Wireless Microphones for Higher Education
May 11, 2012 2:42 PM, By Bennett Liles
Solutions for today’s classroom environments.
The higher education environment is a hugely challenging place for AV people and gear. Trending toward more students, bigger classrooms, and tighter scheduling, colleges and universities are requiring more sound reinforcement for instructors. The equipment used for this must function in the same high up-time and mega-multi-user arena where the rest of the hardware must perform. High reliability and user friendliness are basic and essential requirements, especially where AV staffing is kept to a bare minimum.
In the multi-classroom campus, sound reinforcement for speech tends to be required by the same group of instructors in different classrooms on a varying schedule, while visiting lecturers are likely to require wireless microphones in the more spacious lecture halls. All of this introduces issues of microphone/transmitter mobility, permanent versus temporary installation, equipment security, and user friendliness. The typical solutions for wireless microphone use in the campus environment fall into three technical approaches. These are radio frequency systems, infrared equipment, and electronic field production (EFP) RF systems.
RF, IR, or EFP
Each of the classroom microphone solutions has its own distinct set of advantages and challenges, so it is not unusual to see two or all three of them used in various places on the same campus. The traditional RF wireless mic systems are well-known to most AV techs, and they are simple to set up in temporary installations. As long as the classroom podium or instructor’s console is non-metallic, an RF receiver can rack or shelf-mount with other gear. It does work better if enough room is available inside the rack for the local UHF antennas on the unit to be extended up and out at 45-degree angles. If they are metallic whip types, direct contact with other metallic gear in the rack should be avoided. This will usually work for the minimal distances required by most instructors and visiting lecturers. Larger lecture halls with multiple receiver systems will normally require external antennas and coax connections to the receivers through antenna signal distributors, a type of which can be used in IR installs as well. The 50Ω coax normally used for RF antenna connections is RG-8/U, RG-58/U, or for long runs, the thicker RG-213/U.
The big downside to RF on campus is, of course, frequency coordination. Who is using which mic in what room and when? The microphones are not interchangeable between classrooms, so professors will have to switch mics between rooms because the prospect of users attempting to reset their own frequencies is too gruesome to contemplate. Does the mic stay in the classroom? Probably not for long unless it’s under lock and key. Who gets keys and does the microphone cabinet stay locked? With RF microphones issued to individual faculty members, they could quickly end up lecturing by radio to each other’s classrooms. The prospect of networked control over classroom RF mic receivers and setting their frequencies on a time schedule is an interesting scenario, but it gets less practical as the scale increases. With the development of drivers for networked receivers, their operation and frequency scheduling could be worked into the campus-wide AV network control application. For a small to medium-sized campus, this could enable each professor to be issued one microphone while the RF coordination is done on the receivers. For larger universities, RF microphone coordination can be a much more complex problem.
Enter infrared. Line of sight transmission beautifully solves the RF coordination problem and provides a low-cost signal security feature. This is the primary reason why IR wireless microphone systems have been strongly aimed at the academic market, but IR mic systems are not without their limitations. The signal might not only stop at the walls. It might stop at the presenter’s elbow when it’s in the wrong position or it could go silent when the lecturer turns to use the blackboard or to point at the projection screen. Whiteboards, however, can exhibit a limited amount of infrared reflectivity.
Infrared wireless mic transmitters use an LED to emit IR radiation to the sensors. The beam is modulated, in most cases switched on and off, to encode the voice data onto the IR carrier. The receivers use a photodiode to convert the photo beam to an electric current. From that point, the signal is carried over coax to the receiver in a method similar to that for RF.
IR systems typically require somewhat extensive sensor signal distribution systems on coax cable. In-line and rackmounted sensor distribution units can be used to allow more sensors to be either ceiling- or wall-mounted. IR sensor mounting locations must be more precise than those of RF antennas because reception can fall victim to a variety of interference sources including plasma displays, IR-assisted listening systems, infrared LAN links, incandescent lighting, and sunlight. For this reason, the light acceptance angle on many IR sensors can be adjusted to reduce interference from these infrared light sources. To save on coax, signal distributors can be mounted in the ceiling to run multiple IR sensors into a single coax to the receiver, but in addition to the loss in the cable, each distributor will also exhibit several decibals of loss. Seventy-five-ohm coax is normally used in IR installations, and the same signal loss characteristics apply here as with RF remote antenna installations. With RG-59/U the 100 meter loss is about 3.3dB. With RG6/U, the loss is 2.7dB and with RG11/U, it is 2.0dB. The thicker the cable, the lower the loss, the harder the runs are to make and, of course, the higher the price per foot.
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