A wireless microphone is a microphone without a physical cable connecting it directly to the sound recording or amplifying equipment with which it is associated. Also known as a radio microphone, it has a small, battery-powered radio transmitter in the microphone body, which transmits the audio signal from the microphone by radio waves to a nearby receiver unit, which recovers the audio. The other audio equipment is connected to the receiver unit by cable. Wireless microphones are widely used in the entertainment industry, television broadcasting, and public speaking to allow public speakers, interviewers, performers, and entertainers to move about freely while using a microphone to amplify their voices.
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 (or specialist) 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.
Various individuals and organizations claim to be the inventors of the wireless microphone.
Figure skater and Royal Air Force flight engineer Reg Moores developed a radio microphone in 1947 that he first used in the Tom Arnold production "Aladdin on Ice" at Brighton's sports stadium from September 1949 through the Christmas season. Moores affixed the wireless transmitter to the costume of the character Abanazar, and it worked perfectly. Moores did not patent his idea as he was illegally using the radio frequency 76 MHz. The producers of the ice show decided that they would not continue using the device; they would rather hire actors and singers to perform into hidden microphones to "dub" the voices of the other ice skaters who would thus be freed to concentrate on their skating. In 1972 Moores donated his 1947 prototype to the Science Museum in London.
Herbert "Mac" McClelland, founder of McClelland Sound in Wichita, Kansas, fabricated a wireless microphone to be worn by baseball umpires at major league games broadcast by NBC from Lawrence-Dumont Stadium in 1951. The transmitter was strapped to the umpire's back. Mac's brother was Harold M. McClelland, the chief communications architect of the U.S. Air Force.
Shure Brothers claims that its "Vagabond" system from 1953 was the first "wireless microphone system for performers". Its field of coverage was a circle of "approximately 700 square feet" which corresponds to a line-of-sight distance of only 15 feet (4.6 m) from the receiver.
In 1957 the 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. The pocket-sized Mikroport incorporated a dynamic moving-coil cartridge microphone with a cardioid pickup pattern. It transmitted at 37 MHz with a specified range of 300 feet (90 m).
The first recorded patent was filed by Raymond A. Litke, an American electrical engineer with Educational Media Resources and San Jose State College, who invented a wireless microphone in 1957 to meet the multimedia needs for television, radio, and classroom instruction. His U.S. patent number 3134074 was granted in May 1964. Two microphone types were made available for purchase in 1959: hand-held and lavalier. The main transmitter module was a cigar-sized device which weighed 7 ounces (200 g). The Federal Communications Commission (FCC) granted Litke 12 frequencies at his approval hearing. It was first tested at the Olympic trials held at Stanford University in 1959, then Vega Electronics Corporation manufactured the design later that year, the product called the Vega-Mike. The device was first used by the broadcast media at the 1960 Democratic and Republican National Conventions. It allowed television reporters to roam the floor of the convention to interview participants where Presidential candidates Kennedy and Nixon spoke into the wireless microphone. Television anchor John Daly praised the microphone during a TV news broadcast in July 1960.
Introduced in 1958, by 1960 the Sony CR-4 wireless microphone was being recommended for theatre performances and nightclub acts. Animal trainers at Marineland of the Pacific in California were wearing the $250 device for performances in 1961. The 27.12 MHz solid-state FM transmitter was capable of fitting into a shirt pocket. Said to be effective out to 100 feet (30 m), it mounted a flexible dangling antenna and a detachable dynamic microphone. The tube-based receiver incorporated a carrying drawer for the transmitter and a small monitor loudspeaker with volume control.
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, through the efforts of Academy Award-winning Hollywood sound engineer George Groves.
Wider dynamic range came with the introduction of the first compander wireless microphone offered by Nady Systems in 1976. Todd Rundgren and The Rolling Stones were the first popular musicians to use these systems live in concert. Nady joined CBS, Sennheiser and Vega in 1996 to receive a joint Emmy Award for "pioneering [the] development of the broadcast wireless microphone".
Advantages and disadvantages 
||This article contains a pro and con list. (November 2012)|
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 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 or, more often, from other radio equipment or other radio microphones, though models with many frequency-synthesized switch-selectable channels are now plentiful and cost effective.
- Operation time is limited relative to battery life; it is shorter than a normal condenser microphone due to greater drain on batteries from transmitting circuitry, and from circuitry giving extra features, if present.
- Noise or dead spots (places where it doesn't work, especially in non-diversity systems)
- Limited number of operating microphones at the same time and place, due to the limited number of radio channels (frequencies).
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. (See antenna diversity).
Another technique used to improve the sound quality (actually, to improve the dynamic range), is companding. Nady Systems, Inc. was the first to offer this technology in wireless microphones in 1976, which was based on the patent obtained by company founder John Nady.
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.
