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Guide to Using Wireless Audio Systems Onstage

June 30, 2014
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Fig. 1. The Shure ULX2 Handheld Transmitter can be ordered with a choice of capsules.

AH, THE freedom of wireless microphones. The thrill of standing next to the drummer on a wobbly riser. The excitement of walking into a crowd and singing with drunken patrons. The wonder of hearing a taxi dispatch through your P.A. system. . . . Well, no gig is perfect, but using a wireless mic or instrument system definitely beats the confines of a wire.

This month, I’ll help you choose a wireless system that fits your needs and offer tips to keep the taxi dispatcher from crashing your party. (Note that the principles here hold for wireless mics, guitar/bass, and IEM systems.)

How High Is Ultra High? The majority of wireless instrument systems operate via Frequency Modulation (FM) in the Very High Frequency (VHF) and Ultra High Frequency (UHF) bands. There was a time when many people in the pro audio community snubbed their noses at VHF wireless, deeming it inferior to UHF—but that’s simply not the case. Each has advantages and disadvantages, and may be more appropriate for a given application. UHF transmission tends to suffer less interference due to the fact that traditionally the UHF band has been less crowded. On the other hand, the inherent construction costs for UHF systems are higher. Battery life of a VHF system tends to be better than that of a UHF system. The point here is not to rule out a wireless mic simply because it’s VHF. In fact, you’ll find that manufacturers are revisiting the VHF band for new products because the UHF bandwidth available to pro audio is shrinking. (See the “Buyer Beware.”)

The 2.4GHz “ISM” (Industrial, Scientific, and Medical) band is rising in popularity, since it has the advantage of being license-free to use worldwide. (A license is currently not required for use of wireless mics/instruments/IEMs in the United States, providing that the equipment is used in the core TV bands and is transmitting at 50mW or less.) Manufacturers such as AKG and Line 6 are using 2.4GHz technology for digital transmission of full-bandwidth, uncompressed audio. (See the “Wireless Audio Processing” sidebar.) Other advantages of digital wireless include increased dynamic range and data encoding to prevent interference and unauthorized eavesdropping (!) from third parties.

Digital wireless may take various forms. Some systems employ frequency-hopping, spread-spectrum technology similar to that used for cordless phones. This technology requires more bandwidth than an FM signal, so they usually operate in the 900MHz or 2.4GHz unlicensed bands. Lectrosonics’ Digital Hybrid Wireless® combines traditional FM transmission with digital audio: Analog audio is converted into digital information; the digital audio is encoded back into an analog signal, which is broadcast via FM. The information transmitted is digital (and encrypted), yielding an inherently low noise floor, but since the transmission itself is analog, efficiency and transmission range are increased.

Arguably the single most important wireless feature is frequency agility. A fixed-frequency system will cost far less, but you’ll be stuck with a predetermined operating frequency. At some point, you may need the ability to change the channel to avoid interference—even in a relatively immobile installation (such as a church). If you are traveling with a wireless system, then fixed-frequency operation should be considered a deal-breaker.

Frequency agility varies with price. Some receiver/transmitter combinations offer a choice of four or five frequencies, while others can tune across thousands. Initially, this might not seem important, but take a look at the wireless requirements of your typical four- or five-piece bar band: a wireless mic for the lead singer, two wireless guitars, and one bass, plus several sets of wireless IEMs. All of a sudden, you need eight frequencies that won’t interfere with each other or anything else in the neighborhood and vice versa. You definitely want a wide choice of frequencies.

Speaking of interference, some wireless receivers have the ability to scan the airwaves and automatically choose a vacant frequency, a very useful feature. You must realize that all such technology is only capable of detecting RF gear operating when the scan takes place—so if you are performing in a venue where (for example) security uses radios to communicate, ask them to turn their radios on during soundcheck. When this feature was in its infancy, you’d need to manually tune the transmitter to match the receiver, but more and more systems use infrared remote to “auto-sync” the transmitter to the correct frequency.

