Optimizing Wireless Microphone Systems for the Real World, Pt. 1

In today’s sound reinforcement, broadcast, and even recording landscapes, wireless microphones and IEMs are a heavily-used tool. Cleaner stages, freedom of movement and mobility are all possible because of wireless technology. This series of articles aims to provide readers with some of the basics involved with getting the most out of your wireless systems. Experience shows that proper planning, system design and some audio basics go a long way towards success in this area. Frankly, if you’re having trouble with your wireless mics, it is not a matter of VooDoo, black magic, or the fluorescent lights – it is more likely good, old-fashioned interference.

Gain Structure

Let’s start with the basics – these are the things that apply to any wireless system, no matter how big or small. First off, gain structure is very important with wireless microphones in order to get the best sound. There is another benefit though – better range. When you are properly modulating the RF carrier (the actual transmission signal) the receiver has more to “lock onto” and thus your range improves.

So here are the steps towards setting the proper gain structure. First, start with the transmitter itself. If it is a handheld microphone, make sure it has the capsule on it that you plan to use. Next, ideally you’ll want the person using it to give you their vocal level – either speaking or singing – not the “rehearsal” level, but the real performance level if you can talk them into it. Or, if the performer or speaker is not available, use your best judgment and simulate what you think the talent will do.

What you want is to bump into the limiter on peaks. How this will be indicated differs with different manufacturers, but you should basically see that your audio signal fills up the available range and, on peaks, gets a red light to light up or some other kind of indication that your are out of dynamic range. All analog wireless units have a limiter in the signal path to prevent over-modulation, as specified by the FCC. Digital wireless units are different – they may not have a limiter. Still – it is important to get a good audio signal at the transmitter.

For body packs, connect the microphone element you intend to use – either a lavaliere mic, ear-worn, or headset. If it is a guitar pack, use the instrument cable and the guitar, bass or other instrument that will be used in performance. Again, what you want to see is full audio modulation with peak signals triggering the limiter.
As with any gain structure setup, you have now optimized your signal-to-noise in the transmitter/receiver system. Next comes the receiver. Ideally, you’ll want to connect the receiver to the mixing console with a line level input.

Many consoles only offer a line input as a 1/4″ TRS jack, so you may want to make or buy a custom cable for this purpose. It is OK to use a mic input, but you will sacrifice a small amount of signal to noise, on the order of 3dB. The reason for this is that you will need to pad down your wireless receiver output (usually by 40 dB) then re-boost it using the mic preamp. A slight amount of noise is added in this process. For lower-quality mic preamps, you may lose more than 3dB – perhaps up to 6dB or more. Nevertheless, some engineers like to use the mic preamp for the wireless mic input for the way that it affects the sound. We won’t get into matters of taste here.

Assuming you are connected to a line level input on your mixer, your wireless receiver should be set at 0dB (no attenuation). This combination of transmitter setup and receiver to mixer connection should yield the signal with the lowest amount of noise and distortion. Note that some digital wireless mic systems do not have level adjustments at the transmitter or receiver. In such cases, you may need to boost the signal at the mixer by some amount, depending on the original source level at the transmitter microphone.

Transmitter Placement

Let’s get one thing out of the way right now: handheld microphone placement matters. If you are in a corporate AV setting, it is to your advantage to coach the speaker (I don’t care if he or she is a CEO, General, or just an HR droid) to hold the dang microphone up to their mouth! This is all about gain before feedback, people. I’m sure as a sound professional, you already know this. But in my experience, these people do not. I think everyone who speaks publicly should have to go through microphone boot camp where there is a drill sergeant screaming “HOLD THE MICROPHONE UP TO YOUR MOUTH, YOU MAGGOT!!!” OK, rant over. But it is a pretty big deal. Again, if you don’t have a good solid audio signal going in, you will have noise and potentially more interference, not to mention good, old-fashioned feedback.

The more subtle art involves placement of the bodypack transmitter. One thing to keep in mind is that you don’t want the antenna to contact skin or even a sweaty shirt. My colleague Joe Ciaudelli at Sennheiser always suggests clipping the transmitter to the talent’s belt so the antenna is pointing down. Not all transmitters allow this, but most do. You also want to place the transmitter so it is not distracting to the talent – usually this means on the back of their belt or pants. It is not great to put it in their pocket, but in some situations, this is all you’ll get.

Keeping the antenna away from the skin or sweaty shirt can also be accomplished with tricks such as a piece of gaff tape covering their shirt just under the antenna. James Stoffo has suggested using a length of plastic tubing such as from an aquarium. I’ve seen techs use a piece of large shrink tubing that is shrunk over the antenna connector and then again at the tip to “seal” the antenna in a volume of air. Whatever works for you – do it. Otherwise, a very large percentage, perhaps most of your RF signal from the transmitter will be absorbed – this cuts your range and invites interference.

That’s it for now. Stay tuned for antenna systems (part 2) and frequency coordination (part 3).

1 COMMENT

  1. [QUOTE=;][/QUOTE]
    “There is another benefit though – better range. When you are properly modulating the RF carrier (the actual transmission signal) the receiver has more to “lock onto” and thus your range improves.”

    Not really accurate. First, the carrier’s amplitude remains the same regardless of deviation (it is FM after all) and thus the receiver front end will see the same signal strength regardless of the audio level. Secondly, the broader band the signal whether in the form of static or dynamic deviation, the more RF power required to transmit the same distance. Even if there was some anomaly whereby the amplitude was increasing with deviation, it would be so little as to be inconsequential in terms of increased range. I suspect what you’re trying to say is that with optimum carrier to noise ratio (CNR), the receiver will not squelch as early thus providing the full distance of operation the carrier can provide.

    “Otherwise, a very large percentage, perhaps most of your RF signal from the transmitter will be absorbed . . .”

    I understand what you’re trying to say, but the reality is a very large percentage of a bodypack’s RF energy IS absorbed as long as the salty water bag is within the reactive near field of the antenna, considered to be one wavelength, and less than 1/4 wavelength is truly detrimental. The only key points are to keep the antenna dry, not touching any part of the skin or sweaty clothing, and as straight as possible. You can do the test yourself: Put a beltpack in free space and measure it’s signal strength at a measured distance (at least 10 wavelengths). Then put the beltpack on someone (non-sweaty), measure out the same distance and measure the resulting signal. You’ll see a drop on the order of 20-30dB when the antennas have CLOS to each other. Now have the person turn around.

    Henry Cohen

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