There are few things in professional audio as frustrating as a wireless microphone system that is performing poorly or experiencing RF dropouts or interference. The reality is that wireless mic technology has advanced greatly in recent years, and when truly understood and properly applied, wireless mic systems can be relied upon to perform to the highest professional standards. But…
It is my observation that many competent audio operators and mixers have mastered many skills and techniques in other facets of sound production, but still lack a good grasp of how wireless mics work and how to really optimize them – particularly multi-channel setups. Sure, we are relieved when our wireless systems work, but what happens when they don’t? Do we really know how to effectively troubleshoot them? What should we know if we are planning to purchase and install one or more new wireless systems in our worship space?
Part of being responsible for worship sound means understanding the gear we’re using and ensuring it is set up and used properly. As the vast majority of worship centers now use at least one, if not several wireless mics, the church technologist that takes time to be at least familiar with the basics of RF wireless microphone technology is wise.
Here we’ll discuss some wireless system components and how they work, and then look at practical ways we can employ them in our worship services. We’ll discover that with careful planning, proper consultation, the selection of well designed equipment, and correct installation techniques, we can operate a number of wireless systems together with confidence and excellence. This discussion is arranged in two sections: I. RF Microphone Basics, and II. RF Microphone Applications.
Part I: RF Wireless Microphone Basics
A wireless mic system (or “channel”) is one complete system consisting of the sound source (microphone, or instrument cable), a transmitter with transmitting antenna, and a receiver with receiving antenna(s). This RF transmitter, receiver, antennas, and associated accessories are necessary for each and every wireless mic channel. The wireless microphone system is essentially a miniaturized radio station, broadcasting its signal through the airwaves as a radio signal. The audio output of the receiver is then wired into the sound system just like any other sound source.
The wireless mic transmitter (TX) is portable and may be in the form of a handheld device, a bodypack device with lavalier/headset mic or instrument input cable, or a plug-on “cube”. Handheld transmitters integrate a mic capsule, transmitter, and antenna all into one handheld “stick”. Bodypack devices are often clipped on the belt or worn in a pouch or pocket, and the audio source (lavalier, headset, or instrument cable) is wired to its input. The plug-on type of TX is a cube or tube with a female XLR connector on top that accepts standard professional wired microphones. Plug-ons trade aesthetics (they’re often a bit bulky) for the convenience of converting most standard professional wired mics to wireless. (These are the squarish devices you’ve probably seen on the bottom of some handheld mics used in television news by location reporters). Some plug-ons work only with dynamic mics, while others provide phantom power for use with condensers.
Whatever form it is in, the transmitter simply takes the original audio and modulates it onto an RF carrier frequency and radiates it as an electromagnetic signal from its transmitting antenna.
The wireless mic receiver (RX) is a fixed location (often rack-mountable) device or a miniaturized version for use on video cameras. The RX antenna(s) receive the radio signal from the corresponding TX and feed it into the receiver, which processes it and outputs the original (hopefully unchanged!) audio signal. A successful receiver design and operation will receive only the desired TX’s signal, rejecting any other radio signals present in the air.
Diversity reception is not a new development, but is still one of the most important advancements in wireless mic receiver technology. Consider this: Picture one single receiving antenna in an installation. The radio waves from the corresponding transmitter reach the receiving antenna by both the direct line-of-sight path and by multiple reflected paths from walls, floor, ceiling, and other objects in the venue. The waves that are reflected actually travel slightly longer distances to the RX antenna than do the ones that arrive by direct path. This is called multi-path propagation and can lead to reception problems – “holes” in the reception strength. The signal may be diminished or cancelled out in these instances, and the result is loss of reception/dropouts.
A careful repositioning of the receiving antenna to a “better” location might seem to solve this dropout problem – but remember our wireless mic transmitters are portable by design and are constantly moving! To combat this, diversity reception makes use of multiple antennas (usually two) at different points in space, increasing the likelihood of receiving a good, usable signal at any given time.
There are variations of “diversity” designs. “Real” diversity or “true” diversity (sometimes termed differently) actually employs a dedicated receiver circuit for each antenna. A microprocessor then monitors both of these diversity channels and chooses the best signal (or combination of the two) many times per second. True diversity remains a major innovation in wireless mic reception technology and when designed and set up properly it can greatly reduce RF dropouts. Some designs employ “antenna diversity” which simply uses two receiving antennas, but not two actual receiving circuits.
We are talking about analog FM wireless microphone systems, which make up the vast majority of the systems currently in use in our industry. There are also other wireless technologies emerging on the market (digital RF, for instance) for which our discussion points may not necessarily apply.
In frequency modulation, the transmitter modulates the audio onto the RF carrier frequency (fig. 4) and radiates it from it’s TX antenna. This frequency is expressed in megahertz (MHz), and is the frequency to which the TX and the corresponding RX must be tuned. The RF carrier is slightly modulated up and down in frequency, in accordance with the applied audio signal. The range of modulation is termed deviation, and the greater the deviation the more audio information can be represented in the RF signal – i.e. better sound, bigger dynamic range, better frequency response, etc.
If we are using more than one wireless microphone system simultaneously in the same environment, they must be on different operating frequencies. (That is, the TX and RX for system #1 must operate on one system frequency, and the TX and RX for system #2 must operate on another system frequency, and so on). But, not only that, they must also be using “clean” frequencies, that are free from radio traffic in the local environment (from devices such as cell phones, TV broadcasts, comm. radios, and so on). And further, our multiple wireless frequencies must be mathematically compatible with each other to avoid intermodulation interference among them! (This requires frequency coordination, and we’ll talk about it in Part II). For now, the point is that each system in use must have its own assigned frequency that is free from local interference and that is compatible with the rest of the system.
These analog FM systems usually employ a compander to improve (potentially double) the audio dynamic range of the wireless link, allowing a much greater signal-to-noise ratio and a cleaner and quieter audio signal (less noise/hiss!). While we don’t have time for a discussion of companders, just note that these systems manipulate the dynamic range of your audio signal by compressing it at transmission and then expanding it after reception to its original (hopefully!) dynamic range. Each brand or wireless family uses its own compander scheme.
Companders DO influence sound quality! A great compander may be nearly transparent to the user, while a poorly designed one will certainly inject undesirable artifacts into the audio during transmission. Companders differ significantly and the audible results may be subjective. Learning to read “between the lines” on RF product cut sheets is helpful, but there is still nothing quite like listening when considering the sound quality of a wireless audio system.
That’s the bare basics of RF mic components and how they operate. In Part II (next issue) we’ll discuss the real applicable stuff – frequency planning and coordination, interference, and tips on installation and troubleshooting.