C HAPTER 3 SELECTION <strong>and</strong> OPERATION OF WIRELESS MICROPHONE SYSTEMS U.S. VHF T.V. CHANNEL MAP 25
SELECTION <strong>and</strong> OPERATION OF WIRELESS MICROPHONE SYSTEMS F REQUENCY BANDS FOR W IRELESS M ICROPHONE S YSTEMS 26 C HAPTER 3 l<strong>and</strong> mobile <strong>and</strong> paging services. Direct pickup or intermodulation from these sources is possible in this b<strong>and</strong>. Symptoms may again include noise, loss <strong>of</strong> range or actual audio pickup. Of course nearby use <strong>of</strong> other (unexpected) wireless microphone systems can create interference through intermodulation or direct frequency conflict. Given the finite number <strong>of</strong> wireless frequencies available, especially in the “travelling” channels, it is always possible to encounter other wireless microphone users in locations such as hotels, convention centers, <strong>and</strong> news events. Remedies for such interference involve identifying the interfering source if possible <strong>and</strong> relocating the source or the wireless microphone system to reduce proximity. If this is not sufficient it may be necessary to change the operating frequency <strong>of</strong> the wireless microphone system especially if the interfering source is a primary user. NON-BROADCAST SOURCES Non-broadcast sources are those that produce radio frequencies as a by-product <strong>of</strong> their operation. There are three main types which are <strong>of</strong> concern to wireless use: digital audio devices, digital computers, <strong>and</strong> certain AC power equipment. Digital audio devices include: digital signal processors (DSP’s) such as delays, reverbs, pitch shifters; digital recording <strong>and</strong> playback devices such as DAT recorders, CD players, hard drive recorders <strong>and</strong> samplers; digital electronic musical instruments such as synthesizers, organs, <strong>and</strong> MIDIcontrolled instruments. Digital computing devices include: microprocessor equipped units (PC’s, calculators), minicomputers (workstations) <strong>and</strong> main-frame computers. In addition, controllers for lighting, AV presentations, industrial equipment <strong>and</strong> certain video equipment contain microprocessors. Digital devices can produce broad b<strong>and</strong> RFI (radio frequency interference) in close proximity to the source. Any device which carries an FCC type approval label such as “Class B computing device” can be assumed to be a potential source <strong>of</strong> interference. The audible effect is usually high frequency noise or distortion <strong>and</strong> it generally only occurs when the receiver is close to the digital device <strong>and</strong> the transmitter is at a distance. Unfortunately, this is <strong>of</strong>ten the case when wireless receivers are located in or near racks <strong>of</strong> digital gear while the transmitters are being used on a stage. The best remedy for this type <strong>of</strong> interference is to locate the receivers <strong>and</strong> antennas at least several feet from any digital device. In a rack <strong>of</strong> different equipment this would suggest mounting the wireless receivers at the top, analog equipment below that <strong>and</strong> digital equipment at the bottom. In extreme cases, choosing higher wireless frequencies may improve matters. However, as the speed (clock frequency) <strong>of</strong> digital equipment increases this technique will be less effective. Finally, any equipment that uses or controls high voltage or high current AC power can generate radio frequency interference (RFI). Examples include lighting dimmers <strong>and</strong> some types <strong>of</strong> gas discharge lamp supplies such as neon or fluorescent ballasts. Audible effects <strong>of</strong> this type <strong>of</strong> source include buzz or hum in the signal. Again, the first remedy is to relocate the <strong>of</strong>fending source or the wireless equipment to minimize pickup. In some cases special filtering may be applied to the various power <strong>and</strong> connecting cables <strong>of</strong> both the source <strong>and</strong> the wireless equipment to block RFI from leaving the source or entering the wireless equipment. A property <strong>of</strong> FM reception which can reduce the audibility <strong>of</strong> many types <strong>of</strong> interference is the so-called “capture effect.” When multiple signals (close to the operating frequency) are present, the strongest signal will capture or lockin the receiver. If the desired signal is sufficiently strong, the interfering signals may not be heard. Since the strength <strong>of</strong> the desired signal (the transmitter) is dependent on the operating distance, a nearby transmitter can <strong>of</strong>ten overcome weak or distant interference sources. SPREAD SPECTRUM A transmission technique which has some future application to wireless microphone systems is known as “spread spectrum.” The object <strong>of</strong> this technique is to improve performance by reducing interference effects <strong>and</strong> increasing efficiency <strong>of</strong> b<strong>and</strong> utilization. Instead <strong>of</strong> concentrating the information <strong>and</strong> energy <strong>of</strong> the transmission at a single, continuous frequency, the signal is spread out over a wide radio frequency range. The two most common methods are “frequency hopping” <strong>and</strong> “direct sequence spreading.” Frequency hopping systems utilize a transmitter which changes its operating frequency many times per second according to a predetermined pseudo-r<strong>and</strong>om pattern. The receiver is synchronized with the transmitter so that it changes its operating frequency in exactly the same pattern. At any instant the system is operating on only one frequency but over time the range <strong>of</strong> frequencies used may be several MHz. The information may be carried as an FM signal or as a digital signal. Direct sequence systems operate around a center frequency but the effective total modulation (b<strong>and</strong>width) <strong>of</strong> the signal is significantly increased. This is accomplished by modulating the phase <strong>of</strong> the carrier with a high-speed, predetermined pseudo-r<strong>and</strong>om digital sequence (pattern). Again the receiver is synchronized with the transmitter according to the same pattern. In this system the information can be carried as an analog FM signal or as a digital signal, mixed with the phase modulating sequence. By spreading the transmission power <strong>of</strong> the desired signal over a greater portion <strong>of</strong> the radio spectrum, the average energy <strong>of</strong> the desired signal at any one frequency is reduced.
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