Wireless microphones

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.

History
Various individuals and organizations claim to be the inventors of the wireless microphone.

From about 1945 there were schematics and hobbyist kits offered in Popular Science and Popular Mechanics for making a wireless microphone that would transmit the voice to a nearby radio.

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 free 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 ft 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 ft.

The first recorded patent for a wireless microphone 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 oz. The Federal Communications Commission (FCC) granted Litke twelve frequencies at his approval hearing. It was first tested at the Olympic trials held at Stanford University in 1959. Vega Electronics Corporation manufactured the design later that year, producing it as a 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, including presidential candidates John F. Kennedy and Richard Nixon, who spoke into the wireless microphone. Television anchor John Daly praised the microphone during a TV news broadcast in July 1960.

Introduced in 1958, the Sony CR-4 wireless microphone was being recommended as early as 1960 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 ft, 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, claims that the first wireless microphone was invented by Hung C. Lin. Called the "transistophone," it went into production in 1962. The first time that a wireless microphone was used to record sound during filming of a motion picture was allegedly 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


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
 * Galvanic isolation of microphone, avoiding ground loops between microphone and other electrical instruments on stage

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).
 * Higher cost in proportion to fewer other features

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 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 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
AKG Acoustics, Audio Ltd, Audio-Technica, Electro-Voice, Lectrosonics, MIPRO, Nady Systems, Inc, Samson Technologies, Sennheiser, Shure, Sony, Wisycom 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, 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 clothing or on the body and has a wire going into a 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.

Receivers


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 currently (Jan 2014) extends from 470 MHz to 790 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 analogue 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 analogue domain such as predictive algorithms, thus achieving a flatter frequency response in the audio spectrum and also further reducing noise and other undesirable artifacts when compared to pure analogue systems.

Another approach is to use DSP in order to emulate analogue companding schemes in order to maintain compatibility between older analogue systems and newer systems. Using DSP in the receiver alone can improve the overall audio performance without the penalty of increased energy consumption and resulting battery life reduction that is incurred by incorporating DSP into a battery-powered transmitter.

Digital
A number of pure digital wireless microphone systems do exist, and there are many different digital modulation schemes possible.

Digital systems from Sennheiser, Sony, Shure, Zaxcom, AKG and MIPRO use 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 channel, or in some cases two channels, of digital audio. Only the Sennheiser Digital 9000 system, introduced in 2013, is currently capable of transmitting full-bandwidth, uncompressed, digital audio in the same 200 kHz bandwidth UHF channels that were used by analogue FM systems. The advantages offered by purely digital systems 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 and radio-controlled models. 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 offer 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. All are using different digital modulation schemes from each other.

UK
In the UK, use of wireless microphone systems requires a Wireless Telegraphy Act license, except for the license free bands of 173.8–175.0 MHz and 863–865 MHz, sometimes referred to as "Channel 70" (This is not to be confused with TV Channel 69, which was 854–862 MHz. In the UK Channel 69 frequencies always required a license from JFMG Ltd. Use of Channel 69 is no longer permitted and licences are no longer available for that band.)

Arqiva purchased PMSE band manager JFMG 19 February 2009. JFMG were contracted by communications regulator OFCOM to provide spectrum management and licensing services for programme making and special events (PMSE). In May 2015 Ofcom made the decision to end the contract with Arqiva and to insource the existing services.

Channel 69 was replaced as the UK mobile radio microphone band by channel 38 (606 MHz to 614 MHz). Licences to use this band are issued on a Shared basis which means that any frequency coordination between multiple users in or around a particular location must be done by those users themselves. All Shared Licence holders have the same rights as each other.

In 2013 the UK communications regulator, Ofcom, held an auction in which the UHF band from 790 MHz to 862 MHz was sold to be used for mobile broadband services. Objections had been raised by Andrew Lloyd Webber and many others.

The Interleaved (also known as White Space) UHF spectrum between 470 MHz to 606 MHz (Channels 21 - 37) and 614 MHz to 790 MHz (Channels 39 to 60) may be licensed on a Site Specific Coordinated basis. Coordinated licences grant the holder exclusive use of particular frequencies or blocks of spectrum at a particular location for a specified period of time. Coordination between users is carried out by Arqiva PMSE as part of the licensing process.

USA
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.

Changes beginning in 2017 concerning operation on 600 MHz frequencies. Beginning in 2017, the amount of TV band spectrum available for wireless microphone use is decreasing as a result of the incentive auction, which was completed on April 13, 2017. A significant portion of the TV band spectrum in the 600 MHz band, including most (but not all) of the spectrum on TV channels 38-51 (614-698 MHz), has been repurposed for the new 600 MHz service band for use by wireless services, and will not continue to be available for wireless microphone use. Specifically, wireless microphones that operate in the new 600 MHz service band (the 617-652 MHz and 663-698 MHz frequencies) will be required to cease operation no later than July 13, 2020, and may be required to cease operation sooner if they could cause interference to new wireless licensees that commence operations on their licensed spectrum in the 600 MHz service band. FCC 14-50, FCC 15-140, DA 17-314 Spectrum will continue to be available for wireless microphone use on the other TV channels 2-36 (TV band frequencies that fall below 608 MHz), on portions of the 600 MHz guard band (the 614-616 MHz frequencies) and the 600 MHz duplex gap (the 653-663 MHz frequencies), and in various other spectrum bands outside of the TV bands. FCC 15-100, FCC 15-99

Australia
In Australia, operation of wireless microphones of up to 100 mW between 520 MHz and 820 MHz is on unused television channels and 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. After 31 December 2014 operation will not be allowed in the frequency range 694-820 MHz. See item 22A in the schedule in the class licence

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 a move to allow the operation of personal unlicensed wideband digital devices using the UHF television spectrum in the United States. These 'white space' devices (WSDs) would 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 showed that, in some cases, prototypes of these devices were unable to correctly identify frequencies that were in use, and might therefore accidentally transmit on top of these users. Broadcasters, theaters, and wireless microphone manufacturers were firmly against these types of devices ostensibly for this reason.

Later tests by the FCC indicated that the devices could safely be used. This did not reduce the opposition by broadcasters who might also have been concerned by the possibility of entertainment delivery competition from high-speed mobile Internet access delivered in the white spaces.

On September 23, 2010 the FCC released a Memorandum Opinion and Order that determined the final rules for the use of white space for unlicensed wireless devices. The final rules adopt a proposal from the White Spaces Coalition for very strict emission rules that prevent the direct use of IEEE 802.11 (Wi-Fi) in a single channel effectively making the new spectrum unusable for Wi-Fi technologies.

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.