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Age Of Wireless


THE AGE OF WIRELESS

If you're old enough, you probably remember the precursor of the modern music video. Rock groups in the late '60's and early '70's would appear on television variety shows and lip-sync their current hit wearing peace symbols, bell-bottoms, and carefully coifed hair on a stage decorated in psychedelic paisley. Ah, the good old days!

One dead giveaway that they weren't really playing was the common absence of cables leading from the guitars and even the microphones. Apparently, producers wanted a clean, uncluttered look on camera. Besides, who would notice? Everyone would be listening with rapt attention to the canned music, right?

Wrong. Armed with a little knowledge of electronics, I remember thinking how strange this looked. Even stranger, why did they stand so still once they were unencumbered by cables? I thought rock music was supposed to be wild and free. Except for Mick Jagger and Pete Townsend, most of those guys were glued to their spots. Maybe it was force of habit; after all, when they played for real, they could get hopelessly tangled up in cables if they moved around too much.

These days, live performers needn't choose between movement and music. The advent of wireless technology has freed musicians from the cable's leash. Today's concerts feature lead singers and guitarists running amok on stage while belting out their licks. Welcome to the good new days.

TUNING IN

The concept behind wireless technology is simple: A transmitter converts a signal into electromagnetic waves, which radiate from an antenna into the surrounding environment. The antenna on a nearby receiver detects these waves and converts them back into the original signal. This is the basis of all wireless communication, including radio, television, and satellite transmissions, as well as cordless and cellular phones.

Electromagnetic waves (also called radiation) occur at different frequencies; the entire spectrum encompasses an extremely wide range that includes the radio frequencies (RF), microwaves, infrared (heat), visible light, ultraviolet, x-rays, and gamma rays. Wireless audio transmitters convert audio signals into the RF portion of the spectrum before broadcasting them into the air using a technique called frequency modulation.

Everyone is familiar with FM radio, which works in exactly the same way as wireless audio systems. An audio signal is fed into a transmitter, which varies, or modulates, a carrier frequency above and below its nominal value in a pattern that corresponds to the audio signal. The receiver, which must be tuned to the same carrier frequency, extracts the modulated information and converts it back into an audio signal.

The FCC approves, assigns, and licenses specific radio frequencies for a variety of uses. The frequencies available for wireless systems differ from place to place depending on local television, radio, and other signals. If you try to operate a wireless audio system on a frequency already occupied by another signal, interference may render your system unusable.

Most wireless audio systems offer a selection of frequencies to choose from; if one doesn't work due to excessive interference, you can try another one. In addition, most wireless audio manufacturers offer a frequency coordination service that identifies locally used frequencies.

STRIKE UP THE BAND

There are several bands of frequencies in the RF region assigned to wireless audio and other applications. In an ironic twist of terms, the VHF (very high frequency) bands occupy the low end of the region. These bands include low (49 to 88 MHz), mid (150 to 174 MHz), and high (174 to 216 MHz).

The audio portions of television channels 2 through 6 use the low VHF band; wireless auditory assistance for the hearing-impaired uses 72 to 76 MHz. Cordless phones, radio-controlled toys, walkie-talkies, and other consumer products use 49 MHz, which doesn't require a user license from the FCC. Making equipment that works in this band is inexpensive, but few wireless audio products use it because it's so crowded. The mid band is also quite crowded with industrial and government signals. In 1988, however, the FCC allocated eight frequencies in the mid VHF band that can be licensed by commercial users of wireless audio systems, including musicians, churches, theaters, etc.

The best price/performance ratio for wireless audio systems is in the high VHF band, but broadcasters use it extensively (it carries television channels 7 through 13). These frequencies can propagate through walls and sets, making them well-suited to live performance work. However, only broadcasters and video production companies can legally license this band.

The high end of the RF region is called UHF (ultra high frequency). UHF equipment is more expensive to produce, but more frequencies are available for wireless systems. UHF signals can be transmitted with greater power, which increases the operational range between a transmitter and receiver. However, this draws more current, reducing battery life or requiring a larger, heavier battery. In addition, these signals are reflected more easily, which is a blessing and a curse. Although the reflections can extend the operational range, they also contribute to some forms of interference.

The low UHF band encompasses 450 to 614 MHz, although the FCC forbids its use between 608 and 614 MHz for wireless mics. The UHF mid band, which has been approved for wireless use, extends from 614 to 806 MHz. Wireless audio systems don't use the low UHF bands much, but crowding in other bands will soon lead manufacturers and users to seek licenses for these frequencies. The newest generation of cordless phones shares the high UHF band (806 to 960 MHz) with several wireless audio systems. The 902 to 928 MHz region of this band is license-free.

