Using Wireless Mics
by Fred Ginsburg, C.A.S.
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 fishpole 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.
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 $40 per unit.
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 frequency?
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 $2.50 each.
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 battery, or a couple of days on two or three.
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 Radio Shack and other places for under twenty dollars.
A fresh 9-volt battery puts out around 9.30 volts. Replace your batteries at around 8.5
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, or audience Q & A. It is
possible to request both styles of transmitters on the same frequency, for use with a
single receiver.
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.
ENG vs. Rack mount:
ENG receivers are designed for field or camcorder use and have been miniaturized and
designed for internal battery power.
Rack mount receivers intended for theater or concert performances 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 television
channels 7 thru 12, as per FCC regulation.
That means that many radio mics will only operate interference free in some cities,
dependent on the local TV channel line-up.
VHF units tend to offer greater range, longer battery life, lower purchase price, but
are more susceptible to interference.
There are 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 will work in roughly 90% of the cities nationwide.
UHF frequencies are much higher up on the spectrum.
UHF's tend towards less working range, higher battery drain, much more expensive
manufacturing costs, but are less prone to interference.
The majority of radio mics used in our industry are VHF, but recent improvements
in both the pricing and performance of UHF are changing that statistic.
Single antenna vs. Diversity:
Most ENG receivers utilize a single antenna.
Diversity systems deploy two antennas, and internally switch back and forth to
whichever antenna offers the better signal.
Diversity units are usually rack mount size, although there are some ENG sized brands
available.
For diversity antennas to be 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.
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.
Diversity antenna systems are more commonly chosen for presentation, stage and concert
performances where the electronic environment is controlled but the talent's movements
across the stage are not.
Single antenna systems are fine for shot by shot production, since dropouts can be
dealt with by relocating the receiver in relation to talent.
Quad-Box:
A quad-box 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.
Rigging the Talent
Attaching the body pack transmitter:
Hiding the transmitter under the wardrobe of most male performers is usually fairly
simple, due to the fact that men generally wear looser fitting clothing.
Bulging pockets are common: stuffed with wallets, keys, handkerchief, comb, coins, etc.
The presence of a small transmitter case rarely upsets the visual lines of the fashion.
Where the transmitter will be hidden is dependent on a couple of factors, such as the
physical actions or stunts, and the contours of the wardrobe.
Common sites for the transmitter include: inside the waistband of trousers, and the
inside pocket of a jacket or sport coat.
Other sites are: inside the trouser leg, under the armpit (like a shoulder holster),
across the small of the back, or inside of a hat.
A safety pin can be attached to the transmitter by means of tape or a thick rubber
band, allowing the transmitter to be easily pinned onto wardrobe.
ACE bandages are another convenient way of securing the radio mic.
Professionals usually carry a variety of custom elastic belts and cloth pouches (such
as those made by Equipment Emporium) to facilitate rigging. The transmitters fit inside of
the pouches, which can then be pinned directly to clothing or slipped over thin,
lightweight elastic belts.
The Velcro closured belts can be worn around the waist, thigh, etc.
A couple of quick notes about belt and pouch kits. The belts should be thin; porous if
possible. Velcro and elastic can cover a wide range of sizes.
White usually conceals easier than darker colors.
Very importantly, keep the belts freshly laundered.
Hiding the transmitter on a female is often more complicated, due to the differences in
fashion.
Whereas men's clothing is commonly loose-fitting and lumpy, women's fashions tend to be
closely contoured to the body.
A variety of elastic belts are much more important, since the thinner fabrics may not
support the weight of a pin-on transmitter.
Choice of sites are based on wardrobe style, camera angle, and physical action.
Places to rig transmitters include the small of the back (waist belt), the back of the
bra, upper back (X-shaped rig), under the arm, on the inside thigh (intimate, but works
for short, tight fitting skirts), the back of the neck (under long hair), inside a leg
warmer, under a hat, or even under a wig (to accommodate on-screen nudity).
Be considerate of talent's privacy when preparing to rig them. Don't advertise all over
the set that you are about to hide a transmitter under someone's clothing!
Avoid the use of camera or gaffers tape directly against the skin. Use some sort of
cloth liner, first aid gauze, or even toilet paper to protect the skin from these tapes.
If you are in a situation that requires taping directly to skin, then use a medical
surgical tape designed for that purpose, such as 3M Micropore Action tape.
Remember to clean the surface of the skin first with an alcohol pad, in order to remove
oils and dirt.
Antenna considerations:
A prime consideration when selecting the body site to hide a transmitter is the antenna
path.
We want the transmitter antenna to have optimum "line of sight" to the
receiver antenna.
