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Selection and Operation Selection and Operation of Wireless Microphone Systems A Shure Educational Publication.
3 Selection and Operation of Wireless Microphone Systems T ABLE OF C ONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 P ART O NE W IRELESS M ICROPHONE S YSTEMS : H OW T HEY W ORK C HAPTER 1 B ASIC R ADIO P RINCIPLES . . . . . . .
Selection and Operation of Wireless Microphone Systems 4 I NTRODUCTION The many uses of wireless microphone systems can span applications from live entertainment to earth-orbit communications. It can include devices from a single "Mr . Microphone" to a 60 channel theme park system.
RADIO W A VE TRANSMIS SION Radio refers to a class of time-varying electromagnetic fields created by varying voltages and/or currents in certain physical sources. These sources may be "artificial," such as electrical power and electronic circuits, or "natural," such as the atmosphere (lightning) and stars (sunspots).
The speed of radio waves (through a vacuum) is equal to approximately 3 x 10 8 meter/second, or about 186,000 miles/ second. This is also known as the "speed of light," since light is just one part of the radio spectrum.
ambient radio "noise," that is, general radio energy produced by many natural and man-made sources across a wide range of frequencies. The strength of ambient radio noise is relatively constant in a given area, that is, it does not diminish with distance.
SYSTEM DESCRIPTION The function of a radio or "wireless" system is to send information in the form of a radio signal. In this presentation, the information is assumed to be an audio signal, but of course video, data, or control signals can all be sent via radio waves.
microphone level, and usually low impedance. Since the "wireless" part of the wireless microphone only serves to replace the cable, ideally , the characteristics and performance of a particular microphone should not change when used as part of a wireless microphone system.
Handheld transmitter controls are generally limited to a power switch, a mute switch, and gain adjustment. Again, tuneable models include some provision for frequency selection. Indicators are comparable to those in bodypack transmitters: power status, battery condition, frequency .
A variation that is found in a few compander designs is to divide the audio signal into two or more frequency bands. Each band is then pre-emphasized and compressed independently . In the receiver , de-emphasis and expansion are applied separately to these same bands before combining them back into a full-range audio signal.
A few tuneable transmitters use multiple crystals to obtain multiple frequencies. However , the base frequency of the VCO for most tuneable systems is adjustable by a technique known as frequency synthesis.
is not tuneable. Filter circuits of various types ranging from simple coils to precision "helical resonators" are used in front end filters. The second receiver section is the "local oscillator" (usually abbreviated as "L O").
only of frequency variations of the original input signal. This effectively eliminates the (high-frequency) carrier frequency leaving only the low-frequency modulation information (the original audio signal).
This effectively prevents the possibility of noise from the receiver when the desired transmitter signal is lost, even in the presence of a (non-tone-key) interfering signal at the same frequency . T urn-on and turn-off delays are incorporated in the transmitter tone-key circuits so that the transmitter power switch operates silently .
These multiple paths result in differing levels, arrival times and phase relationships between the radio waves. The net received signal strength at any location is the sum of the direct and reflected waves. These waves can reinforce or interfere with each other depending on their relative amplitude and phase.
The next variation, "antenna switching diversity ," again consists of a single receiver with two antennas. The receiver includes circuitry that selects the antenna with the better signal according to an evaluation of the radio signal.
output will usually consist of a mix of the two audio sections. In the case of loss of reception at one antenna, the output is chosen from the other section. Excellent dropout protection is obtained with no possibility of switching noise since the diversity circuit is essentially an intelligent panpot, not a switch.
In all of these designs, the radio wave pattern emitted by the 1/4 wave antenna is omnidirectional in the plane perpendicular to the axis of the antenna. For a vertically oriented 1/4 wave antenna the radiation pattern is omnidirectional in the horizontal plane, which is the typical case for a trailing wire antenna.
varies in a logarithmic progression so that at any given frequency one or more dipoles are active while the others are functioning as reflecting or directing elements, depending on their size and location relative to the active element(s). The longer the boom and the greater the number of elements the greater is the bandwidth and the directivity .
usually operate at "unity" gain overall, that is, no net amplification occurs. Though a multi-coupler is generally a separate acces- sory , some receiver designs are equipped with internal antenna distribution when multiple receiver sections are incorporated in the same chassis such as modular or card-cage systems.
FREQUENCY BANDS FOR WIRELES S SYSTEMS Existing wireless microphone systems transmit and receive on a specific radio frequency , called the operating frequency . Individual radio frequencies are found in frequency "bands" which are specific ranges of frequencies.
