No matter how the details of a technology change, the basic functions and applications of audio equipment remain the same—music and voice must be clearly heard and understood from a distance. Specific to wireless applications, the technology allows mobility and flexibility without the entanglement of wires and the benefit of being able to engage the congregation more directly.
One of the key drivers for innovation in wireless microphones is the changing environment prompted by the wide variety of wireless devices now in common use, and their effect on the RF landscape, as well as the legislation put in place so they can all “play together.” For some of you, this has meant that your existing wireless solutions are experiencing new problems. So how will these innovations—including digital wireless—help you continue to spread the Word?
The Wireless Environment – An Example
As the wireless spectrum has become more crowded with diverse applications, frequency bands that were once clear, or at least practical for wireless microphone use, are experiencing widespread interference. An example of this evolution is those highly touted frequency-agile UHF wireless units from not too many years ago, operating in the 700 MHz band. First came the mandated digital television conversion to HDTV, which rendered the former clear spaces between the video and audio signals useless (resulting in high background noise and shortened range for your wireless transmitters), and then the more recent clearing of large swaths of that band for public safety and wireless carrier services. With billions of dollars of revenue involved, the fate of your wireless microphone investment was lost in the noise.
Is it Interference or Something Else?
If your wireless microphones have been experiencing more problems lately, try to isolate whether the system itself is faulty for some reason, or if a new source of interference has arisen. Sources of interference include digital television (DTV), computers and digital processors/equipment, faulty electrical equipment and lighting fixtures, intermodulation from multiple wireless transmitters, other wireless systems in use, such as in-ear monitors or wireless intercoms, and smart phones and cell phones worn by a presenter and in close proximity to a beltpack or other transmitter (or the cable to a lavalier or headset mic).
Test a problem wireless channel in isolation with all other transmitters and receivers off, and see if anything changes. If you are hearing audio anomalies, temporarily remove the receiver antennas and bring the transmitter closer to the receiver; this should lower the background interference from other sources and allow you to hear what is coming from the transmitter by itself. If it's OK, it is likely RF interference from an outside signal. Use remote antennas to lessen the distance to the transmitter, reposition the receiver for better line-of- sight, or try another channel.
Remember that interfering signals could also be entering through the cables, between the receiver and the next stage in the audio chain, or in the cables of a lavalier, headset, or instrument mic. Try repositioning or replacing them.
Have you added a new channel of wireless lately? Perhaps this new frequency is interfering with your existing systems via a phenomenon called intermodulation. When two or more frequencies transmit together in the same location, additional frequencies are generated which can fall on or near one of the channel frequencies you use. Symptoms can include reduced range, frequent dropouts, noise and distortion, or crosstalk between systems.
If it is a frequency-agile system, change to another channel/frequency and see if the problems go away. Most of the more modern wireless systems have pre-coordinated channels, but if you are mixing brands and technologies, intermodulation interference is a possibility. Additionally, when this signal hits an open receiver that is set or tuned close to its frequency, the resulting noise signal can be demodulated by the receiver and from there become audio; whenever you can, mute the receiver channel when its associated transmitter is off.
Some transmitters have high and low power settings. Use the lowest available transmitter power to effectively cover the particular situation; it will increase battery life, as well. The lower power setting will lessen the possibility of intermodulation, but could make the signal more prone to interference from outside sources if the transmitters are at too great a distance from the receiver antennas. Walk-test the area to make sure.
Solutions to Interference
As a start, keep computers, digital signal processors, and other digital devices several feet away from your wireless receivers and their antennas, as well as the audio cables that connect them. This includes cell phones and other personal digital devices. Know the frequencies of any fixed wireless systems you have, and the ranges of any frequency-agile systems. Respect the standard advice of having a clear, line-of-sight path between the transmitters and the receiver antennas whenever possible— or as close to this ideal as possible.
Remote antennas, both passive and active (containing electronic circuitry to make up for the signal loss in the cable going from antenna to receiver), can be used to shorten the distance between transmitter and receiver, resulting in increased signal strength. Directional antennas will provide increased gain in one direction and greater attenuation of unwanted signals on the rear of the antenna.
