shavlogo2.gif (3085 bytes)Wireless Systems for Stage Use

8/04 - Larry Mundy - http://www.colomar.com/Shavano/wireless.html


Note: Larry Mundy is the author of Design and Build Your Own Live-Sound Speakers and an occasional contributor to this site. 


Some performers don’t like to have a cable attached to their microphone or instrument at all.  Transmitting your signal wirelessly frees you to move around the stage, twirl, walk into the audience, and generally project a more animated appearance without being tethered to a cable.  Wireless systems are not cheap and bring their own raft of problems, but sometimes they are worth it for the convenience of being “freed.”

Just like any radio-based system, a wireless setup consists of a transmitter and a receiver.  The transmitter connects to your microphone or instrument with whatever type of plug you normally use (or with some microphones, the transmitter is built-in).  Some transmitters are so tiny they’re built into the connector plug; most are the size of a cigarette pack or larger and are physically worn by clipping them to a belt or strap (as a result this is sometimes called a “body pack”).   All transmitters use power to send the signal, so they are generally powered by standard-sized batteries.

TIP: With most battery-powered sound equipment, the quality of the sound depends on the state of charge of the battery.  As the battery becomes weak, the sound becomes weak and distorted, long before the component actually stops working.  Plan on carrying spare batteries and change them often.  If your transmitter has an on-off switch, that is intended to preserve battery life, so use it.  If you unpack your transmitter before a performance and find that it’s been left switched “on” for days, replace that battery right then and there.

TIP: Rechargeable batteries can save you money in the long run, but they can take hours to recharge – not a good thing when your performance starts in 10 minutes.  Some electronic equipment isn’t designed to use them, either (check your manual).  If you use rechargeable batteries in your equipment, keep spare conventional batteries, which are ready to go right out of the package.

The signal is encoded into radio waves and travels to the receiver, which is basically a radio tuned to receive only the “channel” the transmitter is broadcasting.  It will likely have some sort of antenna to assist reception, and if you’re lucky it will be AC-powered so you only have one battery to worry about and replace.  It may be a small box that can be set on top of other equipment, or may be designed for rack-mounting in some way that keeps the antenna(s) exposed to the transmitter.  The receiver will then have some sort of output jack(s) from which a short cable carries the signal to your other equipment.  This signal is generally the same strength as the original signal, i.e. it may require “preamplification” to raise it to the “line level” your other equipment expects, just as your microphone or instrument would.  Basically you have exchanged a single, long conventional cable for (1) a short cable to the transmitter; (2) the transmitter; (3) the air the radio signal travels through; (4) the receiver; and (5) the cable from the receiver to the rest of the system.   In return for wireless freedom, you have greatly multiplied the number of things that can go wrong.

One thing that can go wrong is that the direct signal from the transmitter to the receiver is somehow interfered with, most likely by a “bounced” signal from the same transmitter reflecting from a wall or ceiling and confusing the receiver.  The likelihood of this is greatly reduced by a “diversity” receiver design, which is just a fancy word for equipping the receiver with more than one antenna and spacing them some distance apart, so they are more likely to receive a strong direct signal.  In its simplest form this means the receiver simply has two antennas, one at each end of its cabinet.  This is such an inexpensive improvement that it’s worth the few extra dollars.  The next step up is a “true diversity” receiver in which each antenna has its own associated receiver electronics, and some sort of “brain” decides from moment to moment which one has the cleanest signal.  This design is more expensive but again well worth it, unless it’s not critical that your audience hears uninterrupted sound.

Like any other sort of wireless or radio equipment, your transmitter puts out a signal at a frequency that shouldn’t be “heard,” except by its receiver.  The age-old problem is that in certain circumstances the receiver can “hear” other signals – from another wireless transmitter, from a radio or TV station broadcasting an errant frequency, or from some other source of interference.  To deal with this problem, wireless systems have evolved to use ever-higher frequencies to avoid collision with other types of signals.