Most VHF and UHF wireless audio systems multiply the frequency of a crystal oscillator until they reach the desired carrier frequency. However, a new technique called frequency synthesis uses electronic oscillators to directly generate UHF carrier frequencies. Frequency synthesis can help reduce the problem of interference by providing a greater selection of frequencies. However, these systems generally require more power, which, once again, translates to larger, heavier batteries. Examples of these systems include the Nady 950GS (40 channels in the 800 MHz range) and Samson UHF Synth series (74 channels in the 800 MHz range).

DON'T GIMME NO STATIC

As we mentioned earlier, RF signals at nearly identical frequencies can interfere with each other. This interference, called heterodyning, results in a windy or whistling noise that sometimes exhibits a repetitive beat pattern. Manufacturers generally select frequencies that don't coincide with radio or television frequencies, but other sources of RF radiation abound, especially in performance venues. These include digital music and sound gear, computers, fluorescent and neon lighting, and large motors and generators. Walkie-talkies used by backstage security personnel can also cause problems. Broadway represents the worst-case scenario, where almost every theater up and down the street uses wireless audio systems.

Intermodulation is another form of interference. In this case, a receiver picks up two dissimilar frequencies that interact within the electronic components to produce sum and difference frequencies, which also results in a windy or whistling noise. With a careful receiver design that includes high rejection and narrow input filters to attenuate other signals other than the intended carrier, this effect can be reduced. Unfortunately, receivers that offer several channels must have filters wide enough to accept all specified frequencies, which increases the possibility of unwanted signals entering the system.

In order to minimize both types of interference, frequency coordination is essential. Everyone using wireless systems in a venue, including on stage artists and technical crews, must select their frequencies so that they don't interfere with each other, or don't coincide with other local RF sources. This is always difficult, and sometimes impossible, when many wireless systems are in use at once.

Another common problem is dropout, in which the signal fades or disappears into a sea of static noise. This inevitable when the transmitter is too far away from the receiver, so it's important to minimize the distance between them as much as possible. The practical range of a wireless system depends on many factors, including transmitter power, type of antenna, and environment.

Among the most common obstacles to RF are walls, sets, scaffolding, lighting grids, and bodies. Solid metal objects tend to cast a "shadow" in the presence of short-wavelength UHF signals; if the receiver antenna is located in this shadow, a dropout is likely. However, nearby surfaces reflect UHF signals more easily, which might allow the signal to reach the antenna around the obstacle.

Reflections can also cause dropout. RF signals radiate from the transmitter's antenna in all directions. One part of the signal reaches the receiver's antenna directly, while the other parts are reflected toward the antenna from surrounding surfaces, particularly those made of metal. If the difference between the lengths of the direct and reflected paths is a multiple of 1/4 to 3/4 of a wavelength, the two signals will partially or completely cancel each other out, resulting in a dropout. This is called multipath cancellation.

The exact locations of multipath cancellation zones depend on the relative positions of the transmitter, receiver, and reflective surfaces. In an on stage wireless audio system, the receiver and surface are generally stationary, while the transmitter is moving around with the performer. This causes the cancellation zones to move around, as well. When a cancellation zone coincides with the receiver antenna's position, the signal level drops. It's possible to map out the transmitter locations that cause cancellation at the receiver ahead of time, but it's hard to avoid these spots when you're running around during a performance.

LOUD AND CLEAR

Wireless receivers fall into two basic categories: non-diversity and diversity. Non-diversity receivers use a single antenna to pick up RF signals. This setup does nothing to reduce multipath cancellation, but it's inexpensive and works reasonably well with low-power transmitters over short distances (FM radios are non-diversity receivers that work over long distances, but the transmitters operate at very high power level.) The Nady Wireless One and 101, Samson VLP and Stage 2, Sennheiser VHF 1B, and Shure L3 are non-diversity systems that operate in the high VHF band.

Diversity receivers use two antennas to reduce the effect of multipath cancellation. If the antennas are placed at least 1/4 wavelength apart, it is highly unlikely that both will fall within a cancellation zone at the same instant. There are several types of diversity, each with their own ardent supporters.

Phase diversity uses two antennas and one receiver. If the signal level at the receiver drops below a threshold value, the phase of the signal from one antenna is shifted to compensate for the assumed multipath cancellation. In some systems, the phase immediately inverted by 180 degrees, while other systems continuously shift the phase according to the instantaneous signal level at the receiver. For example, the Telex FMR-100 continuously shifts the phase of one antenna to obtain the strongest possible signal.

