Recognizing DTV interference. Interference from DTV transmissions is completely different from interference caused by analog TV and most other sources. The symptoms are a constant high background noise level and unusually short range. In some cases, the audio will also cut in and out even when the transmitter is near the receiver.

A quick check for DTV interference is to remove the receiver antennas and move the transmitter close to the receiver. If the noise level is reduced and the audio is clean, there is a good chance that DTV is the problem. Changing the frequency of the wireless system will probably be the only practical solution. However, it is very unlikely that other frequencies within the same TV channel will work any better.

Grounding and hum. If the wireless receiver is some distance away from the mixer or on a different ac circuit, hum and noise often results. And some mixers just don’t handle inputs for ac powered equipment very well.

To solve this problem, it may be necessary to break the connection between the audio cable shield and the wireless receiver chassis. Some receivers have a switch to do this; otherwise, special cable wiring or a ground lift adapter can be used. DO NOT remove or disconnect the ground pin on the ac cords - this can be life threatening!

Another possible solution in mild cases is to use the line-level output of the receiver and the line-level input to the mixer. This can lower hum and noise by up to 20 dB. Don’t use the unbalanced output of a wireless receiver if at all possible.

Because few wireless receivers have output isolation transformers, elusive hum and noise problems can sometimes be difficult to solve. Some experimentation may be necessary to arrive at a solution. In stubborn cases, an in-line isolation transformer may be the easiest - and best - solution.

Wall-type power supplies. Many lower-cost wireless systems use wall-type (“wall wart”) power supplies to cut cost. In most cases, these units do not provide a connection from the ac power ground to the receiver chassis. This is no safety hazard, due to the low voltages, but hum and noise are a possibility. If problems are encountered, a low-resistance grounding wire between the ac power ground and the receiver chassis can sometimes be helpful. However, this can also create problems rather than solve them, so again, it may be necessary to experiment a bit.

Electrical interference. Another type of RF interference is caused by heavy electrical equipment, defective lighting systems, bad power wiring and similar sources. Often, it appears only when certain equipment operates and also may have a strong line frequency component. However, interference from analog TV transmitters can sound similar – it’s not always easy to tell them apart.

Finding a solution may prove difficult. In general, UHF equipment is less susceptible to such interference than VHF, and higher quality wireless receivers are usually more resistant than low cost gear. But there are few guarantees. Resolving problems of this type often requires the services of a good electrician.

Noisy AC power. In older buildings and facilities with lots of electrical equipment and machinery, the ac power service may generate considerable noise. Not only does this increase the risk of RF interference, it can get through the power supplies in the wireless receivers and into the audio. Sometimes the ground itself is noisy. The result can be clicks and pops, hum, buzzes, and similar problems.

Installing a low resistance connection between the local power ground and earth ground is almost always helpful. Making certain that power connections are tight and in good connection all the way back to the main facility panel is also recommended. If the problem can be isolated to one or two pieces of equipment, repairs and thorough maintenance of the wiring is highly advisable, not only to stop the interference but for safety reasons.

If the noise is getting into the audio via the receiver power supply, “filtered” power strips or even ac line filters will likely be useless. For dc powered equipment, a high-quality dc power supply in place of a wall-type supply will often work. For ac powered gear, all solutions are likely to be expensive.

Lavalier mic electrical noise. In addition to noise caused by static electricity, electret lavalier mics are susceptible to high voltage electrical fields. Neon lighting is a particular problem, but some other types of lighting equipment, computer monitors, large screen TVs and other high voltage equipment can be troublesome.

Except for very large neon systems, simply keeping the mic at least 24-in to 36-in (60-cm to 90-cm) away from the source will usually eliminate the problem. Susceptibility also varies greatly between mic models; try using a different mic. Be careful not to mistake this problem for RF interference; the symptoms may appear similar.

A regular contributor to ProSoundWeb Install Sound, Gary Stanfill is principle consultant for Colmar Systems, based in Southern California. His company has provided engineering and marketing consulting services to wireless microphone manufacturers for several years. Stanfill was president and general manager of Vega, a leading manufacturer of wireless microphone systems, for a total of 18 years. He has over 30 years experience in audio, RF technology, communications and wireless systems. He can be reached at gjstanfill@earthlink.net

Related Wireless Articles by Gary:

Wireless Handbook

Chapter 1: Antennas, Intermod, Frequencies and Batteries.
Chapter 2: Clear Path, Antenna Cables & Splitters, Use Of Remote Whips
Chapter 3: Digital Interference, Lav Mic Issues, Overall Sound Quality

And…

“Max” Wireless Audio Quality
The Bottom Line: Legal Use Of Wireless Systems
Requirements For Using Wireless Systems On TV Channels