Sound Track: Pull the Plug

How to unplug those mike cables and get the most from your wireless microphone system.

There's something magic about throwing a signal through the air and having it arrive intact somewhere else. Wireless mikes work this magic with apparent ease, freeing the videographer from a tangle of mike cables and the accompanying headaches they can cause.

Unfortunately, getting reliable audio from a wireless mike takes more than just a casual "plug it in and go" approach. In this column, we'll cover some of the tips and techniques that will make your experience with the sometimes finicky wireless mike a fruitful one.

A Wireless Overview
Before we launch into specific techniques for improving your wireless sound, let's do a brief overview of the wireless microphone market. There's a wealth of different product types and price points under the "wireless" heading, so we'll divide the market into more manageable pieces.

The first place we delineate wireless mikes is by frequency. The FCC (Federal Communications Commission) divides the radio frequency spectrum into various bands, giving them scientific names like "Low Frequency" (LF) and "Ultra High Frequency" (UHF). Wireless mikes operating in the 30-300MHz band fall in the Very High Frequency (VHF) band; mikes operating in the 300-3,000MHz range are in the FCC's UHF band. UHF mikes use a larger slice of the frequency spectrum (a wider channel) than their VHF counterparts, and so generally offer better fidelity. But to operate in the UHF band, they also need more sophisticated circuitry and they use more power. To go along with this increase in sound quality, circuitry complexity and power requirement, UHF mikes can cost two or three times more than VHF models.

Wireless microphone receivers (VHF and UHF alike) come in three different styles--standard, diversity and true diversity. A "standard" wireless receiver uses one antenna attached to a single receiver circuit, an affordable approach that's often prone to audio dropout from phase cancellation (more on this later). A diversity receiver has two antennae attached to the receiver circuit, decreasing the chances of dropout. True diversity receivers, the most expensive of the bunch, use two antennae attached to two discrete receiver circuits. The true diversity unit monitors the two receiver circuits for the strongest signal, switching silently between them as conditions change. Of the three types of wireless mike receivers, true diversity receivers are the most expensive, but offer the best insurance against dropout.

Wireless receivers are available in both stand-alone (or rack-mount) and camcorder-mounted styles. The former usually plug into an AC outlet for power; most camcorder-mounted models use a 9-volt battery. Controls on stand-alone or rack-mount receivers are sometimes easier to operate because of their larger size, but their audio performance is comparable.

Because the small size of camcorder-mounted receivers makes mounting two antennae impractical, most diversity or true-diversity receivers are stand-alone or rack-mount units. Some lower-cost dual-frequency receivers look like a diversity design (because they sport two antennae), but they're really not. Instead, they hold two standard receivers in one unit, allowing you to record two wireless microphones with a single camcorder.

Wireless mike transmitters come in several flavors as well, the transmitter with permanently attached lavalier being the most common. Transmitters built into handheld mikes are also very useful in video applications. Perhaps most flexible of all wireless mike transmitters is the transmitter that attaches right to the back of a traditional wired mike. You can attach these small transmitters to handheld mikes, boom mikes, instrument mikes, lavaliers--pretty much any professional balanced mike with XLR connector is fair game. You can also bounce the snap-on transmitter from mike to mike as your shooting needs change.

The final factors left to distinguish one wireless mike system from the next are the quality of the sound it delivers and how far it will transmit that sound. Design and construction, components used, antennae size, noise-reduction circuitry--all these play a part in a wireless mike's performance. And though the rule doesn't apply universally, you tend to get what you pay for in wireless mikes. Professional filmmakers use expensive true-diversity UHF units for a reason--these designs deliver the best-quality sound, consistently.

Making the Most of it
But what if you can't afford a professional UHF wireless mike? Is it possible to get good, reliable performance from an affordable VHF system? You bet. Apply a few simple principles of wireless mike use, and you may never again suffer from spotty, inconsistent audio from your wireless system.

There is one constant to mike performance, and it applies to all wireless mike systems regardless of price and environmental conditions. That constant is the battery. Without a strong battery, no wireless system will perform adequately. Some systems chew through batteries at an alarming pace--all the more reason to replace batteries frequently (and at the beginning of any crucial shoot). Don't try to keep track of how long a given battery has been used--it's not worth risking an audio catastrophe in the middle of the shoot. When in doubt, throw it out.

An important but confusing factor in wireless mike performance is its operating range: the maximum distance allowable between the transmitter and receiver. Don't get hung up on a manufacturer's printed maximum-range specification. Range is a complex equation with dozens of variables, from battery strength to antenna placement to the amount of competing radio-frequency noise. When the variables line up just right, an inexpensive wireless mike system may outperform one costing three times as much.

When a manufacturer lists a given range spec, say 300 feet, this is often the maximum range under perfect conditions. You may enjoy 300-foot range (or even more) in certain locales, while conditions in other places may ransack your signal beyond about 20 feet. Like the low-light lux ratings in the camcorder market, wireless range specs should be taken as guidelines--not as absolute measures of performance.

Beside battery power and distance, the biggest factor affecting wireless-mike performance is phase cancellation. As the wireless mike signal fires off in all directions from the transmitter, it ricochets off solid walls and metal objects in its path. Some of these bouncing signals will find their way to the receiver's antenna. Because they've traveled a slightly longer course to the antenna than the direct signal, these reflected signals can arrive out of phase with the direct signal. When these signals mix at the antenna, they tend to cancel each other, causing the dreaded burst of silence (or worse--static) known as audio dropout. A more technical term for this phenomenon is "multipath interference" (see figure 1).

