MULTIPLE WIRELESS SYSTEMS
- A wireless microphone
requires system design and analysis consistent with the channels and
particular design being used. When using multiple wireless mics, the
following interference sources must be considered:
Spurious signals are generated
within the transmitter due to mixing products created in multiplying
the crystal oscillator to the carrier frequency. These mixing
products, if the fall within the bandwidth of the receiver, will be
heard as squeals or chirping sounds. The spurious outputs of the
transmitter are discrete spectral signals (splatter), and typically
cannot be removed easily once a transmitter is designed.
Transmitter intermodulation (IM)
occurs when a carrier frequency from another source is coupled into
the output stage of a transmitter and becomes a second signal source.
The transmitter IM products will overwhelm the receiver and will be
recognized as acceptable signals, thus creating the chirping and
squeals and overall sensitivity degradation.
- In the case of concealed
mics, either suspended round the neck or clipped to clothing, three
things are vital:
The material must not generate
static electricity-- this tends to rule out silk garments. Clothes
with metal supports can also cause problems.
- The antenna lead must be
straight and firm-- not allowed to bend and break, it is best taped
to the skin.
- The mic itself should be as
near the mouth as possible-- unless specified otherwise most neck
mics and omni-directional and will generate feedback if the gain is
really turned up-- which it might need to be if the mic is buried at
chest level. Small cardioid pickup capsules are available but they
can lose some frequency response which might need some correcting at
the mixer. Since these mics are tiny they can also be concealed in
wigs, and in all locations need frequent cleaning to remove
perspiration and make-up.
CLOTHING NOISE & WIND NOISE
- One of the ever-present
difficulties in hiding lavaliers under wardrobe is clothing noise. In
actuality, there are two different causes of "clothing
noise": contact noise and acoustic noise.
Contact noise is the result of
garments rubbing directly against either the mic capsule itself or
the leading few inches of cable (equally sensitive to friction).
Contact noise can usually be controlled-- if not completely
eliminated-- by careful positioning and taping down of the mic and cable.
Begin by securing the clothing on
both sides of the mic capsule. This can be done by sandwiching the
mic between two sticky triangles of cloth camera or gaffers tape.
Form these triangles by folding a few inches of 1" wide tape
corner over corner, similar to folding a flag.
By immobilizing the mic between
both layers of clothing, you have eliminated the possibility of
either layer of clothing rubbing against or flapping onto the microphone.
If the lavalier must be
positioned between skin and clothing, or attached directly to skin,
then a professional medical/surgical tape should be used against the skin.
Once the mic capsule has been
secured, the next step is to form a strain relief for the thin cable.
Make a small loop just under the mic capsule. In the case of very
sensitive mics, such as the ECM-77 and MKE 2, make the loop go around
twice. Tie a small thread of camera tape (sticky side out) to
preserve the loop. Tie the loop loose enough so that it can
"breathe" (change diameter to absorb tugs).
Apply a few inches of tape along
the cable below the loop. Any tension on the cable will be absorbed
by the garment, rather than by the microphone (which is somewhat
isolated by the floating loop).
When using an external "tie
clip," it is still important to think it terms of creating a
strain relief. Loop the thin cable up and under the tie clip, forming
a semi-circle, and passing through the wide hinge of the clip.
Continue the loop behind the garment, and bring the cable around
downward, thus completing the circle. As the cable loops downward, it
should be inserted between the jaws of the tie clip and the back of
the garment. Hide the balanced of the cable behind the wardrobe.
Not only is this arrangement more
pleasing to the eye than a dangling cable, but the floating loop of
cable isolates the mic while the grip of the tie clip serves as a
Acoustic clothing noise is the
sound generated by the clothing itself as garments or layers rub
against each other when the actor moves. Noise is much more prevalent
from synthetic fabrics than from natural cottons or wools. There is
no simple remedy, only prevention, so it is wise to consult early
with the wardrobe department.
However, here are a couple of
tricks that may help. Anti-static sprays, such as Static Guard (tm),
will reduce static electric discharge, clinging, and reduce friction.
Dry silicon spray lubricants sometimes help, but be careful of
staining. Stiff or starched clothing can be softened with water or
alcohol (make sure the colors don't bleed). Saddle soap, silicon, or
light oil can take the bite out of hard leather.
Another noise problem common to
lavaliers is that of wind noise.
Manufacturers usually supply
small foam or metal mesh windscreens with their lavaliers, but these
are usually more effective against breath pops than against outdoor
gusts of wind.
Lavaliers used under clothing
have the advantage of being partially shielded from the wind, but may
still require added protection.
Clothing rubbing against
windscreens can be extremely noisy, so that great care must be taken
when using hidden lavaliers out of doors. Surrounding the windscreen
with sticky tape and securing it to both layers of clothing, as you
would a bare mic, will reduce the friction noise. However, the tape
may destroy a foam windscreen when it is removed! Inexpensive,
expendable foam windscreens can be made by wrapping the mic in
acoustafoam; or by pulling the foam booties off of video cleaning swabs.
Cheesecloth over a mic works very
well against wind. Another Hollywood variation is to snip the finger
tips off of children's woolen gloves, and pull the wool tips over a
lavalier wrapped in foam or cheesecloth.
WIRELESS BODYPACK APPLICATION
- Without question, the most
difficult aspect in using radio mics is correctly attaching them to
the body of the actor or actress.
Actors have never been amenable
to the idea of welding a body-pack directly to their skin, so...