The original manufacturer of the wireless microphone was Vega Electronics Corporation. AKG Acoustics, Audio Ltd, Audio-Technica, Electro-Voice, Lectrosonics, MIPRO, Nady Systems, Inc, Samson Technologies, Sennheiser, Shure, Sony and Zaxcom 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, several high-end systems can consist of over 100 different microphones operating simultaneously. However, the ability to have more microphones operating at the same time increases the cost due to component specifications, design and construction. That is one reason for such large price differences between different series of wireless systems.
Generally there are three wireless microphone types: handheld, plug-in and bodypack:
- Handheld looks like a 'normal' wired microphone, but may have a bigger body to accommodate the transmitter and battery pack.
- Plug-in, plug-on, slot-in, 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.
Several manufacturers including Sennheiser, AKG, Nady Systems, Lectrosonics and Zaxcom offer a plug-in 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 production engineer may use a plug-in transmitter to enable wireless transmission of a highly directional rifle (or "shotgun") microphone, removing the safety hazard of a cable connection and permitting the production engineer 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-in 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 many types of 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 (that is to say the receiver is enclosed in a box 1U high and half-width, so two receivers can be installed in 1U). 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.
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 into a special housing on many professional broadcast standard video cameras are produced by manufacturers including Sennheiser, Lectrosonics 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.
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. In 2010 the Federal Communication Commission issued new regulations on the operations of TV-band devices. 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.
Intermodulation (IM) is a major problem when operating multiple systems in one location. IM occurs when two or more RF signals mix in a non-linear circuit, such as an oscillator or mixer. When this occurs, predictable combinations of these frequencies can occur. For example, the combinations 2A-B, 2B-A, and A+B-C might occur, where A, B, and C are the frequencies in operation. If one of these combinations is close to the operating frequency of another system (or one of the original frequencies A, B, or C), then interference will result on that channel. The solution to this problem is to manually calculate all of the possible products, or use a computer program that does this calculation automatically.
Digital Hybrid Wireless 
Digital Hybrid systems use an analog FM transmission scheme in combination with digital signal processing (DSP) to enhance the system's audio. Using DSP allows the use of digital techniques impossible in the analog domain such as predictive algorithms, thus achieving a flatter frequency response in the audio spectrum and also further reduce noise and other undesirable artifacts when compared to pure analog systems.
Another approach is to use DSP in order to emulate analog companding schemes in order to maintain compatibility between older analog systems and newer systems.
A number of pure digital wireless microphone systems do exist and there are many different digital modulation schemes possible. Digital audio compression is normally used in order to reduce the occupied RF bandwidth. The RF bandwidth that would be required to transmit un-compressed digital audio with sufficient resolution, dynamic range and audio bandwidth for professional audio applications is generally regarded as prohibitive otherwise.
The Zaxcom, Lectrosonics 700, AKG and MIPRO systems, for example, use 200 kHz narrowband UHF broadcast frequencies, the same UHF frequencies used by analogue FM systems, for transmission of a digital signal at a fixed bit rate. These systems encode an RF carrier with one or two channels of digital audio. Advantages of such system include low noise, low distortion, the opportunity for encryption, and enhanced transmission reliability.
Pure digital systems take various forms. Some systems use frequency-hopping spread spectrum technology, similar to that used for cordless phones. As this can require more bandwidth than a wideband FM signal, these microphones typically operate in the unlicensed 900 MHz, 2.4 GHz or 6 GHz bands. The absence of any requirement for a license in these frequency bands is an added attraction for many users, regardless of the technology used. The 900 MHz band is not an option outside of the USA and Canada as it is used by GSM cellular mobile phone networks in most other parts of the world. The 2.4 GHz band is increasingly congested with various systems including WiFi (also referred to as Wireless LAN, wireless networks, 802.11b/g/n), Bluetooth and 'leakage' from microwave ovens. The 6 GHz band has problems of range (requires line of sight) due to the extremely short transmission carrier wavelengths.
Digital radio microphones are inherently more difficult for the casual 'scanner' listener to intercept because conventional "scanning receivers" are generally only capable of de-modulating conventional analogue modulation schemes such as FM and AM. However, some digital wireless microphone systems additionally use encryption technology in an attempt to prevent more serious 'eavesdropping' which may be of concern for corporate users and those using radio microphones in security sensitive situations.
Manufacturers currently offering digital wireless microphone systems include AKG-Acoustics, Audio-Technica, Lectrosonics, Line 6, MIPRO, Shure, Sony, Sennheiser and Zaxcom.
In the UK, use of wireless microphone systems requires a license, except for the license free bands of 173.8–175.0 MHz and 863–865 MHz (N.B. This is not to be confused with TV Channel 69, which operates on 854–862 MHz. In the UK Channel 69 frequencies require a license from JFMG Ltd.:  ).
