10 Wireless Microphone Problems
We’ve all felt it ? that ugly, sinking feeling you get when a simple AV job mutates into an unpredictable nightmare.
If there is not enough space (in megahertz) between these intermodulation signals and the operating frequencies of the units themselves, the receiver has a hard time picking up the signal from its transmitter. Typical symptoms include crosstalk between systems, frequent signal dropouts, poor range, or excessive noise and distortion.PROBLEM: There are different degrees of frequency compatibility. If you know exactly what the operating situation is, you might be able to be a bit more aggressive and squeeze a few more systems into the space. The key is understanding the trade-offs.PROBLEM: Wireless microphones are also subject to interference from other sources transmitting in the same spectrum. The most significant culprits are typically TV stations. FCC rules require wireless microphone users to avoid frequencies in TV channels occupied by a broadcast TV station in the same geographic area.According to FCC mandates, all analog TV stations will cease operations in February 2009. At the same time, the spectrum above TV channel 51 will be repurposed for use by new services. Wireless microphones operating above 698 MHz may need to be tuned to a lower frequency in order to avoid experiencing interference once new services become active. As the transition continues, the occupied TV channels in a given location may change, so it’s wise to regularly check published information.PROBLEM: Receiver antennas are one of the most misunderstood areas of wireless microphone operation. Mistakes in antenna selection, placement, or cabling can cause short range, dead spots in the performance area or low signal strength at the receiver that leads to frequent dropouts. Modern diversity receivers offer much better performance than single-antenna types, but the right antennas must still be put in the right place to maximize the performance and reliability of the system.If the receiver will be located away from the performance area (in an equipment closet or a closed rack, for example), ½-wave antennas or directional antennas should be remotely mounted (ideally above the audience) in order to have a clear line of sight to the transmitters. (Short ¼-wave antennas should never be remotely mounted, however, because they use the receiver chassis as a ground plane.) Extra distance between the antennas will not significantly improve diversity performance, but may allow better coverage of a large stage, church, or meeting room. If the antennas will be far from the stage, use directional antennas to improve reception by picking up more signal from that direction and less from other angles. If the antennas will be connected to the receiver with a length of coaxial cable, in-line antenna amplifiers may be required to overcome the inherent signal loss in the cable.
WE’VE ALL FELT IT — THAT UGLY, sinking feeling you get when a simple AV job mutates into an unpredictable nightmare. For many systems integrators and technicians, the addition of wireless microphones to the list of AV gear causes just such a reaction.
A glimpse backstage, behind the monitor console, atthe show “Love In ? A Musical Celebration,” at theBirch North Park Theater in San Diego. The wirelesssystem shown is a 12-channel receiver setup, fedfrom a pair of directional antennas.
Getting wireless mics to work reliably sometimes seems to require equal parts rocket science and black magic. One day you can pull off a trouble-free show or presentation with dozens of wireless mics; on another, you can’t even get one bodypack to behave for a two-hour sales meeting. That’s why knowing how these devices work is so critical to make them work.
Let’s take a closer look at 10 of the most common problems that seem to pop up in most wireless mic applications — and what you can do to avoid them.
Wireless Woe 1: INCOMPATIBILITY
PROBLEM: When using multiple wireless microphones, interference between the systems themselves always seems to crop up. Even if each system is on its own frequency and spaced several megahertz apart, the mics can still interfere with each other through a phenomenon called intermodulation distortion (IMD), which occurs because radio transmitters interact with each other to create intermodulation signals.
The receiver’s tuning “window” determines how close frequencies can be spaced.
The minimum separation between frequencies depends on the design of the system’s receiver. An entry-level receiver may require a 1 MHz interval between the nearest adjacent system or intermodulation frequency. A more expensive receiver typically has a narrower tuning “window,” allowing for closer spacing between each system or intermodulation frequency.
SOLUTION: To avoid intermodulation distortion, select only frequencies that have been calculated to be compatible with each other. Because it requires knowledge of the design characteristics of the transmitter and receiver, the wireless system manufacturers provide these calculations.
For example, when just eight wireless microphones are used together, thousands of calculations must be performed to ensure compatibility. As a result, most manufacturers publish lists of compatible frequencies for their systems. In addition, software is available that can help identify compatible frequencies in some cases.
Wireless Woe 2: COMPATIBLE, BUT NOT QUITE ENOUGH
Intermodulation signals get stronger when transmitters are close together.
One important assumption that is made by most frequency compatibility software is that all receivers will be turned on and unmuted all the time (even though some transmitters will occasionally be turned off), making it important that none of the receivers is picking up an intermodulation signal that might be heard as noise.
Therefore, the software needs to leave ample space between the intermodulation signals and the wireless mics themselves.
