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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 memories of ancient disaster in veteran sound engineers. In all its forms, wireless has become a fact of life for people who design and use audio systems. With increased use of wireless microphone systems has come the
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Selection and Operation of Wireless Microphone Systems Part One: Wireless Microphone Systems: How They Work C HAPTER 1 Basic Radio Principles frequencies in the range just below visible light, which are RADIO WAVE TRANSMISSION perceived as heat (infrared radiation). The overall radio spectrum includes both natural and artificial sources as Radio refers to a class of time-varying electromagnetic indicated by Figure 1-2. fields created by varying voltages and/or currents in The amplitude of a
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Selection and Operation of Wireless Microphone Systems C HAPTER 1 Basic Radio Principles 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 and radio noise. Radio noise is nearly always considered to be u
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C HAPTER 2 Basic Radio Systems The output of the receiver is typically monitored through SYSTEM DESCRIPTION headphones or loudspeakers. It may feed a portable The function of a radio or "wireless" system is to send audio or video recorder. This is the configuration of information in the form of a radio signal. In this presentation, wireless systems for in-ear-monitors, (IEMs) interruptible the information is assumed to be an audio signal, but of foldback systems (IFB), assistive listening,
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Selection and Operation of Wireless Microphone Systems C HAPTER 2 Basic Radio Systems microphone level, and usually low impedance. Since the to clothing or belt, or may be placed in a pocket or pouch. "wireless" part of the wireless microphone only serves to In theater and some other applications they may be replace the cable, ideally, the characteristics and concealed underneath clothing. Input is made from the performance of a particular microphone should not source to the bodypack via a ca
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C HAPTER 2 Basic Radio Systems Handheld transmitter controls are generally limited called pre-emphasis, which is designed to minimize the to a power switch, a mute switch, and gain adjustment. apparent level of high frequency noise (hiss) that is Again, tuneable models include some provision for unavoidably added during the transmission. The "emphasis" frequency selection. Indicators are comparable to is a specifically tailored boost of the high frequencies. those in bodypack transmitters: p
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Selection and Operation of Wireless Microphone Systems C HAPTER 2 Basic Radio Systems A variation that is found in a few compander designs is TRANSMITTER: RADIO CIRCUITRY to divide the audio signal into two or more frequency bands. Each band is then pre-emphasized and compressed After processing, the audio signal is sent to a voltage- independently. In the receiver, de-emphasis and expansion controlled oscillator (VCO). This is the section that actually are applied separately to these same ba
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C HAPTER 2 Basic Radio Systems A few tuneable transmitters use multiple crystals to RECEIVER: obtain multiple frequencies. However, the base frequency GENERAL DESCRIPTION of the VCO for most tuneable systems is adjustable by a technique known as frequency synthesis. A control circuit Receivers are available in both fixed and portable called a phase-locked-loop (PLL) is used to calibrate the designs. (See Figure 2-9.) Portable receivers resemble transmitter frequency to a reference "clock" fr
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Selection and Operation of Wireless Microphone Systems C HAPTER 2 Basic Radio Systems is not tuneable. Filter circuits of various types ranging from frequency and still yield the same difference frequency simple coils to precision "helical resonators" are used in front when combined in the mixer. When the LO frequency is end filters. The second receiver section is the "local lower than the received frequency the design is called oscillator" (usually abbreviated as "LO"). This circuit "low-si
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C HAPTER 2 Basic Radio Systems only of frequency variations of the original input signal. followed by high frequency de-emphasis. Again, a This effectively eliminates the (high-frequency) carrier precision VCA with a true-rms audio level detector is required. frequency leaving only the low-frequency modulation Finally, an output amplifier supplies the necessary information (the original audio signal). audio signal characteristics (level and impedance) for In a quadrature FM detector the IF
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Selection and Operation of Wireless Microphone Systems C HAPTER 2 Basic Radio Systems RECEIVER:ANTENNA CONFIGURATION Fixed receivers are offered in two basic external un-muted configurations: diversity and non-diversity. Non-diversity receivers are equipped with a single antenna while squelch RF threshold Level diversity receivers generally have two antennas. Both RF signal muted and noise systems may offer otherwise similar outward features: units may be free standing or rack-mountable;
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C HAPTER 2 Basic Radio Systems RECEIVER: DIVERSITY TECHNIQUES Diversity refers to the general principle of using multiple (usually two) antennas to take advantage of the very low probability of simultaneous dropouts at two different antenna locations. "Different" means that the signals are statistically independent at each location. This is also sometimes called "space diversity," referring to the space between the antennas. For radio waves, this "de-correlation" is a function of wavele
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Selection and Operation of Wireless Microphone Systems C HAPTER 2 Basic Radio Systems 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 because the antennas are never combined. Range is the same as for a si
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C HAPTER 2 Basic Radio Systems output will usually consist of a mix of the two audio The smallest simple antenna that is consistent with sections. In the case of loss of reception at one antenna, reasonable transmitter output is an antenna that is the output is chosen from the other section. Excellent physically (and electrically) one quarter as long as the dropout protection is obtained with no possibility of wavelength of the radio wave frequency being transmitted. switching noise since t
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Selection and Operation of Wireless Microphone Systems C HAPTER 2 Basic Radio Systems 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 along the axis of the antenna. A three-dimensional representati
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C HAPTER 2 Basic Radio Systems 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. Helical antennas are highly directional and also broadband. Although these directional antennas are somewhat large (3-
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Selection and Operation of Wireless Microphone Systems C HAPTER 2 Basic Radio Systems usually operate at "unity" gain overall, that is, no net amplification LARGE ROOM/ occurs. Though a multi-coupler is generally a separate acces- MULTI-ROOM APPLICATIONS sory, some receiver designs are equipped with internal antenna distribution when multiple receiver sections are incorporated in Sometimes it is desired to use a single wireless the same chassis such as modular or card-cage systems. transmitt