Diversify Your Portfolio Radio transmitters broadcast their signals without really caring where those frequencies go. As a result, a transmission may travel directly to a receiver’s antenna or it might bounce off an object and then travel to the receiver (or both). “Multipath” is when a signal reaches an antenna via more than one path. Much like low-frequency audio waves bouncing around a control room, multipath can cause phase cancellation, resulting in dropouts. 
 Line 6 Relay G90.
One of the best ways to combat signal loss due to multipath is through the use of true diversity wireless. True diversity employs two tuners and two antennas for reception. A little troll living inside the receiver analyzes the signal at each antenna/tuner and throws a switch connecting the stronger one to the receiver’s circuitry (okay, it’s not a troll, it’s a comparator circuit). This provides a major advantage in maintaining reception. A non-diversity system utilizes one antenna and one tuner and thus may be more subject to multipath—but again, the application dictates the importance of the feature. If the receiver is located relatively close— within, say, 20 feet and line-of-sight of the transmitter—multipath is less of an issue.

A variation of this concept is antenna switching, when an electronic circuit automatically compares signal strength at the two antennas and connects the one with the stronger signal to the tuner.

This technology costs more than non-diversity technology but is not as prone to dropouts (areas onstage where the receiver loses the transmitter). Part of any soundcheck should be walking the stage with the transmitter and marking the floor at dead spots. (I guarantee the lead singer will stand there all night!)

Keep Your Antenna Up Ideally the receiver is within line-of-sight of the transmitter. This positioning provides the greatest operating distance but is not always practical—in which case, true diversity may provide better performance. (Most manufacturers’ range specifications are measured under line-of-sight conditions, and can be very optimistic.) In rack-mounted wireless systems, the antenna should be placed outside of the rack, because the rack itself can pose an obstacle to reception. Thus, it may make sense to either remove the unit from the rack after transport, or get the antennas out of the rack. Most manufacturers offer panels for mounting antennas on the outside of a rack, or remote antennas that mount on mic stands and can be located on the side of the stage. Try to space the antennas at least 1/4-wavelength apart— roughly four inches for UHF and 16 inches for VHF systems—and at right angles to each other.

Do not underestimate the importance of the antenna. Always use the antenna recommended by the manufacturer. Antenna efficiency is critical because transmitter output power is limited by the FCC, and battery life is a function of output power. Antenna size is directly proportional to wavelength of the system: Lower radio frequencies require longer antennas. Never cut the antenna or substitute an antenna from a different system, and always extend telescoping antenna to maximum length.

Choosing a handheld wireless mic is no easier than choosing a wired mic, due to the large number of choices and the challenge of finding a model that complements a singer’s voice. Lower-priced systems tend to have nonremovable capsules. As you scale the price wall, you’ll find handhelds with interchangeable heads. For example, the Shure ULX2 Handheld Transmitter can be ordered with a choice of capsules (SM58, Beta 58, etc.), or the capsule may be swapped out at a later time. A number of wireless transmitters may even be used with capsules from other manufacturers—providing a wide range of choices. Expect shorter battery life when using a condenser capsule, due to the power requirements of the capsule itself in addition to those of the transmitter.

All wireless transmitters require batteries, and it’s an unfortunate fact of life that nonrenewable alkaline batteries possess the most desirable discharge characteristics for the application. You don’t want to lose a wireless mic to a dead battery in the middle of the show, so as a matter of habit, change the batteries before every show. (Save questionable batteries for soundchecks.) NiCAD rechargeable batteries are a no-no in wireless transmitters because they have a nonlinear drain characteristic, and by the time you get a “low battery” warning, it’ll be too late. Lithium ion rechargables fare much better but beware that not all batteries of this type output maximum voltage when fully charged. Thus, the battery gauge may be inaccurate or the transmitter may not work at all. Fortunately, more companies are offering rechargeable battery packs specifically designed for their products. A battery gauge on the transmitter is a must; remote battery indication on the receiver is strongly suggested.

Fig. 2. The Audio- Technica M2 IEM lets you mix two signals at the belt pack.
The concept of mixing your own monitors is particular to IEM systems. Systems from Audio-Technica and Shure offer the ability to mix two separate signals at the belt pack, so for example, you could have a band mix on one channel and “me” on the other. An adjustable limiter can help prevent audio accidents from hurting your hearing, and a headphone jack on the front of the transmitter is very useful for troubleshooting.

Six Pack? Wireless instrument, lavalier, and headset microphones employ bodypack transmitters, typically with a TA or mini-XLR connector (hopefully locking). The same packs can usually be used with cables that have a 1/4- inch TS on the instrument end, but remember that a guitar or bass does not react to a wireless pack the same way it reacts to the input of an amplifier. To compensate, some manufacturers build cables designed to load the pickups in the same manner as an amp.