The most common type of diversity is called switching, or true diversity. This method uses two antennas connected to their own separate receivers.

The system monitors the signal level from each antenna and uses the audio output of the receiver with the strongest signal at any given instant. This requires sophisticated circuitry to switch with no noise. Nevertheless, most high-end wireless audio systems use switching diversity to minimize multipath cancellation.

Combining diversity is a variation of switching diversity in which the audio signals from the two receivers are combined in the optimum proportion according to their relative levels. If both are strong, both are used; if only one is strong, it is used on its own. This scheme eliminates any noise that might be cause by switching from one receiver to the other. The Nady 201 and Shure EC4 and L4 receivers use combining diversity and operate in the high VHF band.

Another factor to consider is signal-to-noise (S/N) ratio. RF transmissions can achieve an S/N of no more than about 65 or 70 dB. As a result, virtually all wireless systems now use companding to improve the S/N ratio; this technology is probably responsible for the current viability of wireless audio systems. The transmitter compresses the signal, typically with a compression ratio of 2:1. The receiver then expands the signal accordingly, which yields an S/N in the 90 to 113 dB range. Companding also extends the practical range of the system by reducing the required RF level at the receiver.

Of course, companding has its own problems. The S/N ratio from one system to another can vary by as much as 20 dB, depending on the specific design of the companding circuit. Systems with lower S/N ratios may also exhibit audible "breathing." Other anomalies include high-end and low-end roll-off, as well as increased feedback sensitivity. These problems can be reduced or exaggerated depending on factory parameters such as threshold.

APPLICATIONS

Vocal microphones are among the most common applications of wireless technology. Many manufacturers build the transmitter directly into the body of a hand-held mic. For example, the Shure Beta 58 and 87 are available in wireless versions. Another option is a headset mic, such as the Samson system with the AKG 410 used by Garth Brooks. A headset mic is connected to a body-pack transmitter attached to the performer's belt. Tiny lavalier mics are also used with body-pack transmitters for lecturers and stage actors. The receiver is connected to the house audio system with standard cables.

Guitarists are using wireless systems in greater numbers, as well. Like headset and lavalier mics, the guitar output is connected to a body-pack transmitter attached to the performer's belt or guitar strap. The receiver is connected to the guitar amp in the normal manner. Examples of wireless guitar systems include the Nady Wireless One GT, Telex R-10G and R-20G, Samson Concert II, Sennheiser VHF 2G and UHF 2G, and Shure Guitarist.

Wireless audio is now in common use for electronic news-gathering (ENG). These systems include a wireless handheld or lavalier mic and a small receiver that usually mounts directly on a video camera. This receiver can be of the non-diversity type because the distance between the camera and reporter is generally very short. Several companies make ENG systems, including the Sennheiser UHF 2B, Lectrosonics Pro-Mini, Telex ENG-1, and Samson MR-1.

Other applications include wireless intercom systems for the technical crews in a venue. These systems can take the form of walkie-talkies, or headset mic/earphones with belt pack transmitter/receivers. For examples, the HM Electronics Systems 800 UHF and Telex Radiocom VHF intercom systems both accommodate up to four full-duplex (simultaneous send and receive) headset/belt-pack units with each base station.

Large performances often require several wireless mics and/or guitar systems operating simultaneously. In such applications, it's important to remember that each receiver can pick up only one signal at a time; if two or more signals are on the same carrier frequency, interference rears its ugly head.

Practically speaking, ten or twelve channels can be used together in each VHF band, while UHF can accommodate up to 40 channels simultaneously. Some wireless systems offer many more channels in a given band, but it may not be possible to use them all.

If the number of antennas on stage is unwieldy, an active antenna splitter can streamline the system. One set of antennas picks up signals from several transmitters and sends them to their respective receivers. For example, the Nady AD-4, Shure WA-404, Samson DA-4, and Telex AD-200 can each accommodate up to four diversity or eight non-diversity receivers; multiple units can be connected to serve more receivers. The Sennheiser SAS100, 200, 300, and 400 can accommodate up to six, twelve, eighteen, or 24 receivers, respectively.

Several companies offer multiple receivers in a single unit with one set of antennas. The Sennheiser EM 1046 offers up to eight switching-diversity UHF receiver modules in a single card cage and a computer monitor system. The Nady 750 offers two switching diversity, high-band VHF receivers in a single unit; up to five units can be connected for a total of ten simultaneous channels. The Samson UR5D includes two separate UHF synthesized receivers with 74 channels each.

This first appeared in Electronic Musician, July 1993, p. 58 et al.

 

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