Introduction to Wireless Microphones

There is a saying in Hollywood that the use of wireless microphones is more of a mystic art than it is a science. In spite of all of the technological advances, achieving reliable performance from radio mics is still a best case rather than an every case scenario.

The fault lies not with the manufacturers, but with the government. Due to restrictions on power output (a mere 50 milliwatts) and frequency allocation (sharing the wavelength with television channels), professional wireless mic units are readily susceptible to dropouts and local RF interference.

What this means in plain English is that a twelve year old kid can walk into a Radio Shack and buy a walkie-talkie or CB that puts out 5 watts (5000 milliwatts), yet Uncle Sam won't trust professional soundpeople with more than 50 milliwatts!

As for the matter of RF interference, anything that has ever caused your television set to hiccup (such as overhead aircraft, vacuum cleaners, computers, passing trucks) may also interfere with your wireless mic.

Never-the-less, radio mics are definitely an important tool for Production Sound recording. They are often the only practical way to get the dialogue.

Think of them as wireless cables. There is nothing wireless about the microphone itself. The fact is, it is the cable connecting the microphone to the mixer or recorder that is wireless. The microphone remains wired to the transmitter.

You may think that I am indulging in a silly game of semantics, but this is an important concept to understand. The lavalier mic is not the wireless part of the system; the XLR mic cable is what the transmitter/receiver is replacing. Virtually ANY mic, including boompole mounted shotguns, can be used with a wireless system providing that you have the appropriate adapter cable or connector.

Another concept that is important is that because wireless mics (of any brand) are always a bit of a gamble, you should hedge your bet by only deploying wireless when you absolutely have no alternate solution. Avoid reaching for your wireless as a first resort. Using radio mics is sort of like going to the dentist: it is not a fun experience but we all do it when we have to.

Exhaust all of the "hardwire" ways of miking a shot. If you can't boom it from overhead, maybe you can mic it from below. Perhaps a strategically placed "plant" or fixed mic can be rigged outside the frame or hidden within the set. If you do need to resort to a body worn lavalier, it might be possible to trail a mic line from talent's ankle.

Some scenes can be started on a wireless for the master shot, and then switched to boom mics or hardwired lavaliers for the closer angles.

Next we will look at Budget Factors.

Budget Factors

Do wireless mics save the production company money?

There is a popular myth that using wireless mics will save time and money. Not!

Wireless mics cost money to rent as well as to operate. For instance, the average daily rental for a good wireless system is approximately $35 to $75 per unit. That is not taking into consideration the cost of batteries, nor the time spent in rigging them and trouble-shooting.

How many units do you need to budget for?

The answer is: a couple more than you plan on using. If you have only one actor and you bring only one radio mic, then what happens if that radio mic stops working either due to an internal electronic malfunction or on account of local RF interference on its frequencies? Do we all get to go home for the afternoon?

Add to this the cost of batteries.

Radio mics can consume an awfully expensive pile of batteries over the course of a production. Most brands operate from 9-volt alkalines, which cost around $2.75 each. Some of the newer units utilize a handful of AA or AAA batteries per reload, and their cost adds up quickly, as well.

The most common cause of poor radio mic performance is weak batteries! Always begin your shoot with fresh, premium batteries installed in the transmitter and receiver.

The battery in the body pack transmitter should be changed around every four hours, more or less. Even though some manufacturers claim eight hours of life, I don't know of any top notch soundpeople who feel comfortable going that long on one battery. After four hours, battery voltage tends to drop off steeply, along with transmitter range and clarity.

In addition, you do not want to interrupt the flow of activity on the set in order to change batteries while the director is "cooking". Professionally, it is safer to change batteries frequently than to risk an ill-timed delay or a bad take.

Receivers don't eat batteries quite as much. Some receivers can last eight hours on a single set of batteries.

And if you do need to check the voltage of a battery, or to replace it, it is usually easier to access the receiver (sitting out in the open) than to fumble with a transmitter buried under someone's wardrobe.

Battery voltage should be checked with a digital volt meter. Inexpensive digital meters can be purchased at your local electronics store for under twenty dollars.

A fresh 9-volt battery puts out around 9.30 volts. Replace your batteries at around 8.2 volts or slightly lower, based on your experience with the radio mics.

By the way, your "discarded" batteries still have plenty of voltage for most consumer devices, so it is not necessary to toss them in the trash. Just don't use them for professional equipment.

There is one more budget factor to consider... time.

Radio mics require fifteen or twenty minutes per unit to properly hide and rig under wardrobe. Longer, if you experience difficulties with clothing noise.