Body pack transmitters either utilize an antenna separate from the mic line (such as
Vega), or they integrate their antenna function with the ground wire of the mic (such as
LectroSonics and Audio Technica).
If talent will be sitting on a metal backed chair, it would be a poor choice to have
the antenna running along the back, say from waist to shoulder.
Similarly, if talent is facing up against a metal filing cabinet, then we would try to
avoid rigging the antenna along the front.
Separate antennas offer greater control over the antenna path. We can rig the antenna
solely on the basis of best line of sight.
The antenna should be kept somewhat taut, with just a little slack, which is best done
by attaching a rubber band and safety pin to the end. The pin is secured to clothing, and
the rubber band acts as a strain relief as the actor moves or bends.
The antenna should not loop over itself. If the best antenna placement is downward,
then invert the transmitter rather than bend the antenna.
The antenna can be kept somewhat vertical (either upward or downward), or can be angled
horizontally from the transmitter up to 90 degrees. If the transmitter antenna is angled,
sometimes reception can be improved by tilting the receiver antenna to match.
The antenna should never cross over the microphone line. It is okay for the mic line to
loop over itself (as when the transmitter is inverted). Always run the mic line and
antenna away from each other; flip the transmitter if necessary so that the lines do not
cross each other.
Moisture will absorb RF energy, and thus weaken the transmission. A rubber sheath of
shrink tubing, fuel line, or surgical tubing can help isolate the antenna from excess
perspiration, rain, etc.
The drawback to separate antennas is that they are an additional element to rig and
hide. However, the disadvantage of combination antenna/mic lines is that the mic line must
be cut to specific (antenna) length, and that the best mic path is not always the optimum
antenna site.
Greater care must be taken to keep the mic cable as straight as possible. Avoid
bunching up the mic/antenna line and "stuffing" it into a pocket or under a
waistband, since this will reduce the transmission signal.
Receiver Antennas:
Good antenna placement is the key to eliminating drop-outs and reducing the chances of
interference.
Keep the antenna path as short as possible, and transmit through as few obstacles as
possible.
Place your receivers as close as you can to the actors. Receivers can be just on the
edge of the set, or even hidden within the set.
It is more efficient to run a long length of audio cable from the receiver back to the
mixing panel than to run a long antenna cable.
Think in terms of clean line-of-sight. The best place for your sound cart may not be
the best place for your receivers!
The antennas might have a cleaner path coming in from the side or rear of the set
Mounting your antennas high will allow them to see over obstacles such as bodies and
grip stands.
Be very careful when mounting receivers onto camcorders. Make sure that the antenna
does not have to "see through" the camera body or VTR.
Be aware that a lot of RF interference can be generated by the video recording heads,
and the viewfinder.
I have found that when mounting small, inexpensive receivers (such as the Audio
Technica Pro88) onto the shoe brackets of Hi8mm and S-VHS camcorders, it is better to
rotate the receiver so that the antenna faces the forehead of the operator rather than
being located directly above the electronics of the viewfinder.
There are different types of receiver antennas.
The most common antennas that come with wireless mics are the stiff wire
"whip" antenna and the short rubber sheathed helicoil "rubber duckie".
The stiff wire antennas are the most efficient, but may not be practical in an ENG
situation. The rubber duckies are safer and more portable, but lose a little in terms of
range.
Early ENG style receivers sometimes came equipped with a limp wire antenna, similar to
those found on transmitters. The limp wire antenna is inefficient unless it is kept taut.
Compared to the wire whip or the popular rubber duckie, the limp wire is a poor choice.
Antennas as accessories:
Ground plane antennas look like little camera tripods and are designed to take
advantage of "ground plane" reflections, sort of the way a pressure zone or
boundary plate mic uses a hard surface to gather sound. Ground planes work very well for
stage shows and the like.
However, most field production involves a lot of electromagnetic equipment (lighting
units, ballasts, coils of electrical cables) strewn on the ground in their path, so ground
plane antennas would not be my personal choice.
"Dipole" antennas look like two wire whip antennas mounted back to back, in a
vertical configuration.
They achieve "higher gain" by polarizing incoming signals (sort of like
sunglasses). Radio signals that strike the antenna at ninety degrees (horizontal) are
passed with greater efficiency than spurious signals bouncing all over the place.
In a sense, these two-element dipoles are 'directional' in that they see a single plane
(picture Saturn's rings).
Dipole antenna systems are very popular on professional motion picture sets.
The yagi type of antenna resembles a two dimensional Christmas tree. It is a smaller
version of household rooftop TV antennas.
