A wireless microphone is essentially a scaled down version of a commercial broadcasting system. The speaker talks into a microphone, the signal is then sent to receivers, and the signal is heard. In a wireless microphone system the components are miniature versions of commercial equipment, but the same principles still apply. The transmitter is small enough to fit inside a microphone or small case. Since the transmitter is battery powered, the user can move freely. The signal is then picked up my a receiver that is ultimately connected to a speaker. There are two types of wireless microphones, handheld, and lavaliere with a transmitter pack.
Wireless microphones are used in almost everything except studio work, television, film, and live sound all benefit from the use of a microphone that requires no cable wrangling.
The Origins of Wireless Microphone Technology
Technological advancements since the 1960s have affected both the size and performance of wireless microphones Until that time wireless microphones were large and used miniature vacuum tubes, and did not sound good by today’s standards. The development of semiconductor technology in the late 1960s helped these problems significantly. Technology in the early 1970s introduced the integrated circuit compandor which was incorporated into wireless microphones to reduce noise. At about the same time, the FCC authorized the use of frequencies in TV channels 7-13 for wireless microphones. Thus the wireless microphone’s most serious problem, radio interference from other services, was virtually eliminated. Later, the application of diversity reception minimized the problem of dropouts (transmission losses due to cancellation of radio waves), greatly improving system reliability.
Today’s wireless microphones perform as well as conventional wired microphones. In the 1980s, wireless microphones were being manufactured with improved dynamic range and smaller transmitters, a result of better compandor integrated circuitry and advanced circuit design techniques. Wireless microphone design has changed very little since the 1980s
Radio Frequencies Used
There are no international standards for wireless microphone radio frequencies. Transmitter power limits, frequency stability, or RF bandwidth occupancy are not controlled. Wireless microphones could theoretically, operate at any frequency. In the United States the FCC regulates what frequency bands can be used. The Federal Communications Commission regulations are as follows:
• Low power communication devices may operate in the 49.81 MHz to 49.90 MHz band (Figure 10-32 (a)).
Power is limited to 10,000 JiVIM radiation at a 3 meter distance (approximately 1 to 5 mW) and with a 5 kHz audio frequency limit. This segment of the RF spectrum is susceptible to man made noise generated by auto ignition, fluorescent lights, dimmers, etc. The restriction imposed by the FCC on low power equipment aggravates the problem of signal-to-noise ratio in this band. Because these frequencies are evenly spaced (15 kHz apart), only three wireless mics can operate simultaneously without RF intermodulation products causing interference.
Wireless mics are permitted to operate in the commercial FM broadcasting band, providing their power is not greater than 50 JiVIM radiation at 15 meters. With this power restriction, it is not practical to use this band for professional applications in which reliable transmission performance is expected.
• Wireless mics may operated on a shared basis with business radio service. Continuous radio transmission is authorized if the transmitter power is limited to 120 mW. The business radio service frequencies for wireless
mics are: 30.76 MHz to 43 MHz, VHF low-band; 150 MHz to 173.4 MHz, VHF high-band: 457MHz to 470 MHz, UHF low-band; and 806 MHz to 866 MHz, UHF high-band (Figure 10-32 (b)). At 150 MHz and higher, man-made noise decreases significantly. With the higher power and larger transmission bandwidth allowed by the FCe, along with many more available frequencies and the shorter antenna requirement, operation in the VHF high-band and higher is desirable. The major disadvantage is interference from other business radio services. An operating station license is required and the transmitter must be type accepted
under FCC regulations. Contact your local FCC office for aForm 25 if you want to obtain a license. Anyone can operate a wireless mic system at these frequencies. Specific frequencies designated in this part ofthe regulation for wireless mics are known as “B frequencies.” The bands are from 169.445 MHz to 170.245 MHz, from 171.045 MHz to 171.845 MHz , from 169.505 MHz to 170.305 MHz and from 171.105 MHz to 171.905 MHz (Figure 10-32 (c». “B” frequency transmissions must not exceed a bandwidth of 54 kHz and output power of 50 mW.
• For broadcasting, video production, and film making applications, wireless mics may operate in the 174 MHz to 216 MHz range (TV channels 7 thru 13), on a non-interference basis. (Figure 10-32(d». This means that for
a given location, wireless mics may operate on unused TV channels. Transmitter power is limited to 50 mW. An operating station license is required for broadcasters and filmmakers and the transmitter used must be type accepted under FCC regulations. This VHF high-band offers the best operating area for wireless mics. It is free of citizens band (CB) and business radio interference, and any commercial broadcast stations that might cause interference operate on a schedule, and thus can be avoided easily.