TIPS FOR WIRELESS MIC USERS
By: Rick Shaw
Inevitably the moment comes in most engineers’ lives when they must use wireless microphones. At that point, it’s not unusual for the engineer to experience premonitions of disaster and be overwhelmed by an impending sense of doom. Will the mic go out in the middle of the show? What if the batteries go dead?
However, the good news is that today’s wireless mics are more reliable than ever before, and with a little preparation the chances of a disaster are diminished further.
My television audio experience began at a large teleproduction facility that used multiple wireless systems–everything from headsets to CB radios’ for security personnel to multi-channel wireless mic systems. We had to make our mics work reliably for live broadcast every day. There were problems, and it took careful planning to ensure acceptable daily results. Finally, the facility manager assigned an engineer to maintain and supervise the use of the wireless system.
Fortunately, you don’t have to put an engineer on the case. There are sensible steps anyone can take to assure success when working with a wireless mic system.
Wireless technology isn’t cheap, and although it’s a cliche, when it comes to wireless mics you get what you pay for. You are likely to find that low-cost, semi-professional system suffer from interference problems, poor construction, inferior audio fidelity, poor head room and noise. In general, more expensive professional models are more durable and reliable. Having confined your wireless mic selection to professional units, let’s build a four-mic system to illustrate the important considerations you must weigh when evaluating wireless mics. Let’s also assume that each mic in this system will operate on an independent channel.
First, I’d recommend that only diversity systems be considered. A diversity system has a special receiver with dual channels. The receiver can home in on signals from the transmitter. In situations where phase cancellation occurs in the RF signal from the transmitter, a diversity receiver can switch to its alternate channel and separate antenna and eliminate any perceptible dropout in the signal.
Signal dropouts can be more than the simple loss of sound. They can be loud, noisy, static incidents that are unacceptable during a shoot. In my opinion, signal dropouts are intolerable and usually can be avoided by using a diversity system.
Second, be sure the receiver provides a professional balanced output. Some systems use a 1/4-inch phone jack for output. This is unacceptable in most cases. A 1/4-inch phone jack only works when the receiver is positioned near the audio console and the console has unbalanced line inputs to accommodate the receiver. I would recommend a professional XLR balanced output because it offers the most flexibility.
It’s also important to have the ability to switch between mic- and line-level output. I usually prefer using the line level output because the receiver may have to push the signal through several hundred feet of cable. Under such circumstances, line-level output offers a better signal-to-noise ratio. When using mic-level output, be sure to switch off the phantom power on the channels of the audio console dedicated to the wireless receivers. Phantom power is great for the condenser mics designed to use it, but it can cause problems with other equipment in the audio chain.
When buying a wireless mic system, you must choose between a bodypack transmitter and accompanying lavalier element and a hand-held mic. Some manufacturers offer a selection of hand-held mic elements. If you have the budget, it makes sense to buy a hand-held and a lavalier/bodypack combination for each receiver channel. This adds flexibility by allowing you to use either on any receiver channel. It also prevents an engineer from having to rely on certain channels for a hand-held or lavalier mic. An additional benefit is that if a system breaks down, you’re covered. Understand that you can’t transmit on a hand-held and a bodypack on the same frequency at the same time. They will interfere with each other.
Another important feature is having either a built-in limiter circuit or commander within the transmitter. This adds cost to the system but is the only way to keep the mic element from overmodulating the entire system. It also improves the dynamic range. Remember that once you’ve handed the mic to the talent and walked into the control room, the mic attenuation is no longer under your control. If our talent decides to shout or sing in the middle of a speaking segment, start looking for another job.
When designing a multi-channel system, you should seek assistance from the wireless mic manufacturer. It takes careful planning to select the right frequencies for each unit. You also need to know what frequencies are active in the vicinity of your studio. Finding out could take a few days of work using an RF spectrum analyzer to monitor the frequencies you plan to use in your system. Sometimes wireless manufacturers have access to this information. Additionally, interference isn’t too much of a problem if your mic system is powerful. Where it becomes critical is in multi-channel systems. A receiver in a multi-channel system can pick up a harmonic from an adjacent transmitter and amplify it. This is an example of an interaction problem and the best reason to let the manufacturer help choose compatible frequencies.
Powering the mic transmitter is another important issue to consider. Normally, wireless mics use 9-volt or AAA batteries. There is nothing wrong with either approach, and professional systems are designed to get the most out of the juice they’re supplied. However, it’s your responsibility to supply reliable power. This area is sometimes the most overlooked.
To go with alkalines or NiCads depends on how much your system will be used. If the system is used once or twice a month, buying a few good alkalines should do the trick. If you use the system regularly, I’d recommend going with NiCads (they’re also easier on the environment) and a good recharging system. You should have a separate set of fresh batteries for both the rehearsals and the performance. These should be kept ready and freshly charged for each session. Always use a fresh set of batteries when you begin a shoot. Take out those used for rehearsal and put them back in the charger. Rotate the batteries in this fashion. It might also be a good idea to color-code or mark them in some way so
that you can identify which set is fresh and which is depleted. Also identify on the batteries the date they were purchased because even the most fastidiously cared-for battery eventually will wear out.