If you assume that the sound system operator will take a more active role, however, you may find that more systems are actually compatible. In this scenario, it’s assumed that the operator will mute any receiver that’s not actually in use at the moment — and that all transmitters will be left on at all times during the show. The distance between each transmitter and the receiving antennas is also assumed to be similar. These assumptions would make sense in a Broadway theater installation, however, the same performance might not reasonably be expected in a school auditorium operated by volunteers with little or no training.
Interference symptoms are much worse when the transmitters are located very close to the receiving antennas or to each other — or when high-powered transmitters are being used. This is why it’s much more difficult to get 40 wireless systems to work in a theater (many transmitters very close together at various distances from the receiver) than it is to get them to work in a school with one system in each classroom (transmitters far apart from each other but fairly close to its own receiver).
SOLUTION: To get a balance of high performance with the maximum number of systems, make sure that the level of compatibility between frequencies is appropriate to the expected use of the systems. Keep transmitters at least 10 feet from the receiver antennas. If the transmitter’s RF output power is adjustable, use the lowest transmitter power that is necessary to cover the expected distance between the transmitter and receiver.
Wireless Woe 3: TV STATIONS
Occupied UHF TV spectrum in Seattle before Feb. 17, 2009.
SOLUTION: When indoors, avoid TV channels active within 40 to 50 miles. Outdoors, a 50- to 60-mile radius should be used. Because active channels vary from city to city, the appropriate frequencies for wireless microphone operation depend on location. Manufacturers usually offer guidance as to which frequencies to use in different cities.
UHF TV spectrum in Seattle after Feb. 17, 2009.
Wireless Woe 4: OTHER DIGITAL DEVICES
PROBLEM: Other wireless audio devices that operate in the TV band — in-ear monitors, intercom systems, etc. — as well as non-wireless devices can also cause interference problems. Digital devices (CD players, computers, and digital audio processors) often emit strong RF noise and can cause interference if they are located within a few feet of the wireless microphone receiver. For transmitters, the most common sources of interference are GSM mobile phones and PDAs worn by presenters (See “Attack of the BlackBerrys,” page 57).
SOLUTION: Be aware of other wireless audio equipment when selecting wireless microphones frequencies. Keep digital equipment at least a few feet away from wireless microphone receivers. Use an AM radio as a cheap RF noise detector; you might be surprised at what the gear in your rack is emitting.
Wireless Woe 5: NONDIVERSE RECEIVER ANTENNAS
Receiver antennas should be angled 90 degrees apart and spaced 1/2-wavelength apart for good diversity performance.
SOLUTION: To ensure good diversity performance, space antennas apart by at least one-half of a wavelength (about 9 inches at 700 MHz). The receiver antennas should be angled apart in a wide “V” configuration, which provides better pickup when the transmitter is moving around and being held at different angles.
Covering, coiling, or looping (left) the transmitter antenna reduces the signal output and therefore the range.
The amount of loss depends on the exact length and type of cable used, so follow the manufacturer’s recommendations. Total net loss should be limited to no more than 5 dB.
Wireless Woe 6: UNINTENTIONAL SIGNAL BLOCKAGE
PROBLEM: The human body can also interfere with wireless signals. Largely composed of water, our bodies absorb RF energy. In addition, if a user cups his or her hands around the external antenna on a handheld transmitter, its effective output can be reduced by 50 percent or more. Similarly, if the flexible antenna on a bodypack transmitter is coiled or folded, the signal suffers.
SOLUTION: Keep the transmitter antenna fully extended and unobstructed to achieve maximum range and performance.
10 Wireless Microphone Problems
We’ve all felt it ? that ugly, sinking feeling you get when a simple AV job mutates into an unpredictable nightmare.
PROBLEM: Despite the fact that transmitter battery life is a top concern with wireless mics, users continue to try and cut operating costs by using inexpensive batteries. Most wireless manufacturers specify alkaline or lithium single-use batteries because their output voltage is very stable over the life of the battery. This is important because most transmitters will exhibit audible distortion or signal dropouts when supplied with low voltage. Rechargeable batteries often seem like the ideal solution, but most rechargeables provide about 20 percent less voltage than a single-use battery — even when they are fully charged.PROBLEM: As good as it is, analog wireless audio transmission has limitations imposed by the inherent noise and limited dynamic range (about 50 dB) of FM transmission. To overcome this, most wireless microphone systems typically employ two kinds of audio processing to improve sound quality. Pre-emphasis is applied in the transmitter (with corresponding de-emphasis in the receiver) to improve the signal-to-noise ratio. A compressor in the transmitter and expander in the receiver can increase the dynamic range to more than 100 dB. This makes it important for audio levels to be set carefully. If the audio level is too low, hiss will be audible. If it’s too high, distortion may result.PROBLEM: Probably the most frustrating problem with wireless is that the airwaves themselves keep changing. The list of analog and digital TV channel assignments has been changing regularly since the DTV transition began years ago. Rights to the UHF TV spectrum above channel 51 is in the process of being auctioned. Some of it (like channel 55) is already being used by the new owners, while the rest may remain unused until 2009.
