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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 . . . . . . . . . . 5 Radio Wave T ransmission . . . . . . . . . . . . . 5 Radio Wave Modulation . . . . . . . .[...]

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. It can evoke visions of freedom in prospective users and [...]

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 electro[...]

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. The wave equation states that the frequency of a radio wave, multiplied by its wavelength always equals the speed of li[...]

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. The total radio field at a given loca- tion consists of direct waves, indirect waves[...]

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. In each case, the information must be converted to a radio signal, tran[...]

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. Therefore, the selection of microphone type for a wireless microph[...]

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 . Handheld transmitter antennas are usually concealed internally , t[...]

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. Though more expensive, [...]

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. A control circuit called a phase-locked-loop (PLL) is used to calibrate the transmitter frequency to a reference "clock" frequency th[...]

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"). This circuit generates a constant radio frequency that is related to the frequency of the rec[...]

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). In a quadrature FM detector the IF signal passes through a circuit which introduces a 90 degree phase shift relative to the original IF signa[...]

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 . When the tra[...]

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 result is substantial variation in [...]

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. Switching noise is possible but this system avoids the possibility of phase cancellation between antennas[...]

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. (See Figure 2-24.) Signal-to-noi[...]

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. There is very little output alon[...]

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. Stand-alone active ante[...]

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. Use of radio frequencies in the United States is regulated by the FCC (Federa[...]

"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 . Propagation characteristics are good, and antenna length is more manageabl[...]

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). As in the high-band VHF region, unused television channels [...]

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. The first class of interactions is the more important one and may occur in any group of wireless microphone system[...]

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. The complexity increases exponentially for additional systems: a group of 1[...]

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 . When these two frequencies are applied to the mixer section (a non-linear circuit) one of the output frequencies of the mixer is this difference frequency (the IF), wh[...]

multipliers. However , both types of transmitters can produce other spurious emissions due to power regulating circuitry , parasitic oscillations, carrier harmonics, etc. These emissions can all be controlled through careful design. It can be seen that the calculation of both local oscillator conflicts and image frequencies depends on the intermedi[...]

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. Other countries, most of Europe and India for example, use a 7 MHz/TV channel bloc[...]

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). In the process, the commission [...]

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. UHF systems are even less likely to respond to commercial radio sources since the UHF band i[...]

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. Instead of concentrating the information and energy of the tra[...]

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 . This means that for a 50 mW 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. In practice, the ba[...]

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 supervised by specific government agencies in each country , with the result that allowable (legal) frequencies and frequency bands differ from country[...]

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. It should be remembered that while wireless microphone systems cannot ultimately be as consistent and reliable as wired systems, the[...]

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. One limitation inherent to a cr[...]

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 . The bodypack[...]

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. However , these high level input sources often require additional (external or internal) attenuation to prevent overload of the transmitt[...]

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 . Tha[...]

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. 4) T urn the receiver powe[...]

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. Receivers with non-detachable antennas shou[...]

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. Alkaline batteries have a much higher power capacity , more favorable discharge rate and longer storage life than other types of single-use batteries such as c[...]

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 . Unless there is audible interference when a[...]

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. Though problems with wireless microphone systems eventually [...]

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. PRESENTERS The most common wireless choice for presentations is a lavaliere/bodypack system, which allows hands-free[...]

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. Frequency response changes are most often due to impedance problems. Make sure t[...]

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 . A lavaliere microphone may be u[...]

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. This number can only be handled with systems operating in the UHF range. 10 to 12 sys[...]

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. A common choice is a camera-mounted receiver used wit[...]

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 . Since most handheld transmitters use unidirectional microphone elements a plug-on transmitter may be ne[...]

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 . (See Figure 5-2.) The modification to the recei[...]

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). Note that although diversity receivers are always preferred it[...]

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. T oday’s equipment has progressed to the point that excellent results can be achieved with minimal input from the ca[...]

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 operating frequencies (f1 and f2) are the sum of the two frequencies and the difference between the two frequencies: f1 + f2 (sum) f1 - f2 (difference) If we [...]

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. A third system that coincided with an IM product may experience interferenc[...]

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 83.25 86.83 87.75 VHF High Band 7 174-180 175.25 178.83 179.75 8 180-186 181.25 184.83 185.75 9 186[...]

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. ambient noise Amplitude magnitude or strength of a signal or wave AM rejection ability of an FM receiver to reject signals from AM transmitters and/or AM noise from ele[...]

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 field at a given point, measured in volts per meter for electric energy FM frequency modulation F requency a measure of the rate of variation of a wave o[...]

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 transmitter or the tuned frequency of a receiver Oscillator a circuit that produces a continuous periodic output Phase-lock -loop (PLL) a circuit which maintain[...]

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 . Stutzman, G. Thiele (J. Wiley & Sons, 1981) • F requency Synthesizers, V[...]

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 . • Selection and Operation of Personal Monitor Systems • Audio Systems Guide for Video Production • Audio Systems Guide for Houses of Worship • Audio Systems Guide for Meeti[...]