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A Wireless Microphone Primer

written by Buzz Turner - courtesy of the Turner Audio Sales website

Why Do They Call It Wireless If They Have All Those Little Wires Inside?
OK. This is a primer so let's start with a quiz. How about a multiple choice? Here goes...
Bill and Jane are experienced sound mixers with many "big time" credits. They use wireless almost every day. It is fair to say that Bill and Jane....

A) Love wireless and can't live without it.
B) Hate wireless and wish it would go away.
C) All of the above.

If you answered "C", you have probably spent your share of time alternately marveling and cursing RF gear. There is probably no single piece of audio hardware, that enjoys such wide use, and is as universally misunderstood as a wireless mic. The purpose of this article is to review some of the basics and take a look at some of the most common problems and solutions.

An Overview
Wireless mics used in production today come in two basic flavors; VHF and UHF. The VHF band for wireless mics falls between 169 to 216 MHz. 169 to 172 MHz are set aside as Travel Frequencies. These are eight specific frequencies assigned by the FCC and dedicated to wireless mics. (Due to frequency spacing you can only use four travel frequencies at a time.) These frequencies are used by mixers that travel throughout the U.S., and are for the most part usually free of on-channel interference. The bad news is that a lot of crews are on Travel Frequencies so if you are shooting ENG, covering a large event or press conference, chances are pretty good that someone else will have a radio on a Travel Frequency. The frequencies between 174 and 216 MHz are used by broadcast television. If you are operating wireless mics on these frequencies, you must be sure to stay away from local TV channels. In terms of operating multiple systems, the theoretical limit depends on the number of TV channels in your area.

UHF wireless mics operate on the higher UHF bands. Frequencies are available from 470 MHz up through 800 MHz. Again, these frequencies are across the UHF broadcast band and care must be taken when determining a radio's frequency. UHF wireless mics offer a wider frequency spectrum and hence, more possible channel allocations. In situations that demand multiple systems, UHF is a bit easier to deal with because of the less crowded spectrum and sheer number of available frequencies.

Audio performance for VHF and UHF is nearly identical with some of the high end UHF systems offering real improvements in audio bandwidth, transient response and system noise floor. In terms of operational range or distance, UHF offers some advantage especially in inhospitable RF environments. Another advantage is that broadband RF interference (compressors, elevator motors, computers etc.) are often below UHF frequencies.

Frequency Agility
Wireless mics are typically crystal controlled and operate on one set frequency. Although frequency agile systems (you can vary the frequency of the transmitter and receiver over a preset range) have been around for a while, they have only recently gotten small enough to be used in field production. The good news is you can change frequencies whenever you need to, the bad news is that the current crop of portable frequency agile units are plagued with less than ideal audio performance. (Ed. note - This situation is beginning to change.)

Diversity
Diversity systems have a receiver with two antennas. The relative signal strength of each antenna is compared and the stronger of the two signals is instantaneously (and silently) selected. Diversity systems are designed to decrease dropouts at close range, not to extend the range. By virtue of improved signal integrity, the effective working range of the system is improved. Theaters, live broadcasts, concert sound and ENG are all candidates for diversity systems.

Antennas
By using the right antenna for the job and optimizing the antenna's position and location, you can improve the performance of your system. If you are using a body pack transmitter with an attached antenna make sure the antenna isn't coiled up in a ball. Where ever possible, keep the antenna away from the performer's skin as perspiration will effect the RF propagation. Try to avoid using rubber duck or limp wire antennas on VHF receivers. A quarter wave whip is a much better choice. For VHF systems a half wave dipole or collinear antenna are inexpensive options that will improve reception. A high gain directional array like a yagi can really save the day if you need extremely long reach.

Problem? What Problem?
Let's start with interference. By definition, direct interference on your operating frequency is an extremely rare occurrence and can best be avoided by proper frequency assignment and coordination. If you are shooting a large event covered by multiple crews, take the time to coordinate frequencies. If for whatever reason that's not possible, keep your wireless in your equipment case and hardwire.

Inter modulation interference is probably the most common form of interference. Inter mod can occur anywhere multiple systems are operating. The sum and difference of operating frequencies interact and create interference. Again, the solution is proper frequency coordination. Interference from outside environmental sources such as broadband RF and neon lights is probably the most difficult to deal with. Try to determine where the interference is coming from and shut that piece of equipment down. If that isn't practical, move your receiver antenna as close to the transmitter as possible. Experiment with transmitter and receiver antenna positioning to optimize the signal.

Turn On, Tune In and Drop Out
The most common type of interference really isn't interference at all. "Drop outs" or "hits" are when the signal briefly disappears. Sometimes a "sizzle" or loud "pop" is heard. Drop outs can happen when the receiver is inches from the transmitter! Drop outs occur as the transmit signal is reflected off objects and combines with the original signal path. When these multiple signals arrive at the receiver antenna out of phase and cancel out, the receiver "sizzles" as it rapidly looses signal strength and may "pop" as it breaks squelch. The only cure for drop outs is to experiment with antenna placement. When moving a receiver antenna to remedy a drop out, a little goes a long way. Without a doubt, diversity systems provide the best insurance against drop outs.

Defense Gentlemen, Defense
Most wireless problems can be avoided with a bit of care and preparation. If you are renting wireless from a rental house let them know in advance where the equipment will be operated and if there is any other RF equipment on the shoot. Install new batteries when you begin a job. If you are using body pack transmitters, visually inspect the mic cable and connector, plug the mic cable into the transmitter and make sure it seats properly. Turn on a transmitter and receiver and adjust the system gain as per instructions. Listen to the output of the receiver through a high quality mixer and headphones. Wiggle the microphone cable and listen for shorts or opens. Gently pull the transmit antenna and look for breaks in the insulation. Speak into the microphone and listen for any anomalies in the audio. If you are operating multiple systems, turn on each system and check them thoroughly. Once all systems are on, have people walk test the systems in the performance area. Listen for drop outs and adjust receiver antenn as as needed.

And In The End....
On those days when you have done everything right but all your system will pull in is "Radio Moscow" you can...

A) Hardwire or boom the shot
B) Casually inquire "How much money is budgeted for ADR?"
C) Check the "I Ching" and call the Psychic Hot line.

If your not sure what to do, try all of the above.

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