"travelling frequencies," because they can (theoretically) be used throughout the US without concern for interference from broadcast television. Legal limits of deviation ( + / _ 12 KHz) allow high quality audio transmission. Once again, power is limited to 50 mw .
THE UHF BAND The low-band UHF range of frequencies may be con- sidered as two overlapping bands: low (450-536 MHz) and high (470-806). The primary users of these bands are business services such as land mobile radio and pagers (450-536 MHz) and UHF television channels 14-69 (470- 806 MHz).
SYSTEM COMP A TIBILITY The two main areas of concern are: interaction between transmitters and receivers related to their operating frequencies, and interactions between transmitters and receivers related to their internal frequencies.
operating frequency . This further restricts available frequency choices as the number of simultaneous systems increases. It should be apparent from this discussion that the prediction of potential compatibility problems due to IM products is best left to computer programs.
received signal by an amount equal to the intermediate frequency (IF). Specifically , the operating frequency is above the local oscillator frequency by an interval equal to the IF .
multipliers. However , both types of transmitters can produce other spurious emissions due to power regulating circuitry , parasitic oscillations, carrier harmonics, etc.
180-186 MHz and so on up to channel 13 at 210-216 MHz. UHF channel 14 begins at 470 MHz and extends to 476 MHz with successive channels up to channel 69 at 800-806 MHz. The 6 MHz/TV channel block is found in the US, the rest of North America, South America and Japan.
Selection and Operation of Wireless Microphone Systems 30 C HAPTER 3 W ireless System Operation DTV VS. WIRELES S SYSTEMS In the United States, the Federal Communications Commission (FCC) is currently supervising the transition of broadcast television from its traditional analog format to an all-digital format (DTV).
BROADCAST RADIO High-band VHF wireless FM systems are not generally subject to interference from commercial AM or FM radio stations. Both bands are well below the VHF band and in particular , these systems are not typically sensitive to moderate AM signals.
SPREAD SPECTRUM TRANSMIS SION A transmission technique that may have application to wireless microphone systems is known as "spread spectrum." The object of this technique is to improve performance by reducing interference effects and increasing efficiency of band utilization.
Important transmitter characteristics are power output and antenna efficiency . Maximum power is limited by government regulations and battery capability . Antenna efficiency is limited by size and design. Recall that the efficiency of typical wireless transmitter antennas is fairly low , about 10% or less for VHF .
width than wireless microphones. Even so, bandwidth limitations necessitate the use of companders to achieve acceptable dynamic range in most high quality analog wireless systems. The bandwidth required for a high fidelity digital wireless system depends on the amount of digital information transmitted and the transmission rate.
35 Selection and Operation of Wireless Microphone Systems C HAPTER 3 W ireless System Operation OPERA TION OF WIRELES S SYSTEMS OUTSIDE OF THE U .S. Allocation and regulation of radio frequencies is s.
SYSTEM SELECTION The proper selection of a wireless microphone system consists of several steps based on the intended application and on the capabilities and limitations of the equipment required for that application.
CRYST AL -CONTROLLED VS. FREQUENCY SYNTHESIS Crystal controlled wireless units can be designed with wide audio frequency response, low noise, low distortion, and relatively long battery life. They are the most cost- effective choice for fixed frequency applications involving a moderate number of simultaneous systems.
required by the microphone (usually between 11 and 52 volts DC). If less than the minimum is available, the condenser microphone performance may be compromised with less headroom or more distortion. This is not a concern with dynamic microphones (which do not require power) or with condenser microphones powered by an internal battery .
Mixers and playback devices produce line level outputs. These sources typically have low-to-medium output impedance and may be balanced or unbalanced. They can sometimes be interfaced with a simple adapter cable.
A udio Interface Here we will discuss the sound system interface. Remember that the basic function of a wireless microphone system is to replace the connecting cable between the source and the sound system. In the typical case, the output of the wireless receiver will resemble the output of the original source both electrically and physically .
The threshold type squelch adjustment procedure is: 1) T urn the transmitter power off to eliminate the desired signal. 2) T urn on all associated equipment in nearby locations to create the "worst-case" signal condition. 3) Set the receiver volume control to minimum to avoid excessive noise in the sound system.
SYSTEM SETUP: RECEIVER ANTENNAS Setup of receiver antennas involves first the antenna-to- receiver interface and then antenna placement. The simplest case is a receiver with the antenna(s) permanently attached. The antenna is typically a quarter -wave telescoping or possibly "rubber ducky" type.