The effects of multipath interference increase with distance, and with the number of reflective surfaces and objects between transmitter and receiver. Multipath interference is less of a factor outdoors, though it can still occur. Wireless mike signals are weak, tiny and fickle. Sometimes, their behavior is almost impossible to predict.

In most cases, you can't eliminate reflective surfaces by removing walls, ceilings or pavement. What you can do in most situations is keep the distance between transmitter and receiver to a minimum, and keep a direct line-of-sight between them. If you're shooting from a handheld camcorder, this may mean selecting different shots to keep yourself close to the action.

If you're shooting from a stationary camcorder, your options increase quite a bit in most shooting situations. Ask yourself this question, "Why does the receiver have to stay on the camcorder?" The answer, nine times out of 10, is, "It doesn't." You can improve your wireless performance by placing the receiver a little closer to the transmitter, running a slightly longer cable to the camcorder. Take this principle to the extreme, and you have one of the most dependable wireless setups going.

Say you're shooting a lecturer or speaker on a stage. She agrees to wear a wireless lavalier mike for the sake of the video. Unfortunately, you have to videotape the proceedings from the back of the hall, a good 80 feet from the stage. Here's the trick: place the wireless receiver (stand-alone style, preferably) on one edge of the stage, or just below the speaker on the ground. With this simple move, the lecturer is still untethered, and you've just cut your transmission distance from 80 feet to around 10 feet. Run a line-level signal (balanced, preferably) from the receiver to your camcorder through a well-shielded cable, and you've eliminated a slathering horde of audio gremlins. If you can only get your receiver 40 feet from the stage, you'll still improve your chances of a dropout-free recording.

Small Moves
Because radio waves are so small, movement of a few inches can make the difference between a strong signal and complete dropout. If you're experiencing dropouts, try placing your receiver in a different location. If the receiver is camcorder-mounted, try moving your tripod a few feet in either direction to get out of a "shadowy" signal area. If you're shooting handheld, listen in your headphones for dropouts. If your subject has moved into a location that causes dropout, move your camcorder location until clean audio returns.

In addition to antenna position, antenna orientation can make a big difference in wireless performance. Try positioning a receiving antenna at a 45-degree angle, or even horizontally. Try shifting the transmitting antenna in similar fashion. On wireless lavs that use the cable between mike and transmitter as an antenna, make sure the cable is stretched out as much as possible. If your transmitter has a flexible wire antenna in addition to the connecting mike cable, be sure it's stretched all the way out. While letting the antenna hang straight down is the norm, running the antenna horizontally (through belt loops, perhaps) may reduce dropout. On one shoot, the talent's body was blocking the wireless signal until we ran the flexible antenna down the front of his pant leg. When using wireless mikes, be ready to try anything to insure glitch-free audio.

Can multiple wireless mikes coexist? Yes, provided they're operating on different frequencies (or channels). This doesn't mean different bands--in other words, numerous VHF mikes operating on different channels will coexist without conflict. Two mikes operating on the same frequency, however, will clash horribly.

Many wireless systems offer a switch that selects between anywhere from two to 15 different frequencies. Other models have only one frequency, which means you must select the frequency of the mike when you buy it. It's unusual to have several mikes fighting for the same frequency, but the odds of this happening go up dramatically if all the wireless systems are from the same manufacturer.

One possible solution to clashing wireless mikes is to simply share one signal. If you and another videographer are using wireless mike systems tuned to the same frequency (especially if they're the same make and model), you should both receive a healthy signal from the one transmitter. Even if you have the option to switch frequencies and use two transmitters on one person, sharing the signal may be the best way to go.

Other Tips
On wireless transmitters that simply snap onto the back of a standard "wired" microphone, you must be sure to set the unit's modulation level (or input level) carefully. This control matches the output of the specific microphone to the transmitter's input sensitivity. Mikes with a high output level can overdrive the transmitter, resulting in harsh distortion. Mikes with a low output level will not adequately drive the transmitter, allowing noise and hiss to infiltrate the signal. For optimum performance, you may even want to adjust the modulation level for each source on which you use the mike. Increase the modulation level for someone with a soft voice; decrease the level to record a rock band in concert.

On the receiver end, take similar care in setting the receiver's output level to match the camcorder or mixer into which it's plugged. Some receivers have a variable output-level knob, others have a simple switch to select between mike- and line-level output. Have this switch or knob in the wrong position, and the result will either be distorted audio or poor noise performance. Use a line-level output signal when plugging the receiver into an audio mixer's line inputs, or the RCA-style audio inputs of a VCR or camcorder. Use mike level from the receiver when running into a camcorder's mike input, or the mike input of an audio mixer. If you're using a camcorder or audio mixer with both line and mike inputs, use a line-level signal for optimum performance.

Using the wrong output level from the receiver has the same effect as setting a transmitter's modulation level incorrectly. Run a mike-level signal into a line input, and the result is a noisy, hissy signal. Run a line-level signal into a mike input, and you'll get massively distorted audio. If you have to use a line-level output with a mike-level input, place an attenuator in line to reduce the signal strength.

Wireless mikes offer a high degree of convenience and mobility, but they're not without their drawbacks. Expect quirky behavior at any moment, and be prepared to resort to a backup plan at the drop of a signal. If you don't use headphones when shooting, now is the time to start. If your wireless system goes down for any reason, you can always pull the jack from your camcorder's mike input and limp along with the built-in mike. Not optimum sound by any stretch, but much better than silence.

These tips should help you get a handle on the sometimes unpredictable behavior of wireless mikes. Properly used and cared for, a wireless mike can be the videographer's best friend.