An important consideration in any professional wireless mic application is the distance between the transmitters and the receivers. This area should be held to a minimum. If your production requires the talent to walk across a large studio or outdoor area, you may need to build an antenna combiner for all your wireless receivers. Master antenna elements then can be placed or hung at the most advantageous place, or at a central position within the range of motion, for best coverage. This method also eliminates having to use a separate antenna system for each receiver.
When you’re on location, examine the placement of your receivers, antennas and transmitters because surrounding structures and radio systems can cause interference. If you’re transmitting near a corrugated steel wall, such as those found in some auditoriums, problems may arise that aren’t easily solved. Move as far away as possible from metal structures. Make sure your antennas don’t touch
anything, and don’t place your receivers with their antennas attached within a steel rack, unless your antennas are remotely positioned in the open. I usually position my antennas on diversity systems so they are 90-degree angles to each other, which provides better coverage.
Always allow enough time to test your systems at the location. You should begin such a test by powering up your receivers and listening for hums and buzzes at the console. If you get a hum, snap an ordinary microphone on the cable and see if the hum goes away.
Once you’re satisfied that the line is clean, switch on the system. Put fresh batteries into the first mic and have someone walk around the location, speaking at various levels. If you hear any clipping distortion, and you know the input is adjusted properly at the console, you may need to adjust the input sensitivity at the microphone. Normally, most manufacturers provide a tweaker adjustment–some use numerical positions on the pot. Don’t be afraid to make this adjustment. That’s what it’s there for, and your system might not work properly without taking the time to adjust it.
Another important adjustment available on some systems is the squelch, located at the receiver. Turn the squelch slowly until all the hash sound goes away and the channel is clean. If you have a pair of headphones, you may be able to monitor the receiver at the source, which is a good way to check for problems. Some receivers have a jack on the front to provide for this monitoring. Likewise, most systems provide metering that can be switched from RF signal strength to audio output so that the receiver operation can be checked. Additional LEDs also are handy to indicate the active A or B diversity channels and other receiver operations.
In most cases, hand-held mic transmitters are superior to lavaliers. This is because lavaliers are temperamental and aren’t always the best choice in a PA environment. A hand-held will deliver better gain before feedback. Elements can be chosen with a cardioid pattern to increase directionality. There are a few lavaliers available that have limited directionality, but this can affect the gain.
Some hand-held mics need to be grasped in a particular area to avoid interfering with their transmission. Antennas are mounted in various places on hand-helds, so a little experimentation might be necessary. Bodypack wireless transmitters that use a lavalier mic element are invaluable in video and film applications where a mic needs to be concealed or where a hand mic is inappropriate to the look of a show. When buying a bodypack system, you may be able to choose among mic elements
to find the one that will best work with a particular transmitter. Many times a lavalier is attached via a Limo snap-in type of connector. Some wireless manufacturers use their own type of connector out of necessity. I like the Limo the best. It is a durable, well-made connector. Sometimes an adapter can be made to convert the Limo to an XLR so that several types of lavaliers can be used on a single system.
Other than battery connections, connectors are the first area of weakness in wireless mics. A loose wire can cause intermittent problems, such as fizzing, popping or other noise in the signal. During your setup, wiggle the connections for the lavalier and the antenna while the transmitter is on to check for
failures. It’s better to find out before the program starts.
Take care when placing a bodypack system on the talent. Clothing can play a big part in whether the system will work successfully. As odd as that sounds, you need a rigid place to attach the transmitter, and there has to be a way to mount the mic element elegantly. Certain fabrics can cause problems.
A silk tie can give you more problems in some cases than cotton. You don’t want the talent to sound as if he’s wrapped in cellophane because of clothing that rubs against the mic or fabrics rubbing together near the mic. If the mic touches anything, it can ruin your recording. One way to avoid problems is to use gaffer’s tape to keep a tie from moving, a collar from touching or a sweater from rubbing. Sometimes you may be required to tape layers of clothing together to keep a sensitive mic from picking up the sound of clothing rubbing against itself.
Normally, the transmitter is hung from the waistband or belt. This is a secure area and will usually support the weight of the system. If you’re working on an exercise show and the talent is wearing stretchy clothing, beef up the waistband with gaffer’s tape and place the transmitter in the small of the back. In normal situations, where the talent is wearing a belt, the transmitter is clipped on the side, hidden by the coat. Most bodypacks have a wire antenna that shouldn’t be kinked or coiled. I usually try to run it through a couple of belt loops to keep it straight and offer some strain relief for the wire.
The next step is to run the mic cable. Usually, this should go under the shirt and appear around the third button hole from the collar on a man’s shirt, if he’s not wearing a tie. The wire should be given a single loop right on the mix clip and then attached under the clip on the back side of the shirt. This helps to prevent rubbing noises from the cable itself being mechanically conducted into the mix
element. The same goes for placing the mix on a tie (see illustrations). The fabric loop on the back of most ties is also a convenient place through which to run the cable.
Wireless mics are complicated devices that can sound almost like their wired counterparts if they aren’t taken for granted. If your studio uses wireless mics, allow enough time for setup and adjustment, and you’ll get better results. Place your antennas carefully and don’t expect great sound to come too easily. The technology has improved over the years, but it still takes some finessing to get the most out of a wireless system. Of course, as part of your preparation, it never hurts to have a wired mic plugged in behind the set–just in case.
(The above material appeared in the April 1992 issue of AV Video.)
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