Wireless microphone
From Wikipedia, the free encyclopedia
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.
Contents [hide]
1 Advantages and disadvantages
2 Techniques
3 Products
4 Technologies
4.1 VHF
4.2 UHF
4.3 Digital and Hybrid
5 External links
[edit] Advantages and disadvantages
Wireless microphones awaiting pickup by performers in a musical.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.
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, 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 they 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, 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.
[edit] Technologies
[edit] 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.
[edit] 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.
Example UHF transmission frequencies Channel Frequency
U1 801.375 MHz
U2 801.875 MHz
U3 803.125 MHz
U4 803.750 MHz
U5 804.500 MHz
U6 805.750 MHz
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.: [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]
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.
[edit] Digital and Hybrid
Wireless microphone signals can be transmitted digitally in an analog
format over normal frequency modulated carriers using predictive
algorithms via DSP(digital hybrid) or fully digitally transmitted
involving digital modulation of the carrier. One advantage of digital
encoding of the signal at the transmitter is that companding
artifacts are bypassed, allowing for more linearity and dynamic
range. Like with multiplexed analog FM (used for IEM systems, for
instance), stereo signals can be sent over one RF carrier. Linear
response down to 20 Hz is possible with digital wireless systems,
unlike analog wireless microphones that often have reduced response
below 100 Hz (though there are a few analog wireless products linear
down to 40 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 don't affect the digital audio
stream. Fully digital systems can also offer data encryption, for
applications where information security is important. A disadvantage
to 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 greater than
for full digital and normal analog wireless.
Spread Spectrum technology can be used to put more wireless
microphones in the same RF space. At least one manufacturer offers a
spread spectrum wireless microphone system that operates in the 2.4
GHz range, the same range as Wi-Fi.[1]
Manufacturers that offer digital wireless microphone systems include
Sony, Audio-Technica, Lectrosonics, MIPRO, and Zaxcom.
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Received on Sat Mar 02 2024 - 00:57:19 CST