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Wireless Microphones for Higher Education

May 11, 2012 2:42 PM, By Bennett Liles

Solutions for today’s classroom environments.

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As long as the local RF antennas are not completely surrounded by metal, an internally mounted RF receiver can provide short distance pickup.

With IR systems particularly, it pays to mount the sensors, turn on all possible sources of interference, fire up the system and perform some experimentation before drilling holes and actually running the coax through the walls so that the location of the sensors can be changed easily during initial testing. Jumping through these installation hoops is the price paid for being able to completely shed any RF coordination problems later. IR channels are usually in the 2-3Mhz range and the wavelength of the IR signal from mic to sensor is around 850 nanometers.

The EFP-RF solution is based on the concept of having both transmitter and mobile battery-powered receiver issued to instructors so that as long as each classroom they use has a sound connection for the computer, it can be used for the wireless mic receiver instead. The hardware used is typical of that used in electronic field production where the small receiver is camera-mounted. The downside of this approach is that it requires a little more setup expertise on the part of the faculty member and there are two units running on batteries instead of only one. Such things tend to get left on until the batteries are dead. The EFP-RF method also requires faculty members to keep up with two pieces of hardware instead of just one, so as convenient as it is for RF coordination, this arrangement is normally offered only to the more tech-savvy professors. Some instructors have even provided their own such equipment, with little or no consideration as to what frequency it operates on and what interference it might cause.

There is another interesting wireless scenario on campus that comes into play on occasions when a video recording is needed for an event such as a conference or a visiting lecturer. A sound feed to the back of the room may not be available if the room sound system is using hardwired microphones. In such cases, there is usually a separate wireless lapel mic clipped to the presenter to provide a wireless feed to the receiver on the video camera at the back of the room. However, if there are multiple presenters to be recorded it gets cumbersome to swap lapel mics between them. One solution is to use a splitter transformer inside the podium and to lead the podium mic signal through the direct leg of the splitter to the PA input as usual. The isolated split output is connected to a short adapter cable and into a bodypack transmitter taped vertically inside the non-metallic podium or lectern. The hardwired podium mic works as before with separate level control while the arrangement provides a wireless video feed to the back of the room for the camera. The direct microphone line through the splitter to the mixer will also allow the room’s mixer to continue providing any phantom power needed for a condenser mic on the podium but will not be affected by any level changes on the mixer.

The splitter and adapter cable can be used to provide a wireless camera sound feed across a room using the existing podium microphone.

Sound Control

The other side of the classroom wireless mic equation is how the sound from the receiver is controlled in reference to other sound sources. Should the wireless microphone receiver gain be set and left at a constant level or should users have the ability to vary it, with possible consequences of audio feedback? In many classroom sound installations, it is left for users to vary the mouth-to-microphone proximity to control the volume, but when a battery-powered EFP receiver is connected to the computer sound cable on a podium, the computer sound level control obviously is applied to the wireless mic receiver output.

Many university lecture halls employ a dual and redundant sound system setup with all of the podium media sources such as mobile computing device, disc player, and other audio sources being on separate mixer or switcher inputs whose selection follows that of the video sources on the podium control panel. The user’s volume control is the switcher’s master gain sent to the amplifier and it applies these sound sources to a set of stereo or surround sound speakers. At the same time, the wireless mic receiver’s output is fed directly to a second amplifier whose gain is set and enclosed in the locked podium. The mic amplifier output is conveyed to ceiling speakers arranged all over the room so as to maximize gain before feedback. In this setup there are two separate amps feeding different sources to separate specialized sets of speakers. This is typically the most initially expensive but most user-friendly arrangement for integration of a wireless microphone with other sound sources.

When an EFP receiver output is connected to the sound cable for the mobile computing device on a podium, the podium AV controller’s programming must reflect the selection of that sound source for normally non-sound items such as document cameras. In this way, the professor can go back and forth between displaying computer and document camera with no interruption of the wireless mic sound. In planning such classroom AV control features, a central consideration is the choice between sound sources being exclusively switched or mixed. With only a few sources such as computer, media player, and visualizer, a switcher can easily handle the job, but if there are many sound sources and live narration over media playback, a mixing scenario is required. The real challenge in this one is in making it user friendly. One of the most profound challenges for AV on a university campus is the huge variety of daily users each system will have and their broad range of technical competency along with their willingness to use the technology.

The electronic field production (EFP) solution uses a mobile transmitter and receiver.

The mixing setup will normally require a combination mixer/amp with a DC remote-controlled master volume feature that uses an external converter to change RS-232 serial commands from the podium AV controller to a DC voltage applied to the remote volume terminals on the mixer/amp. All sound sources, including the microphone receiver, are fed to various inputs on the mixer/amp. The individual gains of the sound devices are initially set on the mixer’s inputs and the master volume is remotely user controlled on the podium AV panel. Any sound source can be narrated over, but the relative gains between microphone and other sources are typically not adjustable except by accessing the mixer—which should be safely locked inside the podium or AV cabinet. This is sometimes known as the “all-on” sound configuration. Any sound source is heard whenever it is played no matter which video source is selected.

For simple classroom AV setups with few sources, a switcher with a dedicated auxiliary input can be the answer. These switch sound from only the video source that is selected on the podium AV controller, but the aux input, used for the mic receiver, is manually set on the unit and applied to the switcher’s output along with any other sound source selected. Users have no control over mic volume, but their mic will be heard regardless of the other single sound source selected.

Higher education is the most challenging AV environment and the right choice for wireless microphone signal conveyance (RF, IR, or EFP) will depend on the uses for each classroom, the type of classes taught, and the number of present and future wireless microphone users. The sound control method employed should remain transparent to users while uniquely serving their needs.

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