Wireless microphones work by modulating audio signals over certain portions of the electromagnetic spectrum. The two most often used portions are the radio frequencies (RF) and infrared (IR) frequencies.
Infrared literally means "below red" - referring to the band of electromagnetic radiation just below visible red light. Infrared radiation has wavelengths between about 750 nanometers and 1 micrometer. RF frequencies,
- on the other hand, have wavelengths between about 0.31 meters and 6 meters.
The part of the infrared spectrum that is used in optical communications is usually between 1530 and1565 nanometers.
Infrared wireless microphone systems are often the best solution for classrooms. In today's RF saturated world, a regular (VHF or UHF) microphone system risks interference from a variety of other devices and sources. Cell phones, radios, TV, PA systems, police, and emergency service systems, and so on can all interfere with VHF and UHF, but not with IR. Unlike the RF systems, IR is relatively short-range, cannot penetrate walls, and is line-of-sight. This means that a school can have as many classrooms as desired with individual audio systems, without fear of interference from adjacent rooms.
HOW THEY WORK
Wireless IR microphones use LEDs to emit infrared radiation. The beam is modulated (switched on and off) to encode the data. The receiver uses a silicon photodiode to convert the infrared radiation to an electric current. It responds only to the rapidly pulsing signal created by the transmitter, and filters out slowly changing infrared radiation from ambient light.
An infrared transmission system is comprised of three components: the transmitter, the emitter (also called radiator), and the receiver. The transmitter modulates the audio signal onto a carrier frequency using FM or digital techniques. The emitter takes the modulated signal and converts it into infrared light. The receiver decodes the infrared signal and coverts it back to an audio signal, which is sent to the loudspeaker or headphones.
ADVANTAGES
- Not prone to interference like RF
- Does not penetrate walls
- More secure - not prone to "eavesdropping"
- Less expensive
- IR signal reflects off surfaces, aiding reception
DISADVANTAGES
- Emitter must be in "line of sight" with receiver
- Can be affected by sunlight or other ambient light sources
- Limited to short distances (relative to RF)
Infrared light lies between the visible and microwave portions of the electromagnetic spectrum.
