Into Thin Air: How Wireless Mikes Work

For many, there’s something magical and mysterious about wireless microphones. The first time you put on a pair of headphones and hear someone talking to you from hundreds of feet away–with no wires attached–is an almost eerie experience. Unfortunately, many wireless users skip right from awe to familiarity, and never really learn what makes a wireless microphone work.

Though wireless microphones may seem like marvels of technology, they’re not all that hard to understand. Knowing what makes a wireless microphone tick will help you make a wise decision when it’s time to buy one, and will help you get the best-possible performance from a wireless system you already own. Stick around for the next few pages, and you’ll come away with a firm grasp on how wireless microphones work.

Air and Electrons

At its very heart, a wireless microphone is just–you guessed it–a microphone. It has a diaphragm that moves in response to changes in air pressure and a conversion system to turn those tiny movements into the flow of electrons we call a signal. Wireless microphones can use a dynamic or condenser element to generate a signal, though condenser microphones are most common.

Condenser microphones are generally smaller, less expensive to build and offer better sound quality than dynamic microphones. Their main drawback is that they require a power source to operate. Since every wireless microphone already has a battery for its additional electronics, it makes sense to just tap this power source and use a condenser element. The main exception to this rule is the "snap-on" transmitter base you can attach to any microphone. Any of these transmitters work equally well with condenser or dynamic mikes.

Once the audio signal leaves the traditional microphone part of the wireless system, the real fun begins. At this point, a special circuit modulates the audio signal, in essence blending it with a high-frequency signal called a carrier. A tiny crystal sets the mike’s carrier frequency, which has to be different from any other mikes or transmitting devices operating in the same vicinity to avoid severe interference. Some mikes offer a choice of carrier frequencies (usually between two and eight) you can select by turning a small dial or flipping a switch.

It’s the carrier we’re referring to when we discuss a wireless microphone’s "frequency." There are two FCC-designated frequency ranges that wireless microphones commonly use: VHF (very high frequency) and UHF (ultra-high frequency). The VHF range covers a span from 174 to 216 megahertz (MHz), while the UHF range sits between 470 and 806MHz (with a few holes). A given wireless system may be using any one of numerous discrete frequencies within each of these ranges. Some multi-channel wireless systems use several different frequencies within the same band.

The difference between the VHF and UHF bands goes beyond the first letters of their acronyms. For starters, the VHF band is getting a little crowded compared to the UHF band, which can spell more potential interference and noise from other devices. Second, the FCC allows UHF wireless systems to use a more powerful transmitter. This usually results in shorter battery life, but can deliver greater range. The main downside to UHF microphones is their higher cost, which is in keeping with the professional market they usually serve.

Once the wireless system modulates the audio signal up to around 200MHz (VHF) or 600MHz (UHF), it’s no longer a simple audio signal. It’s now a radio-frequency (RF) signal, capable of travelling through thin air at the speed of light. All it needs is a springboard to start its travels, a springboard we call an antenna. The antenna’s job is to make the signal’s transition from circuit board to air as painless and efficient as possible.
Antennas come in several different styles, depending on the type of wireless microphone. Wireless belt packs often have a small, flexible antenna designed to dangle beneath the pack. This type of antenna is very efficient, but the nearby human body absorbs much of its output power. Some belt packs use the cable between the microphone and belt pack as the transmitting antenna, with the same loss of power due to proximity to the body.

Snap-on wireless transmitters don’t really have a traditional antenna of their own, instead relying on the transmitter case, the microphone body or the battery itself. Handheld wireless microphones with integrated electronics use the same approach. These schemes aren’t quite as efficient as a longer antenna (especially in the lower VHF band). Because handheld microphones are usually held further from the signal-absorbing body, they deliver range roughly on par with that of a belt-pack transmitter. Handheld microphones with a small protruding antenna, though not fashionable, offer the best range of all.

It’s worth noting that the FCC limits the transmitter power of devices like wireless microphones, not the actual radiated power. Hence a more efficient antenna design will result in a stronger signal with the same transmitter power. The FCC sets these limits at 50 milliwatts for VHF and 250 milliwatts for UHF.

At this point, your audio is a genuine radio-frequency signal, just like the ones received by your car stereo, TV rabbit ears and garage door opener. And, just like these signals, your audio signal needs a place to land. It needs a receiver.

It’s Better to Receive

The receiver makes up the "back half" of a wireless microphone system. Its job is to pick up the signal generated by the transmitter, separate the audio information from the high-frequency carrier and output the audio signal in a fashion useful to the outside world.

Like the transmitter, every receiver has an antenna of some kind to pick up the transmitted signal. Because we live in a noisy electronic world, the receiver’s antenna also picks a great deal of stray RF signals from other sources. In most cases, the noise hitting the receiver’s antenna is at a different frequency than the desired signal. By filtering out everything not at the transmitter’s precise frequency, the receiver can pluck the wireless microphone’s signal from a cacophony of competing noise. This is why multiple wireless microphone systems can peacefully co-exist, provided they’re all using different carrier frequencies. Each receiver is listening only for its transmitter’s frequency, and filtering out all other signals.

Believe it or not, stray RF noise doesn’t pose the largest threat to clean audio pickup–the transmitted signal itself does. Through the magic of multi-path interference, the transmitted signal can arrive at the receiving antenna having traveled two or more different routes. Called a "dropout," this interference causes your audio to disappear, replaced by silence–or worse–a burst of white noise.

There are several ways to minimize multi-path interference, each resulting in increasingly complex receiver designs. The first attaches two antennas to the receiver, in hopes that at least one will be receiving an interference-free signal at any given time. Because the wavelengths used by wireless systems are so short, antennas separated by just a few inches can give dramatically different results. This type of receiver is called a diversity design (see Figure 4a).

More complex (and effective) is a true diversity receiver. Instead of simply attaching two antennas to the same electronics, a true diversity receiver is like two separate receivers in one box. Each antenna has its own dedicated electronics, and the wireless receiver will actively switch between them as signal conditions change (see Figure 4b). The switch from one antenna to the other is instantaneous, and goes unnoticed to the ear. Some receivers have LED indicators to show which antenna assembly the receiver is using at that moment. Unless the receiver or transmitter (or both) moves, a true diversity receiver may use the same antenna for long stretches of time before conditions force it to switch.

On the Outs

After the signal travels from antenna to receiving electronics and all competing noise is filtered out, the receiver reverses the process used by the transmitter. It de-modulates the signal, removes the high-frequency carrier and restores the audio signal to its original state.

Depending on the type of receiver and recorder in use, this audio signal still may not be in the correct form. At this stage, the receiver amplifies or attenuates (reduces) the audio signal to the appropriate level. It may also balance the signal for professional applications (see "The Balancing Act" in the February, 1998 issue of Videomaker for more information on balanced audio).

Wireless receivers may put out one of three different signal levels. These are microphone level, consumer line level (-10dB) and professional line level (+4dB). Microphone level is appropriate if you’re attaching a wireless receiver directly to your camcorder’s mike input jack. You’ll usually find this type of output on 1/8-inch mini-jack connectors. Consumer line level is the right output for patching into a VCR, audiocassette deck, home stereo receiver or the like. RCA-style jacks typically carry this. The highest signal level is most common for professional applications, such as feeding a professional mixer or recorder. This type of signal usually travels on a balanced XLR or 1/4-inch tip-ring-sleeve connector.

It’s important to make sure you’re running the right type of signal from the wireless receiver into the input of your camcorder, mixer or recorder. If levels don’t match up, you’ll be faced with all sorts of signal maladies. Run a line-level receiver output into your camcorder’s mike input, for example, and the result will be severe distortion. Run a mic-level wireless output into a line-level input, and you’ll have an unusable, ultra-quiet signal that’s swamped with noise. Before you purchase a wireless system, make sure it will interface properly with your video equipment.


Other Performance Factors

With a basic understanding of how wireless microphones work, it’s time to turn our attention to some of the finer points of design. Manufacturers have many tricks up their sleeves to improve wireless microphone performance, some being more effective than others.

As already mentioned, dropouts can be a way of life regardless of the quality of your wireless system. Environments with lots of RF noise or surfaces to bounce the signal can challenge even the most expensive models. The method that the wireless receiver uses to handle dropouts is a major concern. Some systems gracefully mute the audio when the carrier drops out, with a smooth transition between the on and off states. Others create a brief "fffft!" sound as the mute circuit engages. Still other models let noise blast through at extremely loud levels. This burst-of-noise approach is clearly the least desirable. A wireless system’s ability to handle dropouts often improves with price, though not in every case. The best way to learn a system’s characteristics is to test it out yourself.

Many wireless microphone manufacturers use "companding" noise reduction. This system is so named because it combines a COMPression process at the transmitter with an expANDING step at the receiver. Companding noise reduction doesn’t help with dropout noise. Instead, it reduces the low-level hiss that all wireless systems add to the audio. Companding noise reduction can make a significant improvement in the cleanliness of the signal, delivering sound almost on-par with a wired microphone. UHF wireless systems with noise reduction offer the best possible sound, often indistinguishable from that of a wired system.

Remember that the heart of a wireless microphone system is the microphone itself. All the nifty electronics, noise reduction and antenna arrays won’t make a bad microphone sound better. Poorly designed mikes will often sound dark and dull or thin and tinny. If you’re unhappy with the sound of a given wireless system, the problem could be in the microphone itself. Try another wireless microphone at the same price point to see if your sound improves.

Don’t get too wrapped up in maximum range figures as printed by manufacturers. Those figures are often inflated, and represent absolute best-case performance. With so many variables affecting a wireless microphone’s signal, it’s possible that a mike rated at just 150 feet could outperform one rated at 800 feet in the same conditions. Shoot in a different locale, and the latter microphone may soundly outperform the one with a shorter range spec.

Finally, there are a lot of tricks you can use to improve the audio from any wireless system–see "Wireless Wisdom" sidebar for more details. With your new-found knowledge of wireless mikes, you can venture fearlessly into your next video production…with no wires attached.

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