SR/Live Sat, May 17, 2008

Wireless monitor systems are essential for stagebound musical productions. Perhaps the biggest advantage of a wireless monitor system is the ability to use an individual monitor mix for each musician on stage. Furthermore, a wireless monitor system significantly reduces the amount of, or even eliminates, monitor speakers in the performance area. This results in lower risk of feedback, and a more lightweight, compact monitor system.

Some special precautions must be taken before using wireless monitor systems. In most cases, this signal is a stereo signal. This multiplexed signal is more sensitive to dropouts, static and multi-path situations. For longrange applications, mono operation can improve system performance.

If wireless microphones and wireless monitor systems are used in parallel, those systems should be separated in a way that the frequencies are at least 8 MHz apart and that the physical distance between the transmitter and the receiver is maximized. This will reduce the risk of blocking – an effect that desensitizes a receiver and prevents the reception of the desired signal. Therefore, if a bodypack wireless microphone transmitter and a wireless monitor receiver are both attached to the same talent, those devices should not be mounted directly beside each other.

When musicians use the same monitor mix, one transmitter can be used to provide the radio frequency (RF) signal to more than one wireless monitor receiver. If individual mixes are desired, each mix requires its own transmitter operating on a unique frequency. To avoid intermodulation disturbances, the wireless monitor transmitters should be combined, and the combined signal should then be transmitted via one antenna.

Passive combiners suffer from signal loss and high crosstalk. An active combiner keeps the RF level the same (0 dB gain) and isolates each transmitter by around 40 dB from the other. Again, intermodulation is a major issue within the entire wireless concept. When using stereo transmission, it is even more critical.

When considering an external antenna, one important factor must be taken into consideration: the antenna cable should be as short as possible to avoid losses via the RF cable. A directional external antenna is recommended to reduce multipath situations from reflections, and it will have some additional passive gain that will increase the range of the system.

If remote antennas are used for the wireless monitor transmitters as well as wireless microphone receivers, those antennas should be separated by at least 10 to 15 feet. “Blocking” of the receivers, as discussed above, is then avoided. Furthermore, the antennas should not come in direct contact with the metal of the lighting rig. This will detune the antenna and reduce the effective radiated wireless signal.

Overall, the use of wireless monitor systems is more critical than the use of wireless microphones. In the next edition of Live Sound International, we will discuss the topic of frequency coordination of a wireless system. ■