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Tech Talk 201



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Wireless Solutions


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.


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.


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 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 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.


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 elastic bandages.

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.


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.


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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