- Quote: "There is nothing in the world worse than a cheap
wireless system," Hosch says. "It's scary when your
future is hanging on a $ 1.98 battery. Anytime you can, you should
get someone to use a wired mic over a wireless system."
Scheirman adds, "and just remember that even the best wireless
microphone system, a $10,000 system, is almost as good as a mic
cable." (From TCI, May 1993, page 25).
A wireless microphone system is a small-scale version of a typical
commercial FM broadcasting system. In a commercial broadcasting
system, a radio announcer speaks into a microphone that is connected
to a high-power transmitter in a fixed location. The transmitted
voice is picked up by a FM receiver and heard through a speaker
or headset.
In a wireless microphone system, the components are miniaturized
but the same principles apply. The transmitter is small enough
to fit into the microphone handle or into a small pocket-sized
case. Since the microphone and transmitter are battery powered,
the user is free to move around while speaking or singing into
the mic. The transmitted voice is picked up by a receiver that
is wired to a speaker.
Two types of microphones are available with wireless mic systems:
the handheld mic, with a transmitter in its handle; and the lavalier
mic, which is small enough to be concealed as a lapel pin or
mounted on a headset. Lavalier mics are wired to miniature body-pack
transmitters, which fit into a pocket or clip onto a belt. There
are also some transmitter which accepts any three-pin XLR input
and acts as a wireless mic-then you can use any mic you want
with the transmitter.
Wireless microphones are widely used today in the DJ industry.
They eliminate the need for cables around the dancefloor. Wireless
mics make it possible to obtain usable audio from any place on
the dancefloor or table area.
Handheld mics are used by DJs where they provide the freedom
needed to move around the dancefloor and gesture spontaneously.
They are used by speakers and entertainers who need to pass the
mic from one person to another. In concerts, hand-held wireless
mics permit vocalists to walk and dance around the stage and
even into the audience without restriction and with no chance
of shock in the event of rain.
Lavalier and headset mics are used in game shows, soap operas
and dance routines. They eliminate the need for handheld mics
and help to alleviate visual clutter. Lavalier mics are used
by MCs, panelists, lecturers, clergy, stage actors, and dancers
because they can be concealed easily and provide hands-free mobility.
Some lavalier transmitter models have high impedance line inputs
that accept cords to create wireless electric guitars.
Technology in the early 1970s introduced the integrated circuit
component which was incorporated into wireless mics to reduce
noise. At about the same time, the FCC authorized the use of
frequencies in TV channels 7-13 for wireless mics. Thus the wireless
microphone's most serious problem, radio interference from other
services, was virtually eliminated. Later, the application of
diversity reception minimized the problem of dropouts (transmission
losses due to cancellation of radio waves), greatly improving
system reliability.
Today's wireless mics perform almost as well as conventional
wired mics. In the 1980s, wireless mics were manufactured with
an improved dynamic range and smaller transmitters, a result
of better component integrated circuitry and advanced circuit
design techniques. A variety of standard microphones with different
sound characteristics is available.
There are no international standards for wireless mic radio frequency
allocations. Performance is not controlled for transmitter power
limits, frequency stability, or RF bandwidth occupancy. Wireless
mics could therefore, theoretically, operate at any frequency.
Certain frequency bands are more commonly used. In the United
States, the FCC regulates the operation of wireless mics at specified
frequency bands.
WIRELESS PROBLEMS - Transmission Loss
There is a calculated transmission loss between transmitter and
receiver through use of an isotropic antenna. Less transmitter
power is required for an equivalent signal strength at the receiver
as frequency is lowered. One problem with wireless microphones
is the difficulty in designing antennas that are small but efficient
in the VHF low-band area. However, for the VHF high-band, small
and efficient antennas are practical.
Interference from other radio services is the major problem at
both VHF and UHF. The only clear channels available are the unused
TV channels in a given location and the "B" channels.
For touring groups the TV channels become a problem, as a clear
TV channel in one city may not be clear in another. Therefore,
the "B" channels are recommended for this purpose.
[The "B" channels are specific frequency bands designated
by the FCC for wireless mics.]
Dropout
Loss of reception at the receiver of a wireless mic due to radio
wave cancellation called multipath reflection is usually referred
to as dropout. This problem has several possible sources. Dropout
characteristics are different between VHF and UHF frequency bands.
The dropout zones are much shorter at UHF where rapid flutter
is often heard.
Loss of reception may also be caused by a transmitter being too
far away from the receiver. This may be corrected by relocating
either the transmitter and receiver antennas closer to each other.
The power of a signal received by an antenna is a critical factor
in causing dropout. When examining practical solutions and limitations
in alleviating dropout, it is important to consider that not
all of the power transmitted will reach the receiver. A wireless
mic transmitter radiates power in many directions simultaneously,
depending on the specific mechanical configuration of the antenna
system. This makes the transmission vulnerable to many types
of interference.
System performance is degraded by path losses due to interfering
objects between the transmitter and receiver, such as other equipment
or people, as well as by the position of the transmitter antenna
and interfering signals due to multipath reflections.
Several paths can occur when the environment in which the wireless
microphone is operating contains objects such as music equipment,
lighting equipment, or stage props made of metal or other materials
that reflect radio signals. Due to phase differential of the
arriving signal, the resultant signal can be enhanced or totally
cancelled, causing multipath dropouts. These path losses affect
the total power received at the antenna. Multipath cancellation
is the most common cause of dropout.