If you want to look at your local digital television environment, the FCC site (www.fcc.gov) has detailed databases that cover all active channels, from low-power community stations to commercial giants. Follow the Databases section to the Television Station Search link and submit your city and state information. Once you know what’s there, you can select wireless frequency ranges to work around it.
If you are adding more channels, or replacing wireless units that have particular problems, consider the benefits of the newer digital wireless microphone systems.
What About Digital Wireless Systems?
For many of the perennial wireless problems, digital wireless may well provide the solution. With a typical analog UHF or VHF wireless system, a very high frequency radio wave carrier is modulated slightly in frequency by the audio signal, transmitted to the receiver, and demodulated to recover the audio signal. Digital transmission changes that paradigm.
Within the microphone transmitter, the audio signal from the voice or other source is digitally sampled, and the sample is converted into a digital “word” consisting of the electrical equivalent of a string of 1s and 0s. As before, a very high frequency carrier wave is modulated, but in this case with the digital “stream” of samples so that the carrier frequency only has two distinct states that represent the signal in the same manner that the flat areas and pits on a CD represent the music. The receiver retrieves this information from the carrier and decodes it via a D/A converter and outputs an audio signal that is the replica of what was encoded at the mic.
Several manufacturers have introduced digital wireless microphone systems, and more will be available in the next year or two. Digital wireless systems are currently available from Audio-Technica, Lectrosonics, Line 6, MiPro, Shure and Sony. [Editor's note: reviews of the Line 6 XD-V70 and Shure PGX digital wireless systems are available at www.churchproduction.com/line60311 and www.churchproduction.com/shure0511, respectively.]
Why Digital?
Analog wireless transmissions are susceptible to a variety of noise and interference conditions, related to signal strength and/or interference from external electronic devices and other wireless signals. These can ride along with the carrier frequency and its audio signal as added noise, affect the receiver directly because the antennas that pick up the transmitter signal are also wide open to pick up other radio signal in the same general RF band, or interact with the carrier frequency to create additional harmonic frequencies.
While the same physics applies to a digital signal riding on a carrier wave, the digital signal with just two states is more difficult to damage. If the receiver finds that something has come in that is not equivalent to a digital word of 1s and 0s, that information will be ignored. If noise is riding on those digital words, it is still decoded as one of two states—rather than something in-between, if it were analog. As long as the digitally modulated carrier arrives at the receiver's antenna with sufficient level, it will be accurately decoded. And as with CD players and other digital audio devices, error correction algorithms are added to fill in the gaps where there is missing information. Typically with a digital wireless system, the signal will retain its quality until the signal level is too low, and then it's gone.
Digital Wireless Advantages
What are some of the advantages of the new digital wireless microphone systems? First, they operate in different frequency bands than the systems you currently own, so adding channels is not likely to create interference. And those frequency bands, typically the lower 900 MHz and the 2.4 GHz range, are clear of high-power transmitters so you won't have to compete with a TV station or a cell carrier. The systems have much more robust rejection of undesired signals, plus sophisticated error correction algorithms, so disruptive noise bursts and distorted audio are unlikely. And these units provide tools for detecting the clearest frequencies and then sync'ing the transmitters with the receivers.
Though these systems offer many performance improvements, you still have to deal with antenna placement and spend some time determining range and potential dropout areas. The system is digital, so you do have to look at gain-staging at the transmitter and receiver to avoid clipping. When in the 2.4-GHz range, you do have to work around existing wi-fi signals from your wireless router; several digital systems have scanning features to help the coordination and provide trouble-free operation of multiple channels of wireless.
The Future Will Keep Changing
Expect that more wireless applications and devices will come into being and the RF spectrum will become even more crowded. However, within this environment, manufacturers of wireless microphones will continue to innovate and apply solutions drawn from the computer, cellular phone, digital audio, and other industries to their products. The laws of physics will not change, but the new and coming generations of wireless microphones will be made to work around the edges—and to deliver the audio quality and reliability that the worship setting demands.