Lectrosonics’ MI39ARA cable is specifically designed for use with passive pickups and has an active DI with a J-FET front end built into the 1/4-inch connector (!). More companies are also offering “cable compensation” circuitry in their packs to model the interaction between guitar (or bass), cable, and amplifier input.

 
Sennheiser EW100 receiver.
The output from guitars and basses varies widely depending upon pickup type and design, so it makes sense to “tune” the input of the bodypack to the instrument. Active pickups produce a hotter output level that can potentially overload the input of a pack. Look for an input sensitivity adjustment and overload LED (or meter) on the bodypack. Spend a few minutes playing the instrument aggressively with the volume control turned all the way up while observing the meter. Adjust the sensitivity so that when the instrument is at its loudest the indicator barely blinks red or shows overload; then, back it off a hair.

When using a handheld microphone transmitter, observe the same practice and expect a different output level if the capsule is changed. The audio output of the receiver also requires a bit of attention. Is the output jack operating at microphone level? In that case, you’ll need to connect the receiver’s output to a microphone input on the mixing console; but if the receiver’s output is at line level, you’ll need to connect it to a line input. Just because you see a 1/4-inch jack for the audio output, does not mean that the output is line level. If the receiver has a control for the audio output level, adjust this after you have dialed in the transmitter’s input sensitivity. This is particularly critical for guitar amps, which behave differently depending upon the strength of the signal presented at their inputs.

Guitarists and bassists may notice that as they switch instruments, their stage volume varies. The elegant solution to this is to use multiple packs, each adjusted to the output of the instrument. If you take that approach, ensure that the packs are transmitting on the same frequency and that only one pack is powered at a time. Powering up multiple transmitters on the same frequency is a sure bet for interference. A less-than-elegant solution is to adjust the sensitivity so that the instrument with the strongest output does not overload the transmitter input, and live with the fact that other instruments may produce less stage (and P.A.) volume. Get into the habit of removing the cable from the belt pack when traveling, especially when flying—TSA agents love to pull out those connectors without releasing the locks. I warned ya’.

Steve La Cerra is an independent audio engineer based in New York. In addition to being an Electronic Musician contributor, he mixes front-of-house for Blue Öyster Cult and teaches audio at Mercy College, Dobbs Ferry campus.

 

Fig. 3. Lectrosonics’ MI39ARA cable is designed for use with passive pickups.
HOW WIRELESS WORKS

Wireless microphone and instrument systems operate in the same manner as FM radio and terrestrial broadcast television. An oscillator is used to create an extremely high-frequency tone that we cannot hear (in the MHz range). This frequency, known as the carrier, has the ability to radiate into space when amplified. Audio is mixed with the carrier, amplified, and sent to an antenna, which converts the electrical signal to electromagnetic radiation and broadcasts it into space. A receiver (tuned to the frequency of the carrier) detects the signal and converts the electromagnetic radiation into a small electrical signal. This signal is amplified and the carrier is stripped away using a lowpass filter— leaving the audio signal behind.

 

WIRELESS AUDIO PROCESSING
Most UHF and VHF wireless systems use two audio processes for enhancing signal quality. Companding is compression before transmission and expansion after reception.

The transmission strength of wireless audio systems is limited by the FCC and may not legally be exceeded; however, the dynamic range of UHF/VHF wireless is typically less than 50dB. If a signal is broadcast with a wide dynamic range, noise will be a problem because the quiet parts of the audio signal will fall below the noise floor of the transmission. One solution is to compress the audio, transmit the signal, and expand the audio back to normal when it is received, allowing many systems to achieve a dynamic range close to 90dB.

Pre-emphasis/de-emphasis is a noise-reduction technique that uses equalization to boost the high frequencies of an audio signal before transmission. The receiver has a complementary EQ circuit that restores the high-frequency response to normal. If any static or hiss is picked up during transmission, the noise is also cut, reducing its level by around 10dB.

These techniques vary from manufacturer to manufacturer, which is one reason that units from different manufacturers operating on the same frequency are incompatible. 


BUYER BEWARE
In January of 2010, the FCC banned the sale of wireless microphone and instrument systems operating over the “700MHz” (actually 698MHz to 806MHz) band in the United States. As of June 2010, the FCC made it illegal to use these devices in those bands, which are now reserved mostly for providers of wireless broadband. As a result, you may find shady people trying to sell older wireless mic or instrument systems that are illegal for use in the United States. As the old adage goes: If it sounds too good to be true, it probably is. When purchasing a used wireless system, make sure that it operates outside the 700MHz band.

 

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