Just when everything is going well in a production, the audio operator often becomes the center of unwanted attention. One moment the singer on stage is moving freely around, belting beautiful sounds into a microphone. The next moment errant taxi calls are as clear as the singer. This seldom ingratiates the operator to the producer who is paying an orchestra by the minute. Fortunately it seldom happens at all today with the advancement in the technology of RF microphones. But all of us older than college age can remember the unmistakable sound of a wireless microphone receiver intentionally tuned to over-occupied spectrum.
Wireless microphone transmitters generally have three form factors today. In the first, the transmitter, battery and microphone elements are all contained in a shell about the same form factor as a conventional "wired" mic, just without a cable stuck in the south end. The second is a clip-on microphone (lapel style) connected by a (usually) thin wire to a separate transmitter/battery pack that can be put on a belt, in a pocket or even taped in an inconspicuous spot. The last is a variation on the second, with a lightweight boom microphone wired to a similar transmitter.
Many variations in usage exist. Put a mic on a guitar or violin, a moving calliope in a parade or the carefully tuned exhaust note of a speeding race-car, and the technology is precisely the same, though it is handled in a slightly different manner depending on the application. The logical extension of wireless microphones is wireless intercom. This operates in a "four-wire" mode, with separate transmitters operating in each direction, connecting to a four-wire intercom system directly, or to two-wire systems through a hybrid (transformer or active).
Under FCC rules in the U.S., wireless microphones operate in four bands: FM broadcast, VHF high band and UHF, all in FM, and Low VHF in AM and FM.
For professional use the FM broadcast band is generally not suitable, though in some limited applications it may be appropriate. It is generally not workable in crowed cities due to spectrum congestion. These microphones have limited range and are often used in consumer or "prosumer" applications.
In high favor are the VHF high band and UHF options. VHF systems have some ability to penetrate buildings. This is useful in instances where the receiver is in an audio mixing room adjacent to a theater or studio, or a microphone is used outdoors and the receiver is indoors. As you would expect, the lower the frequency, the better it is for such applications. VHF high band systems operate where the general noise floor is lowest, though it is important to select an unused channel.
The most common VHF systems operate on television channels 7-13. When there are suitable unused television channels in your operating area this is a good solution. When using these channels, be cautious about harmonics from the FM broadcast band, which fall squarely in this range. For instance, the first harmonic of 88MHz falls in Channel 7, and the third harmonic of 107MHz falls inside Channel 13. A little spreadsheet work with the local FM assignments can give you good insight into what will likely be a problem. Be careful to consider intermodulation components caused by multiple microphones operating together in addition to interference from occupied television channels. When in doubt, a spectrum analyzer is a great tool to look for signals trampling on your fragile signal. Remember that channels from other nearby markets up to 70 miles away may cripple your well-considered plans. Interference may show up as an increase in the noise floor, crackling or other audible disturbances. There are commercial services that can help you plan frequency choices and evaluate likely interference using computer modeling. With the implementation of DTV we may well see both fewer unoccupied channels and mysterious interference effects from high power digital transmitters and the intermodulation products associated with them.
Additional factors affect the selection of band. UHF radios can perform over longer distances when operating at maximum legal power. In addition, UHF antennas are shorter and can be easier to conceal. VHF radios are generally easier to implement, though the frequencies available in an area may be congested, especially in urban centers.
In the past, most receivers were crystal controlled and had limited ability to be frequency agile. Now most modern receivers are capable of covering an entire band. For instance, one manufacturer provides 94 channels between 794MHz and 804MHz.
It is best to keep the receiver as close to the transmitter as is practical to minimize signal loss and fade. The power restriction on VHF microphones is 50mW. It doesn't take much distance or absorption to decimate the signal strength. Radios operating in the 450MHz to 488MHz range can use up to 120mW, but interference from other occupants can be a major concern.
Because these radio systems operate in closed environments where the receiver can often see a reflection as strong as the direct signal, they often suffer from phase cancellation, causing a loss of reception.
To avoid this problem it is best to use a diversity receiver. The second antenna does not receive the out of phase signal, only the direct signal. By sensing the signal level at each antenna and appropriately switching to the best signal source the system can maintain quality reception more reliably. Switching between the signals can be done in the baseband audio signal, or in the RF domain. When switching is done in the RF domain the background noise level may well change. When the switching is done in baseband audio it is difficult to prevent detectable differences in received sound. Also, a complete second receiver and external baseband audio switch is required at extra expense. A third technique utilizes a combination of the signals from the two antennas to maximize the received signal. This presents a somewhat less complicated receiver since a second full receiver is not required. The distance between the receiving antennas is dependent on the frequency being used. It is sometimes best to roughly align the two antennas toward the intended transmitter location so that any glancing reflections are forced to be of different path lengths. This distance must of course not be equal to a multiple of the half wavelength. It is generally true that "true diversity," or dual receiver systems, will provide the best performance. Another example of "no free lunch" I am afraid.
Most wireless microphones today also include some type of dynamics processing to help reduce the noise floor. Compressing - or more properly, companding (because the process should be reversed at the receiver ) provides a degree of protection from overmodulation. This is particularly important with wireless systems because it is very difficult to produce an analog transmission system that is linear, free from noise and undistorted at the same time. Designers optimize the companding system to allow the operating level to be near the maximum undistorted transmission level and at the same time not prone to amplifying transmission noise. You may think of this as similar to noise reduction on analog audio tape, such as Dolby noise reduction in its various forms. In effect, it allows a higher average modulation level without creating distortion.
With all the problems with analog transmission, why not move to digital transmission? That is certainly a wonderful idea - provided spectrum can be devoted to the higher bandwidth of digital transmission. But digital transmission suffers from the same cancellation problems as analog, only with much worse effects. The well-known cliff effect can eliminate reception. This achieves a wonderful signal-to-noise ratio - or rather lack of signal to lack of noise. Dare we use the analogy of the emerging COFDM digital video microwave transmission systems for moving transmitters? If a cost-efficient system could be devised it would certainly raise the bar for wireless microphones. I suspect that the rapid advancement of digital transmission techniques will bring new approaches we have as of yet only glimpsed.