Several months ago in "The Bleeding Edge," we discussed the pending problem with the FCC’s threatened release of “White Space” broadcast frequencies for wireless broadband use and unlicensed transmission. [Check out www. fohonline.com/whitespace for a complete rundown —ed.] While this directly impacts wireless pro audio systems, any increased activity in the airwaves — such as that from microwaves, cell phone signals, and emergency-band communications systems — presents a threat to our never-ending quest for interference-free audio. Any wire that is carrying low-level audio — such as from a microphone or mixing console — acts like an antenna, and may actually attract radio interference. Furthermore, junctions where the audio system may have gaps in its shielding — such as where a cable is connected to the chassis of an audio device — are an invitation for EMI to weasel its way in. Several manufacturers are addressing these concerns with new ways of battling RFI and EMI to keep it out of wired audio. Before we go any further, a couple of quick definitions: RFI, or Radio Frequency Interference, is the unwanted reception of radio signals. As mentioned, any wire can act like an antenna, attracting RFI. EMI stands for Electromagnetic Interference. All electronic devices emit electromagnetic fields while they operate. When several electronic devices are in close proximity, the electromagnetic field from one device may interfere with the operation of another device. A physical gap in the shield of an audio system can allow EMI to enter the audio path.
Double Down
Two great examples of anti-RFI/EMI technology come from major audio connector manufacturers Neutrik and Switchcraft. On the outside, Neutrik’s EMC connector looks like a “normal” XLR connector. Inside the EMC’s shell is miniature circuitry designed to prevent RF from entering the audio path, even in the most hostile of RF environments. The EMC contains a circular capacitor network, which surrounds the shield of the audio cable, ensuring a continuous RF shield from the source end of the cable through the connector to the chassis housing. In addition to this capacitive network, the connector also features a ferrite bead between pin 1 and the cable ground for EMI suppression. The EMC is available in male and female inline and panel-mount versions.
The ferrite bead is not a new idea: whether you realize it or not, you see these everywhere lately. They are placed at the ends of computer monitor cables, inside high-end consumer and pro audio components and can be used to protect cables that are carrying low-level signals of a critical nature, as well as cables that carry AC line voltage. Most often, they simply look like a round plastic lump at the end of the cable. Underneath the plastic lump is a hollow cylinder made of ferrite (a semimagnetic material sort of like iron oxide, AKA rust, combined with other metals — seriously). The bead (sometimes called a “choke” since its job is to choke off RFI) can be placed over the cable when it is being made, but sometimes the cable is looped through the bead several times. Some ferrite beads are halves that snap together to surround a cable that has already been terminated. This is a simple, inexpensive device that effectively suppresses high-frequency noise in electronic circuits.
You’ll also find a ferrite noise suppression device in Switchcraft’s F Series of connectors. These are panel-mount XLR male and female connectors using an all-metal housing with a ferrite disk at the interior rear of the connector. The ferrite disk allows the connector to reject EMI, which might otherwise leak into the audio path and cause nasty noises.
Cellular Cacophony
At least two microphone manufacturers have recognized the fact that the conference room is a hostile environment — and not just for opposing business execs. Many conference rooms in large corporate offices employ some sort of modest P.A. to allow a speaker’s voice to be heard over the other folks at the meeting. Of course, all these highpowered execs are carrying cell phones and PDAs with Web access and instant messaging. Often these devices — which generate RFI and EMI — are placed on the conference table, in close proximity to the microphones. When the cell phone or PDA receives a signal (not necessarily a phone call), it will generate a series of intense RF pulses. These pulses can be picked up by active electronics such as transistors or ICs, thus making their way into an audio path.
Shure combats this problem with its CommShield Technology, now incorporated in its Microflex and Easyflex series of microphones. These mics feature multilayer circuit boards for microphone preamplifiers, extensive grounding techniques, tighter enclosures for increased shielding and improved microphone cable and audio connectors, making them highly resistant to RFI and EMI.
Audio-Technica’s approach to killing RFI and EMI is embodied by its Engineered Sound line of microphones, which features UniGuard protection. Microphones employing UniGuard feature a completely shielded enclosure, paint-free connection points, custom RFI shielding and filtering on all input and output connectors. For added flexibility, Audio-Technica has developed a dedicated crimp tool and special RFI shields for connectors that enable contractors to shorten the cables and correctly reinstall connectors while maintaining the highest level of RFI immunity.
One easy way to help reduce RFI and EMI is through the use of balanced audio circuitry, the purpose of which is to reject such type of noise if it should make its way into an audio cable. That, along with proper grounding, should keep your audio clean.
Steve La Cerra is the tour manager and Front of House engineer for Blue Oyster Cult. He can be reached via e-mail at Woody@fohonline.com.
