Wireless microphone
A wireless microphone, or cordless 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. In one type the transmitter is contained within the handheld microphone body. In another type the transmitter is contained within a separate unit called a "bodypack", usually clipped to the user's belt or concealed under their clothes. The bodypack is connected by wire to a "lavalier microphone" or "lav" (a small microphone clipped to the user's lapel), a headset or earset microphone, or another wired microphone. Most bodypack designs also support a wired instrument connection (e.g., to a guitar). 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 without requiring a cable attached to the microphone.
Wireless microphones usually use the VHF or UHF radio frequency bands since they allow the transmitter to use a small unobtrusive antenna. Cheap units use a fixed frequency but most units allow a choice of several frequency channels, in case of interference on a channel or to allow the use of multiple microphones at the same time. FM modulation is usually used, although some models use digital modulation to prevent unauthorized reception by scanner radio receivers; these operate in the 900 MHz, 2.4 GHz or 6 GHz ISM bands. Some models use antenna diversity (two antennas) to prevent nulls from interrupting transmission as the performer moves around. A few low cost (or specialist) models use infrared light, although these require a direct line of sight between microphone and receiver.
History[edit]
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.[1][2]
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.[3][4][5]
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.[6] 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."[7] 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.[7]
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).[8]
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.[9] 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). Vega Electronics Corporation manufactured the design in 1959, 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.
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 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.[10][11]
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.[12]
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. Kate Bush is regarded as the first artist to have had a headset with a wireless microphone built for use in music. For her Tour of Life in 1979 she had a compact microphone combined with a self-made construction of wire clothes hangers, to free her hands for expressionist dance performances. Her idea was adopted for live performance by other artists such as Madonna and Peter Gabriel.[13]
Nady joined CBS, Sennheiser and Vega in 1996 to receive a joint Emmy Award for "pioneering [the] development of the broadcast wireless microphone".[14]
The advantages are:
The disadvantages are:
Techniques[edit]
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 and maximize signal to noise ratio.
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.
Digital[edit]
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.[16] 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 US 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. The Alteros GTX Series is a local area wireless microphone network that overcomes the line-of-sight problem by utilizing up to 64 transceivers around the performance area. It is also the only system employing Ultra WideBand pulsed RF technology which doesn't generate intermodulation products common with FM, QAM and GFSK modulated carriers used by most other systems.
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, Alteros, Audio-Technica, Lectrosonics, Line 6, MIPRO, Shure, Sony, Sennheiser and Zaxcom. All are using different digital modulation schemes from each other.