Wireless microphone
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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 organizations 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.
Modern wireless microphone technology, which for the first time offered performance with audio and dynamic range equivalent to a cord, originated with the introduction of the first compander wireless microphone offered by Nady Systems, Inc in 1976 according to company claims. Nady systems, Inc was honored with an Emmy award for this breakthrough technical achievement in 1996.
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.
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[edit] Advantages and disadvantages
The advantages are:
- The freedom of movement for the artist or speaker.
- Avoidance of cabling problems common with 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 due to battery life.
- Noise or dead spots (places where it doesn't work, in non-diversity systems)
- Limited number of operating microphones at the same time and place, due to the limited number of radio channels (frequencies).
[edit] 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. 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.
[edit] Products
Electro-Voice, Shure, Nady Systems, Inc, Audio Ltd, Sennheiser, Lectrosonics, Samson Technologies, AKG Acoustics MIPRO 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 there 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.
- 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-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 production engineer 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 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-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 theater 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 theater 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.
[edit] Bandwidth and Spectrum
Almost all wireless microphone systems use wideband FM modulation, which requires approximately 200 kHz of bandwidth to operate in. 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 806 MHz. 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 MHz of spectrum available. Starting in January of 2009 UHF channels for wireless microphones will be limited to below 695MHz, due to the FCC auctioning the "white space" frequencies between 695-806MHz for other uses.
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.
[edit] Digital
There are two commonly-used digital transmission schemes in use today: Digital Hybrid and Pure Digital.
Digital Hybrid systems use an analog FM audio signal in combination with digital signal processing (DSP) to enhance the system's audio. Using DSP, it is easier to achieve a flat frequency response in the audio spectrum and to reduce noise and other undesirable effects. Since the audio is transmitted via FM, these systems use 200-500 kHz of bandwidth.
Pure Digital systems may take various forms, but many systems use frequency-hopping spread spectrum technology, similar to that used for cordless phones. As this requires more bandwidth than an FM signal, these microphones operate in the 900 MHz or 2.4 GHz unlicensed bands. Unfortunately, this results in interference to and from wireless computer networks, Bluetooth devices, cordless phones, amateur radio operators, and many other things.[citation needed]
Manufacturers that offer digital wireless microphone systems include Audio-Technica, Lectrosonics, MIPRO, Sony, and Zaxcom.
[edit] Licensing
In the UK, use of wireless microphone systems requires a license, except for the license 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 license from JFMG Ltd.: [1] ).
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. [2] [3]
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.
In many other countries wireless microphone use requires 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.