Solutions
*Use a high gain receiving antenna at the mix position: High
gain antennas can improve the signal-to-noise ratio, and may
thus reduce fades and dropouts if they are due to weak. Signal
cancellations will not be aided. High gain receiving antennas
are generally also a bad idea because: (a) the transmitter is
constantly moving around with the performer so the antenna would
have to be continuously re-aimed, and (b) much of the received
radio signal is actually caught on the bounce from walls, props,
etc., so even if one stood offstage and aimed a beam antenna
at the performer, it could be aiming at the wrong target.
*Place the receiving antenna(s) and receivers near the mic(s)
and run audio signals back to the mix position: With wireless
mics, an alternative is to place the receiving antenna(s) on
or above the stage, run a moderate length of antenna cable to
a nearby wireless mic receiver, and then run a standard audio
cable between the receiver's audio output and the mixer's input.
Most receivers provide line level outputs that are ideal in this
situation. This keeps the mic transmitting antenna(s) and the
receiving antenna(s) reasonably close, which optimizes the RF
S/N ratio.
*Diversity reception: In some wireless microphone installations,
it may be impossible to locate a single antenna to eliminate
multipath dropout or signal fading. The technique that has been
adapted for wireless microphones to minimize multipath dropouts
is called diversity reception. This is the application of two
or more receiving antennas to receive signals that have been
diverted into more than one path (multipath). The idea, in general,
is that if the signal is weak at one antenna, it will probably
be stronger at the other, at any given instant. Diversity reception
enhances the performance of a wireless mic system. It is usually
effective, although nothing will guarantee a total absence of
dead spots. There are a number of different ways to accomplish
diversity reception, and each manufacturer of wireless microphones
tends to favor one approach or another. The conditions required
to achieve this reception are:
*a single transmitter source
*uncorrelated, statistically independent signals
*multiple receiving antenna systems.
-
- This success of any diversity reception system depends on
the degree to which the independently received signals are uncorrelated.
If a diversity reception system cannot produce uncorrelated,
statistically independent signals, then diversity reception does
not exist.
Implementation of a diversity reception system can be accomplished
in several ways, but all system implementations have the need
to combine the received, independent signals in some method.
The major drawback with any multiple reception diversity system
is cost. Combining techniques are chosen based on cost and the
degree of improvement required. The less predictable or less
closely related the signals, the more significant the benefits
of the diversity system.
There are various techniques of diversity reception based on
the exact method for processing and extracting the transmitted
signals. Space diversity is the technique most commonly used
for wireless mics. Space diversity can be implemented in many
different ways, but the three basic requirements of diversity
reception mentioned earlier must be satisfied. Two or more receiving
antennas are required and must be at least one half wavelength
apart (typically three feet). The amount of separation determines
the degree of the uncorrelated signals. Polarization diversity
is a method of space diversity in which the antennas on the receiving
system are placed at angles to each other in order to capture
the uncorrelated, independent signal. Each antenna provides an
independent path that is selected or combined to produce the
desired signal improvement. These selecting and combining methods
of processing the independent signals are shown below:
In space diversity the incoming signal with the best signal-to-noise
ratio is selected from the two or more antennas used. The signal
selection can be accomplished either prior to or after audio
detection.
Another method of signal improvement is that of combining the
incoming independent signals. The two methods of doing this are
called maximal ratio combining and equal gain combining. In maximal
ratio combining, independent signals are combined in order to
derive the maximum signal voltage/noise power ratios from each
of them. A modification of this approach is equal gain combining
in which all incoming signals are set to an average constant
value.
Clearly, the maximal ratio combining method offers the best possibility
for improvement over a non-diversity system, although it is the
most difficult to implement. Wireless mics typically use selection
or equal gain combining diversity. The choice is based on greatest
reduction of the probability of dropouts. Any of the selection/combining
techniques can be implemented in the pre-detection or post-detection
stage of the receiver.
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:
*transmitter spurious emissions,
*transmitter and receiver inter-modulation, and
*splatter
-
- 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.
WIRELESS MOUNTING
In the case of concealed mics, either suspended round the neck
or clipped to clothing, three things are vital:
1] The material must not generate static electricity-this tends
to rule out silk garments. Clothes with metal supports can also
cause problems.
2] The antenna lead must be straight and firm-not allowed to
bend and break, it is best taped to the skin.
3] 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 lavalieres 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 strain relief.
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 wool's. 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 ™, will reduce static electric
discharge, clinging, andreduce 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 lavalieres is that of wind noise.
Manufacturers usually supply small foam or metal mesh windscreens
with their lavalieres, but these are usually more effective against
breath pops than against outdoor gusts of wind. Lavalieres 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 lavalieres
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
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 elastic
bandages.
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.
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 DJ 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 DJ and the receiver. Pay attention to lighting or
speaker stands that may be in the way.
Examine the location itself. Check for additional electrical
lines, especially coiled feeds, which can generate magnetic fields.
Dimmers and special effects equipment (especially neon's) are
always a problem. Motors can produce interference; be aware of
golf carts, forklifts, camera cranes, automobiles, and kitchen
appliances.
WIRELESS OPERATION
Mics with switches should never be purchased-Control should always
be with the operator-but in radio mics sometimes a switch is
an asset since offstage and private conversation will be picked
up by a neck mic which the DJ 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 operator 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.
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