Introduction to Wireless Microphones

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.