You’ve heard it before. Under the right (or wrong) circumstances, some types of cellular phones are capable of interacting with audio equipment and causing audible interference. Since the first GSM-type mobile networks went up, the alien clicks and buzzes (sometimes known as “Blackberry buzz”) have terrorized audio professionals everywhere. I’m sure you know the characteristic blip-blip-blip bzzzzzzz all too well.
The problem isn’t as bad as it once was, because GSM phones, whose handsets are largely responsible for Blackberry buzz, are being displaced by LTE/CDMA models in many countries, but it still happens.
Dispelling Myths About Interference
I’d like to address the confusion around this type of troublesome interference, and introduce a simple, inexpensive device called the ferrite choke that can be used to stop it.
Many wireless audio users erroneously attribute Blackberry buzz to cell phones being “on my frequency.” In fact, active cellular networks are hundreds of megahertz away from the top of the UHF band and cannot directly interfere with wireless microphone receivers, whose front-ends are well equipped to defend against them.
Rather, Blackberry buzz is caused by GSM handsets initiating communication using pulsed data bursts, which inadvertently and adversely interact with electronic components found inside a wide variety of audio equipment — and not just wireless mics. If the rapid on/off signaling between phone and base station is able to enter the signal pathway of an audio system, it can be rectified by non-linear components — usually amplifiers — and create false voltages at audio frequencies, which are converted into audio later on by speakers (Note: I must acknowledge Henry Cohen of CP Communications for helping to confirm this.)
An Ounce of Prevention
You can prevent Blackberry buzz in three ways: Turn off cellphones, keep active cellphones a safe distance away from audio equipment or physically prevent radio frequency energy from entering devices.
The first two are obvious and easy enough, but the ubiquity of cellular and smartphones makes it difficult to exclude cellphones from everywhere audio equipment is used.
Usually, electronics keep ambient radio frequency energy out through shielding and filtering. But even on a well-designed piece of gear, there are opportunities for electromagnetic infiltration.
The most common entry point is a length of unshielded, poorly shielded or damaged cable, like a lavalier mic or intercom headset cord, which acts as an antenna. XLR cables can also unintentionally conduct radio waves and leak them into a system via a loophole called the Shield-Current Induced Noise (SCIN) effect.
Enter the Ferrite Choke
Ferrite chokes are passive devices that selectively block or dissipate high frequencies traveling through a cable. You might think of them like electrical toll booths, allowing audio frequencies to pass through while keeping radio frequencies out. There are lots of different kinds of ferrite chokes, some of which are expensive, finicky, hard to install and poorly suited for suppressing GSM frequencies, which may be as high as 1,800 MHz.
Luckily, New York-based Fair-Rite manufactures an inexpensive snap-on choke made from a material tuned to combat high frequencies, and distributes them through Mouser. To use, the choke is simply snapped around a cable at the input. That’s it!
These Fair-Rite chokes are available in a few sizes. Mouser part No. 623—0461178181 is good for thin cable, like lavaliers. Part No. 623—0461167281 is good for thicker cables, like XLRs. A snug fit is important, so you may have to wrap a bit of gaff tape around the cable where you place the choke to slightly increase the cable diameter, so the choke fits snugly and won’t slide around.
And if you still hear buzz, you can use two or three in a row for even better suppression.
Problem solved!
Alex Milne works for RF Venue, Inc. (a manufacturer of products that make wireless audio systems work and sound better) and hosts a popular blog series on RF topics at http://blog.rfvenue.com.
