Wireless Microphone Problems

ATTACK OF THE BLACKBERRY

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.

Digital Extras 
Learn about the science of radio waves, refer to a chart showing how the type of cable you use can cause signal loss at 200 MHz and 800 MHz, and get links to online scanner and frequency finders.