Attaining the best possible audio from wireless systems isn’t hard; at the same time, it’s also pretty easy to (needlessly) sacrifice audio quality. Here are a few practical tips on maximizing wireless audio quality.

Living with the limiter. Wireless transmitters all have audio limiters of some type, required by the Federal Communications Commission (FCC) and respective government agencies to prevent overmodulation.Excessive modulation can cause several problems, including serious interference for other systems and/or gross audio distortion.

So since they can’t be avoided, how do you live with them? One possible solution is to simply lower the transmitter microphone gain enough so the limiter never operates. Unfortunately, this is almost certain to degrade the signal-to-noise ratio (SNR) of the audio because the ultimate signal-to-noise ratio in a wireless is restricted by ambient RF noise, practical circuit limitations and other factors. So turning the audio down too much may just bring up the noise floor.

The problem then is to balance the audible effects of the limiter against increased noise, trading one for the other by varying the microphone gain. The optimum setting depends upon the characteristics of the wireless systems being used. Some transmitters have very good limiters that operate unobtrusively. Most are not so benign, and a few sound downright nasty. So the first step is to test a system to see how objectionable the limiters are, listening carefully with the mic gain set too high.

If they don’t sound too bad, then the mic gain can be run higher because occasional minor limiting on voice peaks won’t be a serious problem. Many transmitters have LEDs that flash when the limiter is reached, which can considerably simplify things. The correct mic gain setting is when the LED flashes very briefly only on the very loudest voice peaks. If, on the other hand, the limiter has a nasty sound, you’ll just have to sacrifice some SNR (or buy a better system).

Keep in mind that vocal loudness varies widely between individuals, so it is always far better to make the final adjustment with the actual user in as realistic a manner as possible. And keep in mind that most users are louder during the actual event than during sound checks, so keep the limiter setting a bit on the lower side.

Setting transmitter mic gain. The need to balance limiting against SNR has another important consequenceL the transmitter mic gain control should never be used to set the desired receiver output level. Doing so will almost guarantee that either SNR or audio quality will be compromised.

Instead, once the transmitter mic gain is properly set for the best compromise between SNR and limiting, adjust the receiver output level control to set the mixer level. Also be careful to avoid overdriving the mixer input; the output of most wireless receivers can be adjusted high enough to cause clipping in the mixer.

Many wireless receivers have a switch to select between mic level and line level output. Be certain to use the switch setting that matches the mixer input mode and setting.

Receiver line level output. Many receivers have a switch to select either mic level or line level audio output. In situations where a long cable is needed between the receiver and mixer, or when there are hum and noise problems,using the line level output is often very helpful. Since the line level output of receivers is typically 20 dB or so higher than the mic level output, hum and noise can usually be reduced by this amount.

Once again, be sure not to overload the mixer preamplifier. If the line output of the receiver is used, the mixer must also be set to accept a line level input.

Avoiding feedback. Feedback is more of a problem with wireless mics, as opposed to their wired brethren. The freedom and mobility of wireless allows, and even encourages, users to range about widely, often in front of loudspeakers. (This can easily be controlled by cable length with wired mics.)

Another problem is that omnidirectional mics are normally used with wireless bodypack transmitters. These mics lack the controlled directional pattern of typical vocal mics and are inherently more susceptible to feedback. Also with bodypack transmitters, users frequently forget that they are wearing a live microphone.

The limiter in the wireless transmitter can also increase the possibility of feedback. If the limiter is unobtrusive, and the mic gain is set so high that the transmitter is constantly in limiting, the gain increases when the user is momentarily silent. The result is often a “hollow” sound or feedback.

These can be a difficult problems to address. It may not be possible to improve the microphone technique of users, or to restrict their movements. When feasible, using handheld mics with directional elements, as opposed to bodypacks and omni mics, may be at least a partial solution.

In addition, directional lavalier (lapel) mics are available and can help reduce the chances of feedback. However, when worn on the chest, the level usually changes greatly if the user turns his/her head. Another drawback is that directionals can only be worn on the upper chest, and are therefore more difficult to hide. In addition, equalization is usually required because the voice sounds quite different when the mic is below the chin versus in front of the lips.

Sometimes the only solution may be to move the loudspeakers, lower the level of the loudspeakers in sensitive areas where wireless users may wander, or make other acoustic adjustments. For example, it may be possible to place unobtrusive physical barriers such as flower arrangements, furniture, props or other items to discourage users from walking in front of the loudspeakers. For more experienced users, visual cues such as lines on the floor may be sufficient. Where permitted, regular beatings may also be helpful… :>)

Clothing noise. Noise from clothing is a frequent problem when using lavalier mics. Ideally, the element in the mic should be completely mechanically isolated from the mic case and the mic cable. However – easier said than done! In fact, this is more or less impossible with a miniature lapel mic. As a result, fabric rubbing on the mic case or cable is a frequent source of unwanted noise.

A good mic clip is the first line of defense. The clip should hold the microphone case securely away from clothing so that the fabric cannot rub against the microphone case. Certain types of rough fabrics can also cause noise when they rub against the cable near the mic case. The more effective mic clips also secure the first inch or so (3cm to 5 cm) of the cable to isolate it from clothing.

If the microphone must be hidden inside clothing, where it is trapped between layers of fabric, a mic clip may not be adequate. In this case, a small cotton muslin bag or wrapping may be needed. The best muslin is quite soft and causes very little mechanical noise, especially if it is secured tightly to the microphone case. Hidden mics, especially when under thick clothing or protected by muslin, must usually be equalized to restore the high frequencies lost to the clothing.

Clothing can cause non-mechanical noise. Very smooth fabrics such as silk, rayon and certain polyesters can generate static sparks (electrostatic discharges) when the humidity is low. Because the voltages can be quite high, the electronics in lavalier mics can be affected. If cracking noises are present when the user moves about, it is likely that either the mic cable is bad or that static is the culprit. A bad mic cable will have to be replaced, but commercial antistatic sprays can do wonders for clothing static noise.
More soon…

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

For more articles by Gary, including a look at the potential impact that DTV will have on wireless systems, click here.