The UK communications regulator, Ofcom, had said that it would auction that part of the UHF spectrum currently reserved for wireless microphones, to which objections have been raised by Andrew Lloyd Webber   and many others. Following public consultations this decision was changed and the UHF frequencies used for radio microphones are to be licensed to a commercially based Band Manager  with special obligations to "Programme Making and Special Events" (PMSE) users following a "beauty contest" selection process. The future of the UHF spectrum above 798 MHz, including 'Channel 69' has been further thrown into doubt by moves across Europe to create a new 800 MHz band for mobile broadband applications. 
Licenses are required to use wireless microphones on vacant TV channels in the United States as they are a part of the Broadcast Auxiliary Service (BAS). However, this requirement is often overlooked and rarely enforced by the FCC. Licenses are available only to broadcasters, cable networks, television and film producers. However, the FCC has issued a Report and Order stating that they now no longer allow Broadcast Auxiliary Service (BAS) devices to operate in the 698–806 MHz portion of the spectrum due to their auction of the 700 MHz band. This change is unrelated to, but commonly confused with, the White Space device debate that is currently taking place in the U.S.
The same Report and Order, issued January 15, 2010, also permits most wireless microphones and other 'low power auxiliary stations' in the "core TV band" (TV channels 2 through 51, except 37) to operate with transmit power up to 50 mW without a license, under a special waiver of Part 15 rules. A rule change to make this permanent is proposed.
There are currently some wireless microphone manufacturers that are marketing wireless microphones for use in the United States that operate within the 944–952 MHz band reserved for studio-transmitter link communications. These microphones have the potential to interfere with studio-transmitter links, and their use must be coordinated by the Society of Broadcast Engineers. Licenses in this band are only available to licensees of radio and TV stations, and broadcasters are likely to report unauthorized use in this band due to the high potential for interference.
In Australia, operation of wireless microphones of up to 100 mW between 520 MHz and 820 MHz is covered by a class license, allowing any user to operate the devices without obtaining an individual license. The onus, however, is on the user of the wireless microphone to resolve any interference that the use of the microphone may cause to licensed radio communications services.
Other countries 
In many other countries wireless microphone use does require a license. Some governments regard all radio frequencies as military assets and the use of unlicensed radio transmitters, even wireless microphones, may be severely punished.
For further information regarding licensing in European countries try the European Radio Office (ERO) based in Denmark 
White Space Devices (United States) 
There is currently a movement to allow the operation of personal unlicensed, wideband digital devices in the UHF television spectrum in the United States. These devices are backed by firms which seek to develop and deploy these devices as quickly as possible. These 'white space' devices (WSDs) will be required to have GPS and access to a location database to avoid interfering with other users of the band. Initial tests performed by the FCC have shown that in some cases, prototypes of these devices are unable to correctly identify frequencies that are in use, and may therefore accidentally transmit on top of these users. Broadcasters, theaters, and wireless microphone manufacturers are firmly against these types of devices ostensibly for this reason.
Later tests by the FCC  indicate that the devices can safely be used. This has not reduced the opposition by broadcasters who may also be concerned by the possibility of entertainment delivery competition from high-speed mobile Internet access delivered in the white spaces. A decision on whether and under what rules to allow these devices is on the docket for the November 4, 2008 meeting of the FCC.
Cognitive Access (UK) 
A similar class of device to those known in the US as White Space Devices (WSD) is being researched in the UK and probably many other countries. Whilst the WSD situation in the USA is being closely watched by interested parties in the UK and elsewhere, early in 2009 Ofcom launched research and a public consultation on Cognitive Access to the UHF interleaved spectrum . The outcome of this consultation and the related WSD activities in the USA could have far reaching implications for users of UHF radio microphones in the UK and around the world.
See also 
- Boom operator (media)
- Electronic field production (EFP)
- Electronic news gathering (ENG)
- FM transmitter (personal device)
- Professional video camera
- Recording studio
- Television production
- Television studio
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- "Equipment Profile: Sony Wireless Microphone, Model CR-4". Audio (Radio Magazine) 44: 44. 1960.
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- Telex SAFE-1000 wireless microphone
- Lectrosonics UDR700 Encrypted wireless receiver
- Radiocommunications (Low Interference Potential Devices) Class Licence 2000
^ McClelland Sound History: 1940–1950. Retrieved on January 14, 2010. ^ Litke's wireless microphone patent ^ Litke interviews appeared in both the San Jose News of Sept. 10, 1960, and The Alma Signal-Enterprise (KS) dated Nov. 10, 1960, and Feb. 26, 1981. ^ Telex SAFE-1000 wireless microphone ^ Lectrosonics UDR700 Encrypted wireless receiver ^ http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-10-16A1.pdf ^ http://www.fcc.gov/cgb/consumerfacts/wirelessmic_advisory.html ^ Radiocommunications (Low Interference Potential Devices) Class Licence 2000