The yagi works like a shotgun mic. It is very directional and needs to be aimed towards
the transmitter. They are also very common on professional sets.
General concerns with wireless mics:
Don't expect miracles.
Even the best Hollywood sound mixers have to wrestle with their wireless. The range is
never what you expect nor what the spec sheets claim, because you will not be operating in
a perfect environment.
If you need increased range, try utilizing a dipole or yagi antenna system. You can
request them when you rent or purchase your wireless. If the salesperson doesn't know what
you are talking about, go to a real sound house run by mixers!
Another way to increase your range is to shorten the distance between the transmitter
and receiver. Have someone carry the receiver and walk the distance parallel with the
actor.
The more units that are working simultaneously, the increased likelihood of them
interfering with each other.
Actors passing close to each other may generate a buzz. If you know that actors will be
working close, assign them units on frequencies as far apart from each other as possible.
Think carefully before you just grab a radio mic and stick it on someone.
Check your batteries often. Weak batteries in the transmitter or receiver are the main
cause of problems.
Periodically re-check your transmitter rigging. Actors have a tendency of adjusting
their wardrobe, and upsetting your carefully positioned mic and/or antenna placement.
If talent will be perspiring a lot, or working in rain or near water, then it is a good
idea to encase the body pack transmitter in a protective condom and seal it with
electrical tape. Use standard non-lubricated latex condoms. But don't forget to warn your
personal mate as to their professional use, lest they be discovered when you get home!
Clothing noise:
Wireless transmitters do not suffer from clothing noise. However, the lavaliers plugged
into them certainly do!
Rig your lavaliers the same as you would if they were hardwired.
For the benefit of readers who missed the article on lavalier mics, here are a few
pointers: Make a small loop near the mic capsule. Secure the loop loosely with a piece of
thread or a thin strip of camera tape (sticky side out). The loop should be able to freely
open and close if the cable is tugged. This loop serves to cancel out most cable noise
conducted along the rubber sheath of the mic cable.
Eliminate contact clothing noise by securing the wardrobe on both sides of the mic
capsule. If the clothing is not free to rub against the mic head, then there won't be
noise.
A popular technique is to sandwich the mic head between two sticky triangles made from
camera tape. Start with a two inch long strip of (one inch wide) camera tape, and fold it
corner over corner like a flag, sticky side out. Make a second triangle the same way. Then
sandwich the mic between them, being careful not to block the grill.
In the case of a button down shirt or blouse, attach the sticky triangle onto the
fabric overlap, just above one button. Let the tied off loop hang opposite of the button.
Secure the next inch or two of mic line with a simple strip of camera tape along the
overlap, running vertically downward towards the next button. Any tugging on the cable
will be strain relieved by the section taped to the clothing.
The floating loop will isolate the mic capsule, and the twin sticky triangles will
prevent clothing from rubbing across the mic itself.
Wiring a female can be simpler. If she is wearing a bra, arrange the triangles over the
mic so that one point is down. Secure the mic inside of the bra, at the "cross your
heart" juncture in the center of the bosom.
A small strip of surgical tape can be used to anchor the mic cable along the upper
abdomen. The natural swell of the bosom protects the mic from clothing contact, as well as
positioning it out from the chest cavity.
Clothing noise can also be acoustic in nature, and is created by clothing fibers
rubbing against each other.
Starched clothing is very noise conductive, so soften the cloth with a light mist of
water wherever the camera won't notice it, such as under the sports coat. Soften a patch
of clothing around the site where the microphone is to be attached.
Combat noise from clothing friction with Static Guard.
Wind noise can be alleviated by salvaging the foam tip from a (used) video cleaning
swab. Pull the tip off of the stick, and slice the base off. What remains is a foam hood
that will slip over most lavaliers.
These free windscreens can be painted with marking pens to be less visible. And since
they cost nothing, there is no risk of sandwiching them within tape that would destroy the
foam upon removal.
Additional wind protection can be achieved by wrapping some cheesecloth over the mic.
Cut off the fingertip from a pair of child's or woman's knit gloves, and pull that hood
over the mic and cheesecloth.
Another useful trick for rigging lavaliers is to use moleskin and safety pins.
This technique is particularly effective when you have to wire talent quickly, or if
talent is going to be very physically active and might otherwise dislodge a taped on mic.
Wrap a layer of soft moleskin around the head of the mic. Insert an open safety pin,
and then wrap another layer to secure it. The lavalier is now ready to be pinned in place
under wardrobe. The moleskin tends to insulate the mic capsule from most clothing noise.
An additional strip or two of tape or moleskin can be added to help prevent clothing
from rubbing against the mic, and to strain relief the mic cable. | 
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