installed at or near the amplifier, not out in the field. This should render your amplifier relatively immune the next time the weather gets stormy. |
Putting the 'You' in UTP
Most installers know that unshielded twisted pair (UTP) cabling such as Cat-5/6 can be used for more than just Ethernet connectivity. Here are a few other ways to exploit the benefits of UTP in an AV system:
¦ Assuming it's already in place, use UTP cable for serial or IR control (and even low voltage power) instead of pulling a new specialized cable for control.
¦ Use UTP to transmit audio and video in addition to power and control. Just be aware some manufacturers require that their proprietary (and much more expensive) UTP cable be used instead of standard cabling.
¦ Future-proof an AV installation with UTP. Always pull it back to a centralized patch point in order to maximize flexibility in the future and think about how you might use it. If it will be used for Ethernet, make sure the run isn't more than 100 meters (328 feet).
It may already be old hat to mention this, but always be careful when handling UTP cable to maintain its performance characteristics. Don't bend, crimp, staple, stretch, or twist UTP cable–the end users will thank you.
When Cat-5 Meets the Elements
Category-5 cable is pretty robust stuff, but extreme climate conditions, such as high or fluctuating humidity and/or condensation levels, can cause significant signal degradation at the termination points over time. There is a fix, however.
To protect Cat-5 cabling against the elements, consider this relatively simple solution: At one end of the wire run, pressure-terminate the cable to a transmitting balun. At the other end, use a matrix switcher with a receiving balun. Over time, if the cable begins to become affected by the environment and the signal degrades, the transmit/receive balun combination, which has been monitoring the cable run, senses this deterioration as a lengthening of the cable. Such deterioration would produce the same degradation symptoms as humidity or condensation. The combination then auto-compensates for both signal strength and equalization, maintaining a good connection.
 Balun/transmitter from Key Digital |
HDMI: Troubleshooting On The Fly
HDMI is the bane of a pro AV install's existence. But an understanding of its basics can provide you with enough information to do simple troubleshooting without hauling around expensive test equipment. In fact, just understanding the startup process might be enough to keep you out of harm's way.
The system starts by sending a control voltage from the source to the display. Upon receiving this "start" voltage, the display sends back a response called a hot plug. Once this arbitration is completed, the source requests all extended display identification data (EDID) in order to match its video settings to the display's settings. Then HDCP (high-bandwidth digital content protection) is launched. Without all of these elements, your screen will be blank.
If you have a blank screen, step one is to shut everything down. Second, you need a DVD player with no disc inside (not a Blu-ray player)–turn on the display first, then the DVD player. If the display shows any response when the DVD player starts up, it means your start voltage made it through.
Next, see if the DVD logo appears. If it does, insert a DVD disc and start again. If after you fire up the player the disc plays, then you probably have a video integrity issue; if not, you most likely are dealing with an HDCP issue.
 Keep a regular DVD player handy to troubleshoot HDMI issues |
More Than Meets The Eye With HDMI
Installing HDMI distribution systems is very different from analog AV. Many AV professionals don't realize there are many more layers to an HDMI signal than just AV. As a matter of fact, there are 19 wires altogether in HDMI, including two-way embedded data pathways such as HDCP content protection, EDID resolution management, and CEC controls. Those embedded communication streams add complexity that often creates issues with system performance and reliability.
Every source component can accept a specific number of HDCP authentication keys. Manufacturers do not specify key limitations for their equipment, so if a source signal is inadvertently distributed to more displays than it can support, the source component automatically turns off. To avoid such an error, it's important to run an HDCP check to determine the number of keys each source can accept and then configure the system accordingly.
Displays send EDID data so that sources will output the optimal signal resolution. However, in a distribution system where displays have not been standardized, inconsistencies are common. Native resolutions for different displays vary, and this may cause sources to output at lower resolutions than desired. EDID information should be managed to optimize output signal resolution or determine the best common resolution for all sources and displays.
To diagnose and troubleshoot digital systems quickly and reliably, look for solutions that come with or offer robust installation software to manage EDID and to run HDCP and data rate checks.
 Crestron's DigitalMedia platform uses the company's DM Tools setup software |
Screen Blank? Could Be an EDID Issue
Extended display identification data (EDID) exchange is a standardized way for a display to communicate its capabilities, such as its native resolution, to an attached source device, which then generates the necessary video characteristics to match the needs of the display. When successful, EDID data exchange maximizes functional compatibility between devices, helping to ensure optimal image quality as well as overall system reliability.
However, this data exchange often is not successful. The implementation of EDID for display devices varies widely, and in some cases, displays lack EDID information altogether. Such inconsistencies can cause operational issues ranging from overscan and resolution problems, to a display device not displaying an image at all. But it's not always clear that EDID is the problem.
To help determine whether EDID incompatibility may be the cause of an image display issue, a simple software application (free utilities can be downloaded online, or manufacturers often offer them with related hardware) can be used to directly view the contents of the EDID information from a display device. Connecting the display to a computer allows you to access the EDID and store it in the computer's Windows registry. The software then accesses the EDID settings in the registry and lets you examine their contents.
To solve an EDID compatibility problem, use an EDID emulation device (manufacturers such as Extron and others make them). Such a device manages EDID data at the source and can be installed between source and display to communicate the EDID that the source needs in order to provide the proper video output. The devices generate emulated EDID based on a user-selected resolution and refresh rate, or EDID that was captured and stored from the display device. The source then generates the output accordingly, which is passed through the device to the display.
 Extron 101D Emulator comes with management software |
Interpreter Wanted for Digital Audio Devices
Digital I/O is the norm in most audio systems these days. Many devices incorporate a mix of analog and digital signals. But interpreting signal levels on these components can be confusing, mainly because the same meters are typically used for both signal types. To make sense of the information on a digital mixer's main output meter, first you have to determine which of the outputs is being used–analog or digital. Then, if the device is analog, determine its maximum level. This can be found in the device manual's specifications section.
A digital meter is typically referenced in dBFS (also known as digital full scale), while analog metering is measured in dBu increments. So for example, if it were established that the console's maximum analog output level is +18 dBu, that would equate to 0 dBFS on a digital meter. When meters are pulling double-duty as both analog and digital meters, you have to find out from the console's documentation what the maximum analog output level is; this would be equivalent to 0 dBu's on the console in the digital domain. You can now proceed confidently with an understanding of what the meter is telling you about the output level of either the analog or digital signals from the console.
Screen Failure in Digital Signage Systems
A common failure point in digital signage systems is the power supply for the display screen. Often this results from a display network owner's policy of shutting down power to all their screens each night and restoring power each morning using the building circuit breakers. Doing so repeatedly is hard on power supplies and often leads to a breakdown.
To prevent such problems, pay attention to the power-up sequence of a signage network. The typical display screen is 200 to 300 watts or 2 to 3 amps. This allows for, at most, five screens per 20-amp power circuit. Setting the flat-panels' internal on-screen display (OSD) power-up features at intervals five seconds apart will allow the power to be adequately and safely sequenced.
When possible, use the internal scheduling found in the OSD of some manufacturers' screens or an RS-232-based control system that allows for managing the power on a schedule. Controlling power in this method allows you to put a screen in standby mode, thus reducing power consumption to less than a watt per screen. If the installation comprises 300 or more screens and doesn't need to be displaying signage eight hours a day, that's a savings of up to 2,400 watts of power per day, while at the same time enabling quick and efficient startup in the mornings.
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.
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.