Body pack transmitters can be
hidden almost anywhere. The most common sites include the small of
the back, rear hip, inside thigh, ankle, pants pocket, and inside
chest pocket of a jacket, or in the heroine's purse. When talent is
wearing a scant bathing suit, for example, radio mics can sometimes
be hidden under straw hats, or even on the back of the neck under
long tresses of hair. Leg warmers provide a convenient place to hide
radios when dealing with exercise attire.
There are a number of ways
transmitters may be secured. Belt clips work fine under a jacket or
loose top. Special pouches or pockets can be pinned (or permanently
sewn) into wardrobe. Sometimes it is possible to merely hang the unit
with a safety pin that has been taped onto the transmitter casing.
Specially constructed elastic belts can be worn around the waist,
thigh, calf, or ankle. Transmitters can also be held in place by
Anytime camera and gaffers tape
is used, special care must be taken not to tape directly to skin or
delicate wardrobe. Fold the tape over itself to form a non-adhesive
strip to wrap around first. Better yet, use some sort of liner, such
as a strip of cloth, wide gauze, or even a length of toilet paper.
Avoid placing the transmitter
directly against the skin, since perspiration does not get along well
with fragile electronics. Many mixers have found that unlubricated
condoms provide excellent protection from excess perspiration, rain,
or water spray. Normal-size condoms work fine, just stretch them out
a bit before rolling them onto the body-pack.
Kai's note: when buying condoms
for wireless mic body-packs, buy the large size. For some kicks, buy
some cigarettes, No-Doz, and Tylenol at the same time and watch the
cashier's reaction. By the way, you'll actually use the Tylenol,
No-Doz, and cigarettes (if you smoke) during the production runs, so
they aren't just entertainment.
Care should be taken in securing
the flexible transmitter antenna cable. To prevent the antenna from
being torn from its connector the first time the actor moves or bends
over, use a rubber band to provide elastic strain relief. Attach one
of the rubber band to the tip of the antenna. The free end of the
rubber band can be safety-pinned to the clothing or taped in place
(use medical tape on skin). Thus, the antenna can be maintained
reasonably straight (a little bit of slack is okay) yet protected
against tearing. Avoid running the antenna directly against the skin,
since body moisture tends to interfere with (absorb) the outgoing signal.
The transmitter antenna can be
run vertically up or down from the body pack. However, if the antenna
trails downward, then the transmitter should be mounted in an
inverted position to avoid making a loop in the line. The transmitter
antenna can also be run horizontally, such as partially around the
waist. However, in these instances, the receiver antenna may need to
be tilted sideways (matching the angle) to improve reception.
Under no circumstances should the
mic line and antenna wire ever cross. Run the microphone cable out
from the body pack in the opposite direction of the antenna. When the
transmitter is mounted on the body upside-down (the antenna running
downward), it is okay for the mic line to loop upward, as long as it
doesn't cross the antenna.
Install a fresh battery in the
transmitter every time you use it. It sounds like a detail that
should be obvious, but all too often radio mic problems boil down to
a weak battery in the transmitter. Change the battery frequently--
every four to six hours with most brands.
WIRELESS RECEIVER PLACEMENT
- Strive to maintain minimum
distance between the transmitter and receiver. Move the
receiver/antenna from shot to shot in order to achieve close and
clean line-of-sight placement. Given the option, it is better to run
long lengths of audio cable (from receiver to recorder) than to have
long lengths of antenna cable (from antenna to receiver).
Virtually anything can interfere
with good radio transmission and cause bursts of static. Check for
metallic objects of any kind, such as jewelry, zippers, coins, snaps,
and keys. If you cannot eliminate the metal, then at least reposition
the antenna on the actor.
Carefully eyeball the path of the
transmission between the actor and the receiver. Pay attention to
lighting or grip stands that may suddenly have appeared. A new influx
of crew members or spectators can also block the RF signal.
Examine the location itself.
Check for additional electrical lines, especially coiled feeds, which
can generate magnetic fields. Dimmers and special effects equipment
(especially neons) are always a problem. Motors can produce
interference; be aware of golf carts, forklifts, camera cranes,
automobiles, and kitchen appliances.
Video and computer equipment can
create strange fields. Be aware of Steadicams and other camera mounts
relying on high intensity video or radio controlled camera functions.
- Mics with switches should
never be purchased--Êcontrol should always be with the
operator-- but in radio mics sometimes a switch is an asset since
offstage and dressing rooms conversation will be picked up by a neck
mic which the performer cannot easily unplug and which the operator
may have forgotten to fade out, of course he has to remember to
switch it on again! Wherever radio mics are used it is vital that the
mixing desk is fitted with a pre-fade listen push so that the
operator can listen in to the channel before the performer goes on
stage and check that all is well.
If mics with switches are used,
be sure to tape over the switches so that the actor/performer does
not voluntarily or involuntarily switch the mic on or off. Control
should lie solely with the mixing engineer or members of the sound crew.
WIRELESS REPAIR and TROUBLESHOOTING
- If you work with wireless
mics for an extended period of time (i.e. not just one day), you are
bound to come across problems.
Dead mics? Replace batteries. If
the belt-pack is clearly on, check the physical connection point
between the mic and the belt-pack. If the system is detachable, test
the belt-pack connection with a mic that you know works.
Sometimes the mics will
"thunk" on and off. The problem, other than being out of
range, is usually that the battery is too lose in its chamber and is
contacting on and off. Either add padding at the bottom of the
battery case or adjust the battery contacts inside the body-pack.
Other problems? Check the
location of the body pack on the performer. Sometimes metal objects
will interfere with transmission. Check the location of the receiver.
It may be too close to metal objects or sources of interference.
Change the position of the antenna(s) or even the whole receiver.
Other than that? Get a new wireless system. Or a new auditorium.
Whichever is faster.