Wireless Woe 7: NOT ENOUGH JUICE
Rechargeable 9-volt batteries typically provide less than half the operating time of single-use batteries.
SOLUTION: To combat battery problems, carefully compare the transmitter’s voltage requirements with the battery’s output voltage over time to make sure that the battery will last through a full performance. Lithium-ion and rechargeable alkalines usually work well, while Ni-Mh and Ni-Cad batteries may last only a couple of hours. This issue is specific to 9-volt batteries; AA rechargeables offer similar performance to single-use AA batteries.
Wireless Woe 8: UNADJUSTED TRANSMITTERS
With wireless systems that use companding, adjust the transmitter’s input gain as high as possible to improve the signal-to-noise ratio.
SOLUTION: To get the best sound quality, the transmitter’s input gain should be adjusted so that the loudest sound level that will occur produces full modulation but not distortion.
Wireless Woe 9: WRONG OUTPUT LEVEL SET ON RECEIVER
PROBLEM: After so much discussion of frequency, wavelength, and antennas, it’s easy to overlook the most fundamental requirement of a wireless microphone system: to replace the connecting cable between the source and the sound system as transparently as possible.The receiver will usually have an output level control, while most wired microphones do not. This provides the opportunity to more precisely match the output of the receiver to the input to which it is connected.
SOLUTION: Whether microphone level or line level, the output level should be set to the highest practical level while not exceeding the limits of the sound system input. This might be indicated by the peak light on a mixer input channel, or simply by listening for audible distortion.
Wireless Woe 10: SETTING AND FORGETTING
Software can help find open frequencies.
As if that weren’t enough, the FCC is trying to figure out a way to allow a new strain of consumer products (PDAs, smartphones, or home equipment) to use the unoccupied TV channels (also known as “white spaces”) to deliver wireless Internet access.
SOLUTION: It used to be enough to know whether your city had odd-numbered or even-numbered TV channels in the VHF band. Today, however, the people who set up and use wireless microphones (as well as in-ear monitors and intercom systems) need to regularly check local spectrum conditions, even when working at venues they know well.
There are a number of resources that make this a far less intimidating process. First, most wireless manufacturers now offer online frequency-selection tools that are updated with the most current TV channel assignments. Second, external RF scanners and spectrum analyzers that can quickly scan the entire spectrum (including the TV band) have become increasingly capable and less expensive, making them a practical option for people who rely on wireless systems heavily. Finally, the wireless systems themselves are getting more sophisticated. Even some entry-level systems can scan the spectrum and find an open frequency for themselves. Some premium systems can even connect to your PC or Mac, scan the spectrum, and give you a visual depiction of RF conditions, plus calculate the best set of frequencies (taking into account all of your other RF gear as well), and then program all of the receivers automatically.
Tim Vear is a senior applications engineer with Shure in Niles, Ill. He can be reached at Vear_tim@shure.com.
ATTACK OF THE BLACKBERRYS
We’ve all heard it — that angry buzzing sound whenever a BlackBerry gets close to a car stereo, computer speaker, or speaker-phone. In most situations, it’s just annoying, but when the interference is picked up by the P.A. system during the CEO’s speech, it’s a big deal. Here’s how it happens.
BlackBerrys, like all phones that use the GSM transmission standard, transmit on frequencies in either the 800 to 900 MHz or 1,800 to 1,900 MHz range, depending on the country and the carrier. They transmit data in RF energy bursts that are short but powerful. These bursts occur 217 times per second at power levels as high as 2 watts (depending on how far the phone is from the nearest cell tower). This 217 Hz “lightning bolt” can easily induce a ragged-sounding noise (the now-familiar “dit di-dit di-dit di-dit”) into most audio equipment. The noise can invade at almost any point — at inputs or outputs, through a cable, or directly into a component on the circuit board.
Most of the time, GSM interference occurs when the phone is within just a few feet of an audio device. Audio equipment manufacturers are quickly finding that protecting their products from GSM noise requires extensive design changes — not just the addition of a component or two at the connector. Until such protection is universal, AV technicians need to keep GSM phones away from unbalanced audio lines, including lavalier and headworn mics, hanging choir/ audience mics, and interconnect cables between equipment. The only instant sure-fire solution: Make presenters turn off their phones.