SYSTEM SETUP : BA T TERIES Always use fresh batteries of the correct type in the transmitter and/or receiver . Most manufacturers recommend only alkaline type batteries for proper operation.
It should be noted in Step 3 (on pg. 43) that certain combinations of active transmitters and receivers might indicate pickup of an individual transmitter by more than one receiver . However , in Step 7 (on pg. 43), when all transmitters are active, each should be picked up by just its intended receiver .
TROUBLESHOOTING WIRELESS MICROPHONE SYSTEMS Even when wireless microphone systems appear to be properly selected and set up, problems may arise in actual use. While it is not practical here to offer comprehensive solutions for all possible situations some general guidelines are suggested.
F ollowing are some suggestions on wireless microphone system selection and use for some specific applications. Each section gives typical choices and setup for microphones, transmitters and receivers as well as a few operating tips.
Occasionally it is found that certain wireless microphone systems do not initially work well with certain instruments. Symptoms may include poor frequency response, distortion or noise. In most cases this can be traced to an impedance or level mismatch between the two.
AEROBIC/D ANCE INSTRUCTION Aerobic and dance applications most often require body- pack wireless microphone systems to allow hands- free use by the instructor . The microphone is most often a headworn type, with a unidirectional element. This will give the best results for feedback control and overall sound quality .
Receivers for theatrical applications are not unique but they must be of high quality to allow multiple system use with- out interference. It is not unusual to use as many as 30 simultaneous wireless microphone systems in a professional musical theater production.
FILM/VIDEOGRAPHY Film and videography applications usually call for lavaliere/ bodypack wireless microphone systems to minimize the visibility of the microphone. Handheld transmitters may also be used when visual appearance is not anissue. However , the receivers may be either fixed or portable.
51 Selection and Operation of Wireless Microphone Systems C HAPTER 5 Application Notes Lavaliere microphones will require a bodypack transmitter . If a desired handheld microphone model is available in a wireless version it can be used directly .
Using wireless in-ear monitors for point-to-point A wireless in-ear monitor system (IEM) can also be used as a point-to-point system with only one modification at the receiver . At the transmitter , no modification is usually required since it is AC powered and can accept line level signals directly .
Antennas The antennas supplied with most wireless products are omnidirectional. These are suitable for both point-to-point and point-to-multi-point applications. If additional transmission range is required it may be possible to use directional receiving and /or transmitting antenna(s).
Selection and Operation of Wireless Microphone Systems 54 C ONCLUSION It should be apparent from this presentation that wireless microphone systems are a technology that encompasses a very wide range of principles and applications.
55 Selection and Operation of Wireless Microphone Systems R EFERENCE I NFORMA TION Appendix A CALCULA TION OF INTERMODULA TION PRODUCTS The simplest IM products that can occur between any two operatin.
F or determining compatibility of three frequencies (200 MHz, 195 MHz and 187 MHz in this example) the significant combinations become: In this example, the third system frequency (187 MHz) has been chosen to avoid the first two frequencies and their respective IM products.
57 Selection and Operation of Wireless Microphone Systems R EFERENCE I NFORMA TION Appendix B Channel Band Video Chroma Audio VHF Low Band 2 54-60 55.25 58.83 59.75 3 60-66 61.25 64.83 65.75 4 66-72 67.25 70.83 71.75 5 76-82 77.25 80.83 81.75 6 82-88 83.
Absorption the weakening of radio wave strength by losses in various materials AF audio frequencies, typically 20-20,000 Hz. AM amplitude modulation Ambient local or background, ie.
59 Selection and Operation of Wireless Microphone Systems R EFERENCE I NFORMA TION Glossary Field a distribution of energy in space, ie. electric, magnetic, sound Field strength the amplitude of a fie.
Narrow band an FM signal in which the deviation is much less than the modulating frequency Noise undesirable random audio or radio energy Operating frequency the final output frequency of a transmitte.
61 Selection and Operation of Wireless Microphone Systems R EFERENCE I NFORMA TION Illustrations Included In This Booklet.
Suggested readings for more information on radio technology: • Solid State Radio Engineering, H. Krauss, C. Bostian, F . Raab (J. Wiley & Sons, 1980) • Introduction to Communication Systems, F . Stremler (Addison- Wesley , 1982) • Antenna Theory and Design, W .
Addi tional Shure Publications Avail able: These guides are available free of charge. T o request your complimentary copies, call one of the phone numbers listed below .
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