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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. Various individuals and organisations claim to be the inventors of the Wireless Microphone. John F. Stephens developed an FM wireless microphone for a Navy musical show in 1951 on the Memphis Naval base. Each of the principal players/singers had their own microphone/transmitter. Subsequently, the Secret Service had Stephens modify his invention to be used in government "bugging" operations. In the '60s, Stephens marketed his more famous capstanless multitrack recorder/reproducers. Shure Incorporated claim that their "Vagabond" system from 1953 was the first. In 1957 German audio equipment manufacturer Sennheiser , at that time called Lab W, working with the German broadcaster Norddeutscher Rundfunk (NDR) exhibited a wireless microphone system. From 1958 the system was marketed through Telefunken under the name of Mikroport. Another German equipment manufacturer, Beyerdynamic, claim that first wireless microphone, was invented by Hung C. Lin . Called the "transistophone", it went into production in 1962. It is claimed that 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 . 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 radiowaves 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. ADVANTAGES AND DISADVANTAGES The advantages are:
The disadvantages are:
TECHNIQUES 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 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. 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. PRODUCTS Shure , Sennheiser , Lectrosonics , Samson Technologies , AKG Acoustics and Audio-Technica 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 may operate, the new UHF-R series from Shure can have up to 108 different microphones operating simultaneously. However, allowing more microphones to operate at the same time increases the cost. 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 become available. Generally they are two wireless microphone types: handheld and '''bodypack''':
Several manufacturers including Sennheiser, AKG, Lectrosonics and Zaxcom offer a plug-on transmitter for existing wired microphones, which plugs into the XLR output of the microphone and transmits to the manufacturer's standard receiver. This offers many of the benefits of an integrated system, and also allows microphone types (of which there may be no wireless equivalent) to be used without a cable. For example a television, or film, sound recordist may use a plug-on transmitter to enable wireless transmission of a highly directional rifle (or "shotgun") microphone, removing the safety hazard of a cable connection and permitting the recordist greater freedom to follow the action. Plug-in transmitters also allow the conversion of vintage microphone types to cordless operation. This is useful where a vintage microphone is needed for visual or other artistic reasons, and the absence of cables allows for rapid scene changes and reducing trip hazards. In some cases these plug-on transmitters can also provide 48 volt Phantom Power allowing the use of Condenser Microphone types. DC-DC Converter circuitry within the transmitter is used to multiply the battery supply, which may be three volts or less, up to the required 48 volts. There are three main types of receiver, available in two main types of housing. True Diversity receivers have two radio modules and two antennas. Diversity receivers have one radio module and two antennas. Non-diversity modules have 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 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. Receivers specifically for use with video cameras are often mounted in a bodypack configuration, typically with a hotshoe mount to be fitted onto the hotshoe of the camcorder. Small true diversity receivers which slot in to a special housing on many professional broadcast standard video cameras are produced by manufacturers including Sennheiser and Sony . For less demanding or more budget conscious video applications small non-diversity receivers are common. When used at relatively short operating distances from the transmitter this arrangement gives adequate and reliable performance. TECHNOLOGIES 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 commonly 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. 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. Depending on national regulations, which differ for every country, wireless microphones may operate in the various parts of the range between 470 MHz and 865 MHz. (for example, 470 - 806 MHz in the US). These frequencies are shared with over-the-air TV broadcasts, so when selecting a frequency, the user must know what frequencies to stay away from to avoid interference. In the UK, use of wireless microphone systems requires a licence, except for the licence free bands of 173.8 MHz - 175.0 MHz and 863 MHz - 865 MHz (N.B. This is emphatically NOT TV Channel 69. Channel 69 is from 854 - 862 MHz. In the UK Channel 69 frequencies '''do''' require a licence from JFMG Ltd.: {Link without Title} ). The UK communications regulator, Ofcom has said it will auction part of the spectrum currently reserved for wireless microphones, to which objections have been raised by Andrew Lloyd Webber . [http://www.theregister.co.uk/2007/01/04/luvies_disconnected/ In many other countries wireless microphone use requires a licence. Some governments regard all radio frequencies as military assets and the use of unlicenced radio transmitters, even wireless microphones, may be severely punished. Digital and Hybrid Wireless microphone signals can be transmitted digitally in an analog format over normal frequency modulated carriers (digital hybrid) or fully digitally transmitted involving the digital modulation of the carrier. One advantage of both methods of digital encoding of the signal at the transmitter is that companding artifacts are bypassed, allowing for more linearity in dynamic range. Stereo signals can be sent over one RF carrier. Linear response down to 20 Hz is possible, unlike analog wireless microphones that often have reduced response below 100 Hz (though there are a few analog wireless products linear down to 50 Hz.) Also, control and status signals can be sent along with the digital audio data stream, yielding more accurate reporting on battery life and changes in user settings. With fully digital solutions, FM modulation artifacts dosn't affect the digital audio stream. A disadvantage to both hybrid and full digital wireless is that the radio spectrum used by a single transmitter is wider than for analog wireless, making it more difficult to coordinate a large-scale system with many wireless devices. Practical range for digital hybrid wireless is typically less than for full digital and normal analog wireless. Spread Spectrum technology can be used to put more wireless microphones in the same RF space. Manufacturers working with digital wireless microphone technology include Lectrosonics, Sabine , and Zaxcom . EXTERNAL LINKS
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