Wireless systems use “FM modulation” to translate signals into radio frequencies, and operate in the frequency bands shared by VHF and UHF television broadcasts.  Most of the time these work fine, although like FM radio or TV there is an upper limit on the frequency response of such a system.  Such transmitters and receivers are fairly easy to engineer, because manufacturers have been making VHF and UHF equipment for decades, but that’s also the problem with wireless systems.  There are so many RF signals bouncing around in these frequency bands that a wireless system is pretty certain to run into interference sometime as your system is utilized in different halls, buildings or auditoriums.  In extreme situations it can broadcast TV-station audio through your PA or amp, but usually it just picks up noise.

To help avoid this problem if it crops up, many systems are capable of “frequency synthesis,” which simply means they can be switched from channel to channel until you find one that’s free of interference.  Done well, this is a great convenience.  Done badly, the electronic doodads that allow “channel switching” can degrade the quality of the signal.  And as with all components, having the ability to switch “channels” is one more setting to screw up – if the transmitter is sending signal on channel “A” and someone has set the receiver to receive channel “B,” there will be no sound transmitted and someone will have to troubleshoot the system.  Generally, for a fixed-location PA system, single-channel wireless systems are more idiot-proof, and if they work when installed, the chances of a source of interference moving into the neighborhood are minimal.  For a system that’s moved from place to place, the greater complexity and expense of a switchable system may be worth it, because you never know what sort of interference you’re likely to run into.  If your portable system usually stays in the same geographic area, you will come to know the “channels” likely to be interfered with by local TV stations.

The problem is multiplied when you have multiple wireless transmitters and receivers, for multiple vocalists and/or instruments.  Obviously, each must operate on a separate “channel” or they will interfere with each other, and operating several systems at once multiplies the likelihood that at least one of them will experience some sort of external interference.  And if you “change the channel” on one system experiencing interference, that may cause a problem in another nearby system.  These are good reasons to keep the number of wireless systems onstage to a minimum, even if you think they are sexy and cool.  If you must use multiple systems, some more expensive receivers have intelligent circuitry that can “search” for unused and interference-free channels for you.

For a lot of technical reasons, UHF-band systems sound and work better than VHF-band systems.  They tend to have better frequency response, less interference, greater range, bigger paychecks and houses, more intelligent children . . . I could go on and on.  Unfortunately, UHF systems cost more than VHF systems, sometimes twice as much.  If you are a “casual” wireless user and could use a hard-wired cable instead of a wireless system in a pinch (i.e. when your VHF system is having reception problems), the cheaper system may be all you need.  If wireless freedom is a necessary element of your performance, spend the difference for a UHF-based system.

Many transmitters have adjustable “output gain” controls.  This simply adjusts the strength of the signal they transmit, and can be very helpful in obtaining a clean, strong signal.  You want the “gain” control set high enough that the signal is broadcast strongly, the better to compete with any noise that’s introduced by the system, to improve the signal-to-noise ratio.  At the same time, you don’t want it so strong that it electronically “overloads” the receiver input, which can cause all sorts of electronic distortion.  The optimum setting can depend on the strength of the original signal, the system design, and the distance from the transmitter to the receiver.  Systems with this feature will explain in their documentation how to use it, and will generally have some sort of signal-strength meter or blinking lights on the receiver to help you make proper adjustments.

Your receiver may also have a “squelch” control, designed to silence its audio output when no signal is being received, so the receiver doesn’t just sit there and feed random RF noise into your system from an unused signal source.  Used improperly, these controls can also “squelch” your intended signal as you move further from the transmitter.  Test the setting of this control with the vocalist or instrumentalist as far from the receiver as he or she is likely to be during a performance, turned so that the transmitter is on the “wrong” side relative to the receiver.  Or if you know that signal source will be used consistently throughout the performance, consider not using the “squelch” feature at all.

A wireless system simply replaces a signal cable.  It will transmit anything that’s fed into it, and output a fairly good representation of that same signal.  So all the decisions that go into microphone selection (impedance, design, directionality) still have to be made.  Pretty much any signal source (microphone, instrument, etc.) can be used with a wireless transmitter and receiver.  A wireless system is simply a very expensive cable that you can’t trip over - but carry an ordinary signal cable with you as a backup in case of malfunction of your wireless system; it’s cheap insurance.


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