Deciding to use radio mics on a shoot in order to save money is a mistake. It is less expensive and much more reliable to hire a good boom operator.

But bear in mind that there are many situations where wireless is the best, if not only, practical option!

Choosing a wireless mic system.

There are several factors to consider when selecting a wireless mic system.

Handheld vs. Body pack

Most of the wireless transmitters used for Production Sound are of the body pack format. Body packs with lavalier mics are commonly used for dialogue.

Handheld mics are generally used for vocal performance, reporting, or audience Q & A. It is possible to request both styles of transmitters for use with a single receiver.

Handheld transmitters can be either a permanent microphone or a plug-on style. The permanent handheld mic is what you normally see vocalists using. The microphone has a built-in transmitter in the base.

A plug-on style transmitter (affectionately nicknamed butt plugs by the Hollywood industry) consists of a wireless cube or cylinder that has a female XLR connector. This module simply plugs in to the bottom of a conventional hand mic. Although a few brands of plug-on modules can provide Phantom power to condenser mics, most of the modules do not. However, the majority of hand mics used by reporters are dynamic and do not require any external powering.

Plug-on transmitter modules are preferred by reporters, as that allows them the best combination of dynamic mic in a wireless configuration. Plug-on modules are sometimes used with shotgun mics when cables would be impractical (such as sound effects mics at sporting events).

Note that you cannot utilize two transmitters on the same frequency at the same time. When two transmitters are operating simultaneously, the result is not a blending of the audio as some would expect, but rather a jamming of each other's encoded RF transmissions.

However, there are some models of wireless mic that consist of dual receivers in a single metal casing. Each receiver can be set to its own distinct frequency, and has its own separate output. It is like mounting two receivers back to back, except that these dual units are much smaller and lighter, and share common antennas. An example is the 1820 series ENG wireless from Audio Technica

One more note. Plug-on transmitters usually lack the extended reach of a good body pack, due to size and internalized antenna. When boom operators need to be wireless, the preferred technique is to use an adapter cable to allow the XLR cable coming from the boompole to plug in to the small connection on the body pack (where the lav normally goes). If Phantom power is required for the boom mic, then the boompole cable plugs in to a belt mounted battery power supply, such as the Denecke PS-1, and then the mic output from the power supply goes into the wireless.

ENG vs. Rack mount

ENG receivers are designed for field or camcorder use and have been miniaturized and designed to operate from internal battery power (or an optional power feed from some cameras). The receivers are relatively small and can be mounted on the camera (or a mounting rig, in the case of very small cameras).

Rack mount receivers are intended for a more fixed location, such as a soundstage, studio, theater, or concert performances. These not-so-portable receivers may require 120v AC and tend to be physically much larger. Rack mountable units sometimes have the advantage of more sophisticated front end filtering to reduce interference, and often feature diversity antenna systems as well.

Rack mount units, because of their better front end filtering, are preferred in situations calling for a large number of radio units to operate simultaneously.

The smaller ENG units sacrifice some of the more exotic front end circuitry in order to achieve compactness.

VHF vs. UHF

VHF professional wireless mic frequencies (169-210 MHz) overlap the standard analog television channels 7 thru 13.

That means that many radio mics will only operate interference free in some cities, dependent on the local TV channel line-up. There may also be issues with the new frequencies designated by the change-over to digital broadcasting, so be wary. As of now, it is still legal to use VHF channels 7-13 (but the rules could change).

VHF units tended to offer greater range, longer battery life, lower purchase price, but are more susceptible to interference. There were a handful of legal frequencies just under channel 7, roughly 169-174 MHz, referred to as "A" frequencies or "travelers". This narrow range of frequencies used to work in roughly 90% of the cities nationwide. Again, be cautious of the new digital frequency allocations.

UHF frequencies are much higher up on the spectrum.

As of this writing, it is still legal to use UHF channels 14-51, and those well above channel 69.

An article about the use of UHF frequencies can be found here.
http://www.audio-technica.com/cms/resource_library/literature/706eb4edd613bfcd/white_spaces_update_12082008.pdf

The industry has pretty much switched over to the UHF frequencies, as there are more of them and less chance of interference. Although earlier UHF models were notorious for short operating range (compared to VHF) and higher manufacturing costs, the situation has evolved to the point where neither factor is an issue.

Todays UHF wireless are small, power efficient, and more than able to get the job done. In fact, you would have to look very hard to locate professional wireless systems that still operate in the VHF range.

Single frequency vs. mulitple frequency selection

 In the older days, wireless systems generally only operated on a single, factory installed frequency per receiver. If there was any local RF interference in that frequency, you had to set aside that particular unit and go with another.

