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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.

Wireless Woe 7: NOT ENOUGH JUICE

Rechargeable 9-volt batteries typically provide less than half the operating time of single-use batteries.

Rechargeable 9-volt batteries typically provide less than half the operating time of single-use batteries.

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.

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.

With wireless systems that use companding, adjust the transmitter’s input gain as high as possible to improve the signal-to-noise ratio.

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.

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.

Software can help find open frequencies.

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.

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.



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