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WIRELESS MICROPHONE SYSTEM INFORMATION

A wireless microphone, as the name implies, is a microphone without a physical cable connecting it directly to the sound recording or amplifying equipment with which it is associated.

Modern wireless microphone technology more commonly known as a Radio Microphone, there are many different standards, frequencies and transmission technologies used to replace the microphone's cable connection and make it into a wireless microphone. They can transmit, for example, in radio waves using UHF or VHF frequencies, FM, AM, or various digital modulation schemes. Some low cost models use infrared light. Infrared microphones require a direct line of sight between the microphone and the receiver, while costlier radio frequency models do not.

Some models operate on a single fixed frequency, but the more advanced models operate on a user selectable frequency to avoid interference, and allow the use of several microphones at the same time.

The advantages are:

Greater freedom of movement for the artist or speaker.
Avoidance of cabling problems common with wired microphones, caused by constant moving and stressing the cables.
Reduction of cable "trip hazards" in the performance space

The professional models transmit in VHF or UHF radio frequency and have 'true' diversity reception (two separate receiver modules each with its own antenna), which eliminates dead spots (caused by phase cancellation) and the effects caused by the reflection of the radio waves on walls and surfaces in general.

Some models have adjustable gain on the microphone itself, to be able to accommodate different level sources, such as loud instruments or quiet voices. Adjustable gain helps to avoid clipping.

Some models have adjustable squelch, which silences the output when the receiver does not get a strong or quality signal from the microphone, instead of reproducing noise. When squelch is adjusted, the threshold of the signal quality or level is adjusted.

Generally there are three wireless microphone types: handheld, plug-on and bodypack:

Handheld looks like a 'normal' wired microphone, but may have a bigger body to accommodate the transmitter and battery pack.
Plug-on or cube-style transmitters attach to the bottom of a standard microphone, thus converting it to wireless operation (see below).
Bodypack is a small box housing the transmitter and battery pack, but not the microphone itself. It is attachable to belt or elsewhere and has a wire going to headset, lavalier microphone or a guitar.

Receivers

There are many types of Wireless microphone receiver.

True Diversity receivers have two radio modules and two antennas. Diversity receivers have one radio module and two antennas, although some times the second antenna may not be obviously visible.

Non-diversity receivers have only one antenna.

Receivers are commonly housed in a half-rack configuration, so that two can be mounted together in a rack system. For large complex multi channel radio microphone systems, as used in broadcast television studios and musical theatre productions, modular receiver systems with several (commonly six or eight) true diversity receivers slotting into a rack mounted mainframe housing are available. Several mainframes may be used together in a rack to supply the number of receivers required. In some musical theatre productions, systems with forty or more radio microphones are not unusual.

Bandwidth and Spectrum

Almost all wireless microphone systems use wideband FM modulation, requiring approximately 200 kHz of bandwidth. Because of the relatively large bandwidth requirements, wireless microphone use is effectively restricted to VHF and above.

Many older wireless microphone systems operate in the VHF part of the electromagnetic spectrum. Systems operating in this range are often crystal-controlled, and therefore operate on a single frequency. However, if this frequency is chosen properly, the system will be able to operate for years without any problems.

Most modern wireless microphone products operate in the UHF television band, however. In the United States, this band extends from 470 MHz to 698 MHz. Other countries have similar band limits; for example, Great Britain's UHF TV band extends from 470 MHz to 854 MHz. Typically, wireless microphones operate on unused TV channels, with room for one to two microphones per megahertz of spectrum available.

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