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Audio Gain Structure for Wireless Microphones
By Kent Margraves, Sennheiser
I mix several worship services--including seven Christmas services this past holiday season--but recently had a chance to enjoy a couple of Christmas productions from the audience's perspective (a rare experience!). As an audience member, I was soon reminded of the fact that audio techs sometimes forget (or just don't know) about audio gain staging for wireless microphones. The clipped (overloaded, distorted) sound on the wireless head-worn microphones during loud vocal passages was certainly distracting-- no one wants to hear that!
Audio people soon learn that every sound system is a signal chain and that the gain structure for each device in that chain (source, mixer, processor, amplifier, etc.) must be optimally set. If the audio gain structure of just one device is slightly off, a clipped or noisy audio signal can result. Wireless microphone systems are similar in that they also have a signal chain in which each gain setting must be properly set up. The difference is that part of the chain is on stage, and not physically connected (hey, its wireless!).
Whether it is a hand-held, body-pack, or a plug-on style, the RF transmitter has an audio input. Head-worn microphones, hand-held vocal microphones, electronic instruments, lavalier mics and other items might be plugged in. All of these, with their respective sound sources and varied applications, represent a wide range of signal levels.
The radio link is designed to have as much dynamic range as possible. Well over 100 dB is common by current standards. And to take advantage of that, the audio signal applied to the transmitter must be gained appropriately. We want our audio signal to use as much as of the dynamic range as possible, but not quite so much as to clip the signal. (Sound familiar? It's not unlike gain structure in other audio devices.) Manufacturers can't possibly preset a “one size fits all” sensitivity/gain setting here, as the range of applications is too broad. So, the audio frequency (AF) sensitivity control is provided to allow the user to correctly optimize the system for the application. It may be in the form of a switch in fixed steps, a potentiometer, or a software adjustment. It may be a very course adjustment or allow extremely fine control.
If the signal presented to the transmitter input is too low, it sits too close to the inherent noise floor of the wireless system. If the signal is too high, it simply clips. Either way, the resulting poor audio is broadcast to the companion receiver and fed to the sound system as it is. Once broadcast this way, it's too late; this can't be corrected downstream. Therefore, it is critical to adjust the input sensitivity correctly.
No matter the type of adjustment or resolution, the AF meter is the ticket. Just like metering on your mixing console, use this meter to adjust correctly. And if you think you're sound checking a user that is going to get a bit “more zealous” when there is a live audience, leave just a few dB of extra safety headroom below clipping. Once they are on stage with a live audience, it's not too easy to get your hands on their transmitter for a gain change! Some systems provide a useful AF meter at the receiver. Some also show this same meter on the transmitter. Some don't provide a meter at all, but usually provide at least a clip light at one or both ends of the system. In the latter case, the best the operator can do is increase sensitivity while sound checking until the clip light occasionally lights, and then back down slightly.
Correct AF sensitivity is critical for good sounding wireless audio. Even the best, most reliable wireless link you can find will simply reproduce the audio that is presented to its transmitter. If audible clipping occurs, the transmitter input sensitivity is the first place to check. It may need to be reduced. If excessive, constant background noise is audible, the input sensitivity may need to be increased.
Receiver output
Wireless receivers typically have an output gain adjustment, as well. This is designed to optimize their output level to interface with the downstream device, which is usually a mixing console. This adjustment is sometimes confused as being related to the radio link's dynamic range. It is not--this adjustment is only for the output level of the receiver and is unrelated to the AF gain staging of the RF link, which MUST be set at the transmitter.
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"I also would like to thank you and Sennheiser for not only your assistance with the whole TVBD fiasco, but your support of the H.O.W. market in general. It is truly remarkable these days to have a manufacturer of the size of Sennheiser that allows its personnel to interact with the end-user not only through outlets such as CSC, but the rapid personal response to crazy tech guys like me! It is why I can recommend using your products so readily."
"I also would like to thank you and Sennheiser for not only your assistance with the whole TVBD fiasco, but your support of the H.O.W. market in general. It is truly remarkable these days to have a manufacturer of the size of Sennheiser that allows its personnel to interact with the end-user not only through outlets such as CSC, but the rapid personal response to crazy tech guys like me! It is why I can recommend using your products so readily."
Jay Ballard, Director of Technical Ministries, First Baptist Church of Plant City, Florida