Some later VHF and UHF systems were able to incorporate multiple frequencies into their design. A flick of a switch (on the receiver AND transmitter) would allow you to change over to a second frequency.

Today, it is almost the universal norm for wireless systems to be frequency agile. What that means is that you can select from several to several hundred different frequencies (sub-frequencies, to be technical). Some of the better models even offer internal frequency scan, to help you choose a clear operating channel for each of your systems.

Here is a useful operating tip for determining if your frequency choice is a good one. After selecting a frequency (whether suggested by an internal scanner, or just by random guessing), set the receiver and transmitter. Take a listen.

If the system sounds noisy, or the signal breaks up easily, or the range is way less than you usually get -- try another frequency.

When you think that you have found a clean operating frequency, double check it by doing the following. Turn off the transmitter, but keep monitoring the receiver. Pay close attention to the RF indicator (possibly a meter, but most likely just a red LED). When the transmitter is turned off, the indicator light should go out. If, instead of going out completely, the indicator light flickers a bit -- that is an indication that something out there is broadcasting on or close to your chosen channel. Also, listen carefully. If your transmitter is off, you should hear nothing. But if there is any sort of electronic squeal or chatter, you are hearing (non-audio) radio transmissions of some sort.

Being frequency agile not only allows you the flexibility of choosing frequencies, but also the convenience of mixing & matching wireless components. Actor damages a bodypack? Just grab another one from the stack and reset the frequency. Need a handheld mic instead of a bodypack? Just grab one from another system, reset the frequency, and away you go.

Single antenna vs. Diversity

Some ENG receivers utilize a single antenna. There is only one direct path from the transmitter to the receiver; and only one location for indirect (bounced) signals to be picked up. If, for some reason, the direct radio waves cannot strike the antenna (something blocking the signal path, such as a metal object), then there will be a momentary blackout of the audio, known as a dropout. Sometimes we are lucky, and an indirect radio wave will bounce off the ground or something and still manage to be received, but not always. Remember, actors are often moving; sometimes camera mounted receivers are moving as well.

Diversity systems deploy two antennas, and internally switch back and forth to whichever antenna offers the better signal. So chances are, if the direct radio signal misses one antenna, it may still be able to be picked up by the other antenna. Same applies to bounced signals. Even though the two antennas may only be inches apart from each other, it makes a big difference. Diversity systems have way fewer dropouts than non-diversity signals.

There are different types of diversity antenna systems. Cheaper wireless may have two antennas sticking up, but that is just to fool the consumer. The two antennas are wired together, which is better than only having one antenna.

Antenna diversity systems take it a step further. A comparator inside the reciever compares the RF signal from each antenna, and selects to use the stronger signal.

True diversity, which is featured in the better units, actually processes the RF signal into audio, and then compares which one is better. Essentially, there are two miniature receivers in each receiver.

Diversity units can be rack mount size, or miniaturized for ENG camera mounting

For diversity antennas to be fully effective, they should be separated at least a quarter wavelength (around 19 inches).

Some mixers prefer to remove the antennas from the receiver and connect them via a short length (5 to 25 feet) of RG-59 antenna cable in order to increase the likelihood of one or the other receiving antenna finding a clear signal, as well as to extend the operating reach of the system.

The advantages of diversity include less likelihood of RF dropout due to the direct or reflected signal paths being obstructed, as well as increased working range (based on the antenna placement).

The disadvantages of diversity can include having to deal with two antennas; audibly recognizable "switching" (a trait more common in the cheaper systems); and the chance of one of the two antennas locking onto interference.

Quad-Box

A quad-box or quad-pack consists of four individual ENG sized receivers, non-permanently housed in a compact case.

The case includes an antenna "splitter" (RF distribution amp) so that one single antenna (or two for diversity units) provides the feed for all four receivers.

Most quad-boxes also feature a centralized battery power supply that will "externally" power all the receivers.

Quad-boxes are convenient. Their only drawback is that sometimes better performance can be achieved by separating the receivers and placing them strategically closer to the action.

Receivers may be placed in different sites to optimize antenna line-of-site for each actor in the scene.

On reality television productions, the sound mixer often wears a custom apron or chest mounted utility bag that can house a portable ENG mixer along with multiple radio mics.

In some instances, two more radio mics are deployed as a stereo hop. The two transmitters take the audio feed from the mixing board and transmits the signal back to the camcorder. Many professional soundpeople will simultaneously back up their audio onto a portable digital recorder in their bag, just to be safe.