Wireless microphone
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Invented by Hung C. Lin, the first wireless microphone went into production in 1962, called the "transistophone" and put out by the German equipment manufacturer beyerdynamic. The first time a wireless microphone was used to record sound during filming of a motion picture was on Rex Harrison in the 1964 film My Fair Lady.
There are many standards and frequencies in wireless microphones. They can transmit, for example, in radiowaves UHF, VHF, FM, AM, and some cheap models, in infrared light. The infrared microphones have the disadvantage that they require a line of sight between the microphone and the receiver, while more expensive radio frequency models don't.
Some models operate on a single fixed frequency, and the most advanced models are capable to operate under a user selectable frequency to avoid interference and allow the use of several microphones at the same time.
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[edit] Advantages and disadvantages
The advantages are:
- The freedom of movement it gives to the artist or speaker.
- It avoids cabling problems that are very common on wired microphones, caused by constant moving and stressing the cables.
The disadvantages are:
- Sometimes limited range (a wired balanced XLR microphone can run up to 300 ft or 100 meters). Some wireless systems have a shorter range, while more expensive models can exceed that distance.
- Possible interference with other radio equipment or other microphones, though models with many frequency-synthesized switch-selectable channels are now plentiful and cost effective.
- Limited operation time (they are battery operated)
- Noise or dead spots (places where it doesn't work, in non-diversity systems)
[edit] Techniques
The professional models transmit in radio frequency and have diversity reception (2 antennas), which eliminates dead spots (caused by phase cancellation) and the effects caused by the reflection of the radiowaves on walls and surfaces in general. (See antenna diversity).
Another technique used to improve the sound quality (actually, to improve the dynamic range), is companding.
Some models have adjustable gain on the microphone itself, to be able to accommodate different level sources, such as loud instruments or quiet voices. The ability to adjust gain helps avoiding clipping.
Some models have adjustable squelch, which silences the output when the receiver does not get a strong enough (or good enough) signal from the microphone instead of reproducing noise. When squelch is adjusted, the threshold of the signal quality or level is adjusted.
[edit] Products
Shure, Sennheiser, MIPRO, Samson, AKG Acoustics, API Karaoke, Audio2000's, Audio-Technica, and VocoPro are all major manufacturers of wireless microphone systems. They have made significant advances in dealing with many of the disadvantages listed above. For example, while there is a limited band in which the microphones can operate, the new UHF-R series from Shure can have up to 108 different microphones operating simultaneously. However, to allow for more microphones to operate at the same time, you will pay more money. That is one reason for such large price differences between different series of wireless systems. The audio quality has also greatly improved as newer systems have come to the market.
Generally they are two wireless microphone types: handheld and bodypack:
- Handheld is like a normal microphone, but it has a bigger body to accommodate the transmitter and battery pack. An example would be the Shure UHF-R/KSM 9 System.
- Bodypack is a small box housing the transmitter and battery pack, but not the microphone itself. It is attachable to belt or somewhere else and has a wire going to headset, lavalier microphone or a guitar. An example would be the UHF/U1 model from Shure.
[edit] Technologies
[edit] VHF
The VHF band is located in the frequency range between 30 MHz and 300 MHz. This is the other common band used in wireless microphone systems but it is not used as much as UHF. VHF wireless microphones are typically those of the fixed frequency type, meaning that the user has no opportunity to switch frequencies if interference is encountered.
[edit] UHF
The UHF band is located in the frequency range between 300 MHz and 3 GHz and is the most common band used in wireless microphone systems. Wireless microphones are permitted to operate in a range between 470 MHz and 806 MHz. These frequencies are shared with over-the-air TV broadcasts so when selecting a frequency, you must know what frequencies to stay away from to avoid interference.
| Channel | Frequency |
|---|---|
| U1 | 801.375 MHz |
| U2 | 801.875 MHz |
| U3 | 803.125 MHz |
| U4 | 803.750 MHz |
| U5 | 804.500 MHz |
| U6 | 805.750 MHz |
In the UK, use of wireless microphone systems requires a licence, except for the licence free band of 863MHz - 865MHz